T50 on new install?

[siricosm]

I am about to replace my radiators (and piping) and doing the heat loss calculations. I am wondering if the T50 rating on radiators is still reasonable. Wouldn't T40 be more efficient with today's boilers, or would the difference be hardly noticeably?

I guess T40 would also be at the upper limit of what you could use with a heat pump, which could be an issue if(when?) the government starts to apply pressure to get rid of gas boilers.

What would you use in your own house, if installing a new system from scratch? I am not interested in underfloor heating.

 

[SimonG]

I would always want slightly oversized rads. Controlled on the trv. Easier to turn down if too warm but nowhere to go if too cold.

 

[ShaunCorbs]

If you have the space for the size increase why not

 

[Knappers]

Keep an eye on total volume and expansion requirement

 

[siricosm]

Keep an eye on total volume and expansion requirement

I am planning on using cast iron rads and a thermal store, so the expansion is going to be way more than the vessel in a combi. This is yet another part I don't need.

Surprisingly, regular boilers are even more expensive than system boilers.

Boilers are like bikinis, the less you get, the more you pay.

 

[ShaunCorbs]

Things have moved where 15 years ago heat only boiler was standard ish now not many are getting installed compared to combis

 

[siricosm]

I am planning on using cast iron rads, which are not cheap. So at T40 I would need 1.6x the wattage. For a proper heat pump setup say at T30, one would need 2.4x. So it is hard to know where the sweet spot is.

 

[Knappers]

I am planning on using cast iron rads, which are not cheap. So at T40 I would need 1.6x the wattage. For a proper heat pump setup say at T30, one would need 2.4x. So it is hard to know where the sweet spot is.

Would have 1.6x, would need 2.4x?

 

[siricosm]

As I understand it, if I calculate rad sized based on T50 rads (70C flow), I would need to multiply the wattage I need by 1.6 to have a 60C flow (T40), or by 2.4 to have a 50C flow (T30).

 

[Knappers]

And by wattage needed you mean equivalent output of rad at T50?

 

[siricosm]

Yes. At T40 the rads would need to be 1.6 times bigger, or 2.4 times bigger at T30. (I think).

 

[Knappers]

More than 1.6 seems excessive practically especially with cast rads.
You'd probably have the only cast iron heat pump system in the country!

 

[siricosm]

You'd probably have the only cast iron heat pump system in the country!

Haha, that could be true, but I really like them, I think they look so much better than the modern ones.

 

[Knappers]

Regarding efficiency.
If you can keep the return temp below dewpoint (52°C i believe) then in theory you'll always be condensing so more efficient.
How likely is it that you'd move to a heatpump in future?
If very likely, I'd look at building in redundancy elsewhere (perimeter heating, ventilation etc)
Have you got another more detailed thread on the project?

 

[siricosm]

Regarding efficiency.
If you can keep the return temp below dewpoint (52°C i believe) then in theory you'll always be condensing so more efficient.
How likely is it that you'd move to a heatpump in future?
If very likely, I'd look at building in redundancy elsewhere (perimeter heating, ventilation etc)
Have you got another more detailed thread on the project?

So for max efficiency, T40 (60C flow) is probably good enough most of the time.

I am redoing the heating in my house in copper pipe with cast iron rads, and then I am going to lay hardwood floors, so I am hoping the system will last until I croak. However before that happens (the croaking part), I am worried that the government is going to ban gas boilers, as they are already talking about it for new builds. And you know how this goes, first it is new builds, and then a few years later people have to do it when their boiler dies. So probably not so likely for the heatpump.

I did have a couple threads about aspects of the system I plan to install, but it was mostly about thermal stores (sealed), and the pros/cons compared to unvented systems.

 

[Knappers]

Yea 60 is a good design temp.
Gas boilers won't get banned for decades and by the time they do heatpumps may not even be a thing.
I've done some work for these guys, down the road from me, a bit special and probably silly money.

Cast iron radiators from the Beaumont range

Beaumont Cast Iron home page offering wide range of exclusive cast iron radiators and accessories

www.beaumontcastiron.com

 

[siricosm]

My favorite are these (our house is 1930s):
Deco Cast Iron Radiator - https://www.tradscastironradiators.co.uk/radiators/cast-iron-radiators/deco/

but I can't afford them, nor the ones in my profile pic. I was originally going to scrounge around for used ones, but then I found these ones which are pretty reasonable:
Traditional Victorian 4 Column 760mm Cast Iron Radiator - https://www.castironradiatorcentre.co.uk/products.asp?code=CDC-760&name=traditional-victorian-4-column-760mm-cast-iron-radiator

 

[siricosm]

Have you got another more detailed thread on the project?

Here you go, I found it:

Please criticize my plan | | PlumbersForums.net

Find Please criticize my plan Advice and Help. How-to Please criticize my plan in the Central Heating Forum advice boards on PlumbersForums.net | Plumbing Advice for All. Please criticize my plan

www.plumbersforums.net

I guess a lot of folks don't like thermal stores, so it a bit devolved into a discussion of pro/cons with unvented. The plan is to ditch the house thermostat, and have the rads controlled entirely by TRVs. This design should also lower the return temperature a bit on all but the coldests days.

If you have any comments on that thread, or any tips or advice, please feel free to post them there. I am about 1/2 way through the install.

 

[Harvest Fields]

Things have moved where 15 years ago heat only boiler was standard ish now not many are getting installed compared to combis

Tell me about it. I’m tired of seeing combi’s in large 4 bed double bathroom houses.

 

[Knappers]

I've had a look, wouldn't a gledhill boilermate do the job?
I don't love it in principle as I dont think the boiler would be as efficient as id like, that would be overcome by having multiple heat sources (back boiler, solar etc) which is where a thermal store would excel domestically.
Just off the top of my head I'd look at separating heat and hw, something like thermal store temp controlled by weather compensator and HW straight off combi.
Wire solar panel to immersion and I'm there!

 

[siricosm]

Hee-hee, I am a bit farther along then that. I wound up going for a 300l Telford stainless buffer store, which was really inexpensive, and it came with a 3Kw backup immersion, dual stat and TPRV.

I purchased a 165Kw Nordic-tec plate exchanger, so time will tell how high the store temperature needs to be to get decent hot water, I am hoping that 65C will do it. The use of the thermal balancing valve and a smart pump instead of the usual flow switch on the primary side of the DHW exchanger is an experiment, but if it works it would have some advantages.

I would love to put a solar panel on roof and connect it to the immersion, although I should probably update my old boiler first.

 

[Ric2013]

I purchased a 165Kw Nordic-tec plate exchanger, so time will tell how high the store temperature needs to be to get decent hot water, I am hoping that 65C will do it.

I'd be really interested to know how you get on as I've been toying with the idea of a thermal store for myself for some years but am aware that manufacturers often aren't able to give this level of information. Most people run hotter than this as they don't understand the efficiency issue so I doubt it often comes up, but it'd be interesting to know what happens when you actually start pushing the technical limits. Please keep us updated.

 

[king of pipes]

Tell me about it. I’m tired of seeing combi’s in large 4 bed double bathroom houses.

I was the same way of thinking as you H but having recently removed a thermal store unit and fitted my first Viessman 222- f 35kw I would have no issues fitting this product again in a large home ,built in wi-fi , weather compensation tunes the boiler to the optimum energy output what ever the weather outside the digital interface feeds a wealth of information to the LCD screen or via the vicare app to your phone or tablet it just a joy to use and takes all the if what's and maybes out of heating design it's all done in one package and can be tuned and installed in various configurations to suit the property. Thermal stores have there place but they are old technology now in my opinion. Regards Kop

 

[siricosm]

I'd be really interested to know how you get on as I've been toying with the idea of a thermal store for myself for some years but am aware that manufacturers often aren't able to give this level of information. Most people run hotter than this as they don't understand the efficiency issue so I doubt it often comes up, but it'd be interesting to know what happens when you actually start pushing the technical limits. Please keep us updated.

I have it mostly installed and I am just waiting for the heating season to end, and then I will switch it over. However, it will be while after that until I know how well the hot water performs in the winter, and what temperature I need to keep the cylinder.

I should know right away if using a balancing valve on the primary side of the plate exchanger instead of a flow switch on the secondary is a good idea. I have to think the manufacturers of pre-made stores would do that if it worked well, but I don't see why it shouldn't work.

 

[siricosm]

I just thought I would provide an update for anyone interested in the DIY thermal store option.

Here are the parts I purchased for the store:
£430 300l standard Telford buffer, comes with dual stat, TPRV, and a 3kW electric immersion
£145 Nordic Tec Ba-23-40 165kW plate exchanger with insulation
£52 FS-05 flow switch 22mm compression
-------
£627

The 165kW plate exchanger works well with the tank stat set to 60C, at least it works fine now, we will see when it gets colder and the incoming water temperature drops and the heating is on. We don't have such a great flow rate at out place, maybe 10-12l/m, but the shower is alright, much better than the electric shower we had before, and no longer menopausal.

The Rm Cylinders thermostat that came with the Telford tank has a max setting of 65C, but our Ideal FF360 has a max temp of 82C, so I doubt it could go much higher than that anyway without the boiler cycling towards the end. Currently the boiler heats the tank in a single cycle. And it takes a while, the Rm Cylinders dual stat has a large differential, which is nice.

The tank is stainless, and has two 22mm connections at the top and two at the bottom, two stat pockets at the bottom, a TPRV and a spot for an immersion heater. It would have been nice to be able to fit a top and bottom thermometer, and to have one or two connections half way up. Other than that, it seems pretty nice, and is reasonably well insulated, with a modern looking white cover.

Here is the final plan:

 

[John.g]

Very nice schematic of your system must look at it in more detail later but just re the 165kw Hx, what output do you reckon you might/are getting with your flow rate of 10/12 LPM and assuming the cold water temp at 18C (here, just now). Also, be grateful for any info on the design conditions required to give its rated output of 165kw, Amazon state its less than 1 M2 (0.92) of heating surface so seems a hell of a output based on that.

 

[Ric2013]

Thanks so much for the update!

I'm interested as to why you are blending down the DHW to 48°C. It makes sense for effciency but would seem to contravene the requirement for 50°C at the outlets within 30 seconds. That said, as you aren't storing the water then it's effectively like a combi boiler (on which the water regulations blue book makes the 50/30 claim and then says that a combi boiler probably won't comply and then shrugs and wanders off).

So you're getting 10-12 lpm at 48°C with 18°C incoming cold (assumed)? How quickly does that water reach the taps out of interest (I genuinely DESPISE waiting for hot water to run and have got it down to 10-15 seconds in my own house).

 

[siricosm]

Very nice schematic of your system must look at it in more detail later but just re the 165kw Hx, what output do you reckon you might/are getting with your flow rate of 10/12 LPM and assuming the cold water temp at 18C (here, just now). Also, be grateful for any info on the design conditions required to give its rated output of 165kw, Amazon state its less than 1 M2 (0.92) of heating surface so seems a hell of a output based on that.

Thanks. I think the rating is for a 40C difference in temperature across the entire hot to cold side, so I guess it is a maximum rating in ideal conditions with sufficient flow rates. At 10l/m I don't need anything near that. I was also wondering how they get such a large transfer with such a small area, but in the end I took their word for it, and it is a lot larger than a plate exchanger from a combi.

I wanted it to be considerably oversized so that I could run the tank at a lower temperature. I will have to see how low it can go in the middle of winter. I could not really find any definitive answers on the internet, which is a bit surprising. I would have thought that today with heat pumps and buffers a lot of people would be interested in how big a plate exchanger you need if your tank is 55C and you want 12l/m.

 

[siricosm]

Thanks so much for the update!

I'm interested as to why you are blending down the DHW to 48°C. It makes sense for effciency but would seem to contravene the requirement for 50°C at the outlets within 30 seconds. That said, as you aren't storing the water then it's effectively like a combi boiler (on which the water regulations blue book makes the 50/30 claim and then says that a combi boiler probably won't comply and then shrugs and wanders off).

So you're getting 10-12 lpm at 48°C with 18°C incoming cold (assumed)? How quickly does that water reach the taps out of interest (I genuinely DESPISE waiting for hot water to run and have got it down to 10-15 seconds in my own house).

I think it is a legal requirement here that it needs to be max 48C, and the TMV I bought seems to have that as a maximum. I think the idea is to avoid scalding mostly.

The speed depends on the proximity I suppose. The kitchen, laundry and shower are close, but the bath is far away, so it takes some time to get there. Plus the house used to have a vented cylinder, so there is some 22mm pipe still in the hot line for the bath that I need to swap for 15mm when we renovate the bathroom, as that will slow things down.

 

[John.g]

Thanks. I think the rating is for a 40C difference in temperature across the entire hot to cold side, so I guess it is a maximum rating in ideal conditions with sufficient flow rates. At 10l/m I don't need anything near that. I was also wondering how they get such a large transfer with such a small area, but in the end I took their word for it, and it is a lot larger than a plate exchanger from a combi.

I wanted it to be considerably oversized so that I could run the tank at a lower temperature. I will have to see how low it can go in the middle of winter. I could not really find any definitive answers on the internet, which is a bit surprising. I would have thought that today with heat pumps and buffers a lot of people would be interested in how big a plate exchanger you need if your tank is 55C and you want 12l/m.

Its a long long time since my experiences with heat exchangers and LMTDs and all that jazz but one big advantage you have is that you can increase the primary flow rate as its pumped to compensate for lower temperatures but you may already have it running on full speed?, I know you have no temperature gauge on the Hx primary return but it should be pretty cool to the touch IMO for the ~ 23kw you are extracting from it now.

 

[Ric2013]

I think it is a legal requirement here that it needs to be max 48C, and the TMV I bought seems to have that as a maximum. I think the idea is to avoid scalding mostly.

The speed depends on the proximity I suppose. The kitchen, laundry and shower are close, but the bath is far away, so it takes some time to get there. Plus the house used to have a vented cylinder, so there is some 22mm pipe still in the hot line for the bath that I need to swap for 15mm when we renovate the bathroom, as that will slow things down.

This is something I struggled to get my head around. Because there is a legal requirement for scalding protection at outlets (in new bathrooms) and then a legal requirement for legionalla protection.

The understanding I have reached is that the compromise is that the hot water pipe would be heated to 50°C and then mixed down to temperature as close to the terminal fitting (tap) as possible, but that this does not really apply to installations where water is heated at the time of use.

What I meant about speed is, I suppose, how quickly is the pipe directly after the TMV likely to become obviously warm? I'm assuming there must be some lag as the pump kicks in and warms the plate heat exchanger, but it may be only 5 seconds?

 

[siricosm]

Its a long long time since my experiences with heat exchangers and LMTDs and all that jazz but one big advantage you have is that you can increase the primary flow rate as its pumped to compensate for lower temperatures but you may already have it running on full speed?, I know you have no temperature gauge on the Hx primary return but it should be pretty cool to the touch IMO for the ~ 23kw you are extracting from it now.

I am not sure what the flow rate is on the primary side. It is 22mm direct to/from the tank, and has a Grundfos Alpha 2L 15/60. It is set on the middle fixed speed (II), although it also seems to do alright on the lowest fixed speed (I) with current conditions. Again, this is something I can test in winter when the incoming water is colder, as maybe it will need to be increased.

 

[siricosm]

This is something I struggled to get my head around. Because there is a legal requirement for scalding protection at outlets (in new bathrooms) and then a legal requirement for legionalla protection.

The understanding I have reached is that the compromise is that the hot water pipe would be heated to 50°C and then mixed down to temperature as close to the terminal fitting (tap) as possible, but that this does not really apply to installations where water is heated at the time of use.

What I meant about speed is, I suppose, how quickly is the pipe directly after the TMV likely to become obviously warm? I'm assuming there must be some lag as the pump kicks in and warms the plate heat exchanger, but it may be only 5 seconds?

I could be wrong on this, but my understanding was that legionella was only a problem if you had a lot of heated water sitting around, like in a normal vented/unvented cylinder. I thought it was not a worry with combis or a thermal store with a coil or PHE. The idea being that with such a small volume of hot water, the new (chlorinated) water pushes out all the old when it is used.

For the speed, I can measure that and report back. I will wait until no one has used hot water for a while, and then time it.

 

[John.g]

I am not sure what the flow rate is on the primary side. It is 22mm direct to/from the tank, and has a Grundfos Alpha 2L 15/60. It is set on the middle fixed speed (II), although it also seems to do alright on the lowest fixed speed (I) with current conditions. Again, this is something I can test in winter when the incoming water is colder, as maybe it will need to be increased.

By very bacic calcs, you should IMO be able to drop the boiler flow temp to < 55C and still get your 10/12 LPM at 48C, you may be heating ~ 8 LPM to 58C which is mixing with 4 LPM of 18C cold water to give that 10/12 LPM. All you have to do is keep lowering the boiler SP temp, this means the TMV will keep throttling in until fully closed and the "mixed" temp will then start falling below 48C., then just increase the SP temp by a few degrees and you will be opertaing the boiler at its maximum efficiency. Even if my calcs are wrong, it would still be interesting to see how much you can reduce (if any) the SP temp until the TMV is shut.

Obviously, reduce the tank stat setting first and then the boiler SP.

 

[Ric2013]

I could be wrong on this, but my understanding was that legionella was only a problem if you had a lot of heated water sitting around, like in a normal vented/unvented cylinder. I thought it was not a worry with combis or a thermal store with a coil or PHE. The idea being that with such a small volume of hot water, the new (chlorinated) water pushes out all the old when it is used.

For the speed, I can measure that and report back. I will wait until no one has used hot water for a while, and then time it.

That's my understanding too. As I said, does not really apply to water that is heated on demand. Actually, I'm starting suspect some of our laws are, shall we say, manufacturer driven anyway.

Looking forward to your speed test, and again, thank you.

 

[John.g]

I suppose to look at it in perspective, there are hundreds of thousands of electric showers around Europe alone and the showering temp is generally 35c to say 45C, they cannot go higher than 48C because a TCO will cut out the heating elements some of these, especially in the UK, are mains fed so not from stored water but quite a large % are fed from a CWST which often, in the summer, reaches 25C, perfect conditions for legionella, but I don't hear of any laws prohibiting this.

 

[hometech]

If my flow circuit is 43 degrees and the return temperature is 16 degrees, what is my Delta T?

 

[Ric2013]

I suppose to look at it in perspective, there are hundreds of thousands of electric showers around Europe alone and the showering temp is generally 35c to say 45C, they cannot go higher than 48C because a TCO will cut out the heating elements some of these, especially in the UK, are mains fed so not from stored water but quite a large % are fed from a CWST which often, in the summer, reaches 25C, perfect conditions for legionella, but I don't hear of any laws prohibiting this.

There is a law in the UK: we're not supposed to store cold water or fit cold supply pipes where the water is likely to go above 25°C (Water Regulations), and 20°C as a maximum is recommended (blue book). Though quite how effective a thin fibreglass blanket is at keeping water cool in a loft is dubious and the law is often unobserved: I've known a plumber (who actually teaches at a college and should know better) run a cold pipe between a heating flow and return.

I suppose the question is how many bacteria are already in the cold water? People may take a shower at 37°C (which is probably the optimum breeding temperature), but the water heated instantaneously will be at that temperature for mere seconds: not enough time to breed. However, it is a concern that the cistern contents may not be consumed all that quickly since building services are sized for once-a-year events rather than for the actual everyday occupancy and use (and then we say anyone who sizes things smaller is a cowboy).

Anglian Water tells me Legionaires' disease from domestic settings is common in the UK, but I have yet to see the figures that back that statement up.

 

[John.g]

If my flow circuit is 43 degrees and the return temperature is 16 degrees, what is my Delta T?

Your deltaT is 27C (43-16).
Are you thinking about T50 or T40 rads or whatever?.

 

[hometech]

There is a law in the UK: we're not supposed to store cold water or fit cold supply pipes where the water is likely to go above 25°C (Water Regulations), and 20°C as a maximum is recommended (blue book). Though quite how effective a thin fibreglass blanket is at keeping water cool in a loft is dubious and the law is often unobserved: I've known a plumber (who actually teaches at a college and should know better) run a cold pipe between a heating flow and return.

I suppose the question is how many bacteria are already in the cold water? People may take a shower at 37°C (which is probably the optimum breeding temperature), but the water heated instantaneously will be at that temperature for mere seconds: not enough time to breed. However, it is a concern that the cistern contents may not be consumed all that quickly since building services are sized for once-a-year events rather than for the actual everyday occupancy and use (and then we say anyone who sizes things smaller is a cowboy).

Anglian Water tells me Legionaires' disease from domestic settings is common in the UK, but I have yet to see the figures that back that statement up.

These figures are published (as is everything else).
Mean is 256 cases on a 3 year rolling period
That makes it common as .0000035% of the population. As this data includes the infection brought into the UK and not just the perfect scenario of the right nutrients at the right place and time with the right temp etc etc....Deaths are not recorded in the data as it is contributory rather than causal

 

[John.g]

Wet cooling towers are possibly the most dangerous of all.

 

[hometech]

Your deltaT is 27C (43-16).
Are you thinking about T50 or T40 rads or whatever?.

When I was having all of the rads replaced I contacted Stelrad tech support to help me choose. They said with the figures I gave them they could not do any calcs. I ended up installing regular 1100 x 600 double type 22 rads from screwfix. Home was very warm through that harsh winter we had this year. Now I've spent the summer adding another 200mm insulation to underfloor areas, I'm hoping to reduce heat losses even further.

 

[John.g]

I suppose they couldn't do the heat loss calcs so then couldn't recommend the correct output rad.
If you are wondering how the correction factors that Siricosm quoted then they are (obviously) based on a T50 rad or sometimes called a 50 deg rad, this is based on the rad flow and return temps and the required room temp which is often assumed at 20C, the "deg rad" is then the mean rad temp-the required room temp, if you had flow/return temps of 75/65 and a required room temp of 20 then you would have a ((75+65)/2)-20, a
50 deg (or T50) rad, you can see from the table below that a 40 deg rad will give ~ 75% output of a 50 deg rad so the correction factor is 1/0.748 or 1.34 and so on. the output is the ("deg rad"/50)^1.3. You probably know this anyhow but no harm to show that rads have to be very oversized if using very low flow temps like with Heat Pumps.

Flow temp​	75​	65​	55​	45​
Ret temp​	65​	55​	45​	35​
Mean temp​	70​	60​	50​	40​
"deg rad"​	50​	40​	30​	20​
Output​	100.0%​	74.8%​	51.5%​	30.4%​
X Factor​	1.00​	1.34​	1.94​	3.29​

 

[hometech]

I suppose they couldn't do the heat loss calcs so then couldn't recommend the correct output rad.
If you are wondering how the correction factors that Siricosm quoted then they are (obviously) based on a T50 rad or sometimes called a 50 deg rad, this is based on the rad flow and return temps and the required room temp which is often assumed at 20C, the "deg rad" is then the mean rad temp-the required room temp, if you had flow/return temps of 75/65 and a required room temp of 20 then you would have a ((75+65)/2)-20, a
50 deg (or T50) rad, you can see from the table below that a 40 deg rad will give ~ 75% output of a 50 deg rad so the correction factor is 1/0.748 or 1.34 and so on. the output is the ("deg rad"/50)^1.3. You probably know this anyhow but no harm to show that rads have to be very oversized if using very low flow temps like with Heat Pumps.

Flow temp​	75​	65​	55​	45​
Ret temp​	65​	55​	45​	35​
Mean temp​	70​	60​	50​	40​
"deg rad"​	50​	40​	30​	20​
Output​	100.0%​	74.8%​	51.5%​	30.4%​
X Factor​	1.00​	1.34​	1.94​	3.29​

Heating a room is simply replacing the heat lost from the room. Raising the temperature of air or water is a linear scale so for every degree drop you have a set figure to replace it. So in a room with high insulation and controlled circulation, once heated then keeping on top is the balance. Leaving the heating on 24/7 has created a constant house temperature (I don't think rooms) @ 18 degrees (the personal optimum for my home). Gas consumption is down 64% when compared to the old combi that was replaced. BTW, this is not a heat pump, its a combi

 

[John.g]

I'm not so sure about that, if your house was at the same temperature as the outside air then, depending on insulation it will take a certain amount of heat to increase that temperature by say 5C but to increase it by the next 5C will take more energy as the heat loss at ambient+10C will be greater that at ambient+5C? or to think of it in another way, If you switched off your boiler at your desired room temp of 18C it will take a fixed amount of time to fall by 5C to 13C but if you had had your room temp at say 22C then it will IMO fall by 5C to 17C in a shorter period as heat loss is greater. If the temperature rise/fall was linear then by definition once any room is at its desired temperature then it only takes the same amount of heat to maintain it at its desired temperature, my house certainly requires less heat to keep it at 16C which is my set back temperature when we go out than to maintain it at 22C which is our normal desired temperature.

 

[siricosm]

What I meant about speed is, I suppose, how quickly is the pipe directly after the TMV likely to become obviously warm? I'm assuming there must be some lag as the pump kicks in and warms the plate heat exchanger, but it may be only 5 seconds?

It takes less than 2 seconds. With no one using hot water overnight, I find the plate exchanger is warm, so I guess it must thermosiphon.

 

[hometech]

I'm not so sure about that, if your house was at the same temperature as the outside air then, depending on insulation it will take a certain amount of heat to increase that temperature by say 5C but to increase it by the next 5C will take more energy as the heat loss at ambient+10C will be greater that at ambient+5C? or to think of it in another way, If you switched off your boiler at your desired room temp of 18C it will take a fixed amount of time to fall by 5C to 13C but if you had had your room temp at say 22C then it will IMO fall by 5C to 17C in a shorter period as heat loss is greater. If the temperature rise/fall was linear then by definition once any room is at its desired temperature then it only takes the same amount of heat to maintain it at its desired temperature, my house certainly requires less heat to keep it at 16C which is my set back temperature when we go out than to maintain it at 22C which is our normal desired temperature.

so are you saying that the specific heat energy is not linear? Pretty sure there have been no changes to the basic laws of physics in the last few decades

 

[Ric2013]

The heat loss varies with the difference between inside temperature and the outside. So Hometech and John.g are looking through opposite ends of a lens but effectively seeing the same thing (what kind of mixed metaphor is that?).

I've worked it out this way and feel free to correct me if I've made a mistake somewhere... To take John's example, if external is 13°C, the room at 22°C is 9 degrees above it, whereas the room at 18° is only 5 degrees above it. Heat losses for the warmer room will be 80% greater i.e. if it takes 1000W to maintain 18°C, it will take 1800W to maintain 22°C. There is a linear relationship between heat lost and the difference in temperature. In this example, 200W per degree above the external ambient.

This is consistent with heat loss calculations (radiator output is heat lost from the emitter into the room) which are calculated based on building elements having a heat loss expressed in W/m2K (K=inside/outside difference).

Going back to post 9, I don't therefore follow the example of a radiator at T30 having to be 2.4x the size. I make it 1.67x. If you divide the output of a rad calculated at T50 by 50 (the temperature difference between room temperature to be maintained and the mean emitter temperature), so the same rad at T30 would have:

T50 rated output/50x30 = 60% of the output

So if we want, say, 1000W output from a radiator run at T30, surely we need a radiator rated at 1667W (T50)? 60% of 1667W is 1000W.

The above logic conflicts with radiator manufacturers (e.g. Stelrad) who claim a radiator run at T30 will have an output 52% of the same radiator run at T50 and would therefore select a 1923W model rather than the 1667W one my logic would dictate. We cannot both be correct.

 

[John.g]

While specific heat energy may be linear and the amount of energy required to raise the temperature by 1C is the same I would think that as the temp rises the heat losses increase so the total energy required is the specific heat energy+the increase in losses if one wants to keep the same rate of rise, don't know if this is still a linear relationship of energy vs temperature rise??.

 

[John.g]

The heat loss varies with the difference between inside temperature and the outside. So Hometech and John.g are looking through opposite ends of a lens but effectively seeing the same thing (what kind of mixed metaphor is that?).

I've worked it out this way and feel free to correct me if I've made a mistake somewhere... To take John's example, if external is 13°C, the room at 22°C is 9 degrees above it, whereas the room at 18° is only 5 degrees above it. Heat losses for the warmer room will be 80% greater i.e. if it takes 1000W to maintain 18°C, it will take 1800W to maintain 22°C. There is a linear relationship between heat lost and the difference in temperature. In this example, 200W per degree above the external ambient.

This is consistent with heat loss calculations (radiator output is heat lost from the emitter into the room) which are calculated based on building elements having a heat loss expressed in W/m2K (K=inside/outside difference).

Going back to post 9, I don't therefore follow the example of a radiator at T30 having to be 2.4x the size. I make it 1.67x. If you divide the output of a rad calculated at T50 by 50 (the temperature difference between room temperature to be maintained and the mean emitter temperature), so the same rad at T30 would have:

T50 rated output/50x30 = 60% of the output

So if we want, say, 1000W output from a radiator run at T30, surely we need a radiator rated at 1667W (T50)? 60% of 1667W is 1000W.

The above logic conflicts with radiator manufacturers (e.g. Stelrad) who claim a radiator run at T30 will have an output 52% of the same radiator run at T50 and would therefore select a 1923W model rather than the 1667W one my logic would dictate. We cannot both be correct.

Re rad output a 30 deg rad does not emit 30/50 or 60% of a 50 deg rad, it emits (30/50)^1.3 or "only" 51.5% of that of a 50 deg rad.

That's very interesting about the 200 watts/degC even though I still find it hard to grasp that if you keep adding exactly 200 watts/degC constantly that it can be described as linear as I think that the rate of temp rise would/will be slower the higher the room temperature even though, if the above is correct, its easy to see that the temperature will increase from 13C to 19C if the heat input is increased to 1800 watts immediately.