Condensing bolier flow and return temps - effect on rad sizing?

[doitmyself]

15 is the starting room temperature allowing up to 25 which would give a room to rad difference of 40

Sorry, but I don't follow what you are saying.

The accepted method is to calculate the heat required to raise the room temperature from -1C (or -3C) to 21C. I don't see where 15C or 25C fits in to that.

 

[Ollieg]

I have weather compensation in my own house and fitted it for a few people. It is great but requires a lot of time and effort to get set up correctly. It took me a full winter and and a wee bit of the next to get my whole house perfect.
Its a big undertaking to play with the heat curve, balance the rads perfectly and get room temps correct.

By the way, all I mean about paying a bit more is the people who know their stuff generally charge a bit more than *** packet man.

 

[wattsie1987]

Sorry, but I don't follow what you are saying.

The accepted method is to calculate the heat required to raise the room temperature from -1C (or -3C) to 21C. I don't see where 15C or 25C fits in to that.

What I'm saying is who let's there room get to -1 if the heating is set to have a minimum fall back temp of 15 the rads will be over sized not 20% under like you said.

 

[doitmyself]

What I'm saying is who let's there room get to -1 if the heating is set to have a minimum fall back temp of 15 the rads will be over sized not 20% under like you said.

An interesting approach! But how do you heat the house up from cold if it has been empty for a few weeks and the heating has not been left on?

Can you assume that the heating will not be allowed to drop below 15C?

 

[Rory]

Just to throw something else into the mix Rory about your boiler selection & controls, are you aware of Weather Compensation a few of the boiler manufacturers have this already built into the electronics so with a low cost thermistor kit it can be activated.

I had originally thought I'd get a Nest learning thermostat, but the installer has suggested Worcester's Wave. Allegedly it does both weather compensation (getting the temp from a local weather station) and optimisation.

I must admit that (like much of this stuff!) I'm bemused by how these things optimise - surely these remote units (which seem to basically just be fancy thermostats) can't control the boiler flow temp?

 

[wattsie1987]

Because most of my customers ask me to set there heating up for them. I have an off temp of 15 to keep some comfort and avoid large heat up periods. I would hope the boiler doesn't get turned off as there would be no frost protection! If it was off for a while tho it will heat up fine may take a little longer but will be condensing all the way. To be honest I oversize slightly on my calcs anyway and never had a problem.

 

[chris watkins]

I had originally thought I'd get a Nest learning thermostat, but the installer has suggested Worcester's Wave. Allegedly it does both weather compensation (getting the temp from a local weather station) and optimisation.

I must admit that (like much of this stuff!) I'm bemused by how these things optimise - surely these remote units (which seem to basically just be fancy thermostats) can't control the boiler flow temp?

As far as I am aware they are as you say Rory " fancy thermostats" i.e. optimal start / stop & yes the full on W/C controllers can be both expensive & a bit complicated to set up with the right curve to suit your older less insulated house but you may well enjoy the challenge in the knowledge that it is saving you money.

I have the simplest form my Viessmann Vitodens 100-W (heat only) had it built-in as I say, it cost me about £20 for the out side kit + some 1.5mm 3 core flex. I had to alter the controls wiring to suit my existing S plan but you would have a diverter valve 3 port installed with the new system.

I can adjust the compensate curve on mine by altering the position of the boiler thermostat knob, about 2 or 3 times during the last two heating seasons I have had to turn it up (when it was very cold) the rest of the time it looks after its self.
For this amount of outlay & time spent why would you not take advantage of this? Don't over complicate.

 

[Ollieg]

Nest, Wave, hive and Tado are all fancy on off switches. None of them control flow temp. Proper weather comp does.

 

[Worcester]

OK, so our standard retrofit for renewables, (heatpumps and biomass) includes Honeywell Evo throughout AND weather compensation, - Yes our customers are of the more discerning type, (only worked on two semis in 4 years) and they are all over the moon with their finished systems - customer satisfaction rating in the 5 stars bracket.

The Evo controls room by room temps by time of day with room by room override, and the weather compensation controls the flow temperature, so maximising the efficiency of the heat pumps and biomass boilers - especially important as these are not on/off devices like gas and oil boilers.

You'll have trouble beating the simplicity and sophistication of the Honeywell Evo system.

Going back to my earlier post on heating design - for the power output - boiler and rad sizing you add in factors to cover wether you heat from cold and weather the system is run continuously or intermittently (i.e on day off at night, or set back) - Also standard room temperature designs are 21,22 and 18 dependant upon use. - your radiator size is therefore affected by output (kW) required, flow temperature and room temperature.

 

[Rory]

Nest, Wave, hive and Tado are all fancy on off switches. None of them control flow temp. Proper weather comp does.

Worcester say the Wave does control the flow temp (from the installion instruction):

"The Wave uses Load Compensation as standard to
continuously monitor and control the room temperature during
your programmed central heating times. This function offers
energy savings compared to standard on/off controls.
The compensation feature means that, rather than just
switching the boiler on and off, it will adjust the flow
temperature of the central heating water in line with your
central heating set temperature. The smaller the difference
between the actual temperature and the set-point
temperature, the lower the flow temperature from the boiler."



The only thing I'm a bit miffed about now is that we are stuck with 28i Junior - I asked for 29CDi Classic but apparently it's not instantly available as it's not a popular choice at that size (people usually for got the Junior or Compact). Anyway, the Classic has only just been updated so chances are I might have got an "old" one, and then I'd be annoyed about that!

 

[doitmyself]

I would hope the boiler doesn't get turned off as there would be no frost protection!

That' a big hope. What happens when the house gets sold, the old owners move out but the new ones' don't move in for a couple of weeks? The insurers could insist that the heating gets turned off and everything drained down. So the new owners move into a cold house.

If it was off for a while tho it will heat up fine may take a little longer but will be condensing all the way. To be honest I oversize slightly on my calcs anyway and never had a problem.

If you have designed for a temp rise of 10C, it doesn't matter if it's 10 degrees from 0C to 10C, or from 15C to 25C. You will still need the same amount of heat. If you want to raise the temp by 20C you will need double the amount of heat. It will take more than "a little longer" to achieve that.

I would be really interested to see a sample of your calculations.

 

[Rory]

That' a big hope. What happens when the house gets sold, the old owners move out but the new ones' don't move in for a couple of weeks? The insurers could insist that the heating gets turned off and everything drained down. So the new owners move into a cold house.

As a customer, I have to say I would be pretty dismayed if an installation was sized based on such a rare occurance.

 

[doitmyself]

As a customer, I have to say I would be pretty dismayed if an installation was sized based on such a rare occurrence.

That particular scenario might be rare, but it's no different from a new-build house, which has to be heated from cold when the first owners move in, which could be in the winter when the house, and outside temperature, are below freezing.

A system which has been designed to raise the temperature 10C will never be able to raise it 20C.

 

[chris watkins]

That particular scenario might be rare, but it's no different from a new-build house, which has to be heated from cold when the first owners move in, which could be in the winter when the house, and outside temperature, are below freezing.

A system which has been designed to raise the temperature 10C will never be able to raise it 20C.

But you just wait for a time or day when the outside air temperature is not -3 (or what ever it has been designed to) you will then have to less heat going out & the emitter will raise the room air temp or you could turn the boiler stat up to increase the water temp & therefore the output, before resetting.

What I would suggest you don't do is design your system for these one off or very rare occasions as this will effect the day to day efficiency's.

 

[doitmyself]

But you just wait for a time or day when the outside air temperature is not -3 (or what ever it has been designed to) you will then have to less heat going out & the emitter will raise the room air temp or you could turn the boiler stat up to increase the water temp & therefore the output, before resetting.

I can just imagine the conversation between the proud owner of a brand new £350K house and the site's sales rep when he is advised not to move in to the house as the heating system is not powerful enough to bring the house up to an acceptable temperature.

As for the second suggestion, is that always possible? Turning up the water temperature will increase the rad output, but you will still be limited by the boiler output. If you need 10kW for a 10C rise, you will need 20kW for a 20C rise. Who installs a boiler with 50% spare capacity?

What I would suggest you don't do is design your system for these one off or very rare occasions as this will effect the day to day efficiency's.

But isn't that how systems are supposed to be designed, i.e. for an outside temp of -3C?

 

[stevetheplumber]

i remember when we cubed the room in ft and times by 5 to get the rad size then choose the rad that fitted the window

 

[doitmyself]

i remember when we cubed the room in ft and times by 5 to get the rad size then choose the rad that fitted the window

I think some installers still use that method.

 

[kay-jay]

i remember when we cubed the room in ft and times by 5 to get the rad size then choose the rad that fitted the window

If you do that and compare it to results from a mears calculator its always there or thereabouts [emoji89]

 

[wetdrip]

Ok so hear goes I work my room heat losses out @ delta 50 1 degree = 2% i up size by 20% which gives me a delta 40 heat figure for each room. I then work out the system requirement and add 10% for pipework heat loss. I range rate the boiler through the perameters at a Max of this figure. I set flow temp to 65 allowing room temp to be 15...... Delta 40! Completely balance the system for equal heat loss per rad and fit an external weather compensator. Fit a fully modulating boiler with adjusting pump speed and system learning capabilities and there you have it the most condensing system you can get. This will cost you mind and most are not willing to pay for this level of service!

That's the one quirky thing that gets me, not just on here, but youtube videos as well.
You state 10% heat loss for pipework; and here's the thing: a massive percentage of the pictures we see on here and youtube, the installers never bother to insulate the pipework.

P.S. And as someone who still has his WB GREENSTAR 28i j ( Which magazines best buy) still in its box. This thread is a tad painful:-(

 

[doitmyself]

i remember when we cubed the room in ft and times by 5 to get the rad size then choose the rad that fitted the window

I applied that formula to my living and dining rooms; and got the following results:

Living - 7485; Dining - 3903.

I then realized that, if the formula uses cubic feet, it probably calculates heat in BTU. So I applied the necessary conversion and got:

Living - 2.19kW; Dining - 1.14kW

Compare this to the current (original) rads: Living - 2.07kW; Dining - 1.13kW

Coincidence or what?

Problem is: the house has been fully insulated since it was built about 25t years ago, so the current requirements are: Living - 1.09kW; Dining - 1.01kW.

The rads are now oversized by an average of 75%, which means that, even in the coldest weather the flow temp is about 65C and the boiler condenses most of the time.

 

[newcastle phill]

Flow temp isnt the most important factor.
Return temp is the more so.
Whats your return temp

 

[wattsie1987]

That's the one quirky thing that gets me, not just on here, but youtube videos as well.
You state 10% heat loss for pipework; and here's the thing: a massive percentage of the pictures we see on here and youtube, the installers never bother to insulate the pipework.

P.S. And as someone who still has his WB GREENSTAR 28i j ( Which magazines best buy) still in its box. This thread is a tad painful:-(

I just use the 10% as a rule of thumb but it is surprising how much heat can be lost through pipework. Why do you say quirky?

 

[wattsie1987]

I can just imagine the conversation between the proud owner of a brand new £350K house and the site's sales rep when he is advised not to move in to the house as the heating system is not powerful enough to bring the house up to an acceptable temperature.

As for the second suggestion, is that always possible? Turning up the water temperature will increase the rad output, but you will still be limited by the boiler output. If you need 10kW for a 10C rise, you will need 20kW for a 20C rise. Who installs a boiler with 50% spare capacity?


But isn't that how systems are supposed to be designed, i.e. for an outside temp of -3C?

I always size the boiler to the system at Max output so there will be more for the boiler to give if needed. I think you've missed the point where I said I oversize the rads by 20% then lower the flow temp. If the flow temp was at Max the system would be 20% more powerful than needed and have that chilly -3 house up to temp in no time!

 

[wetdrip]

I just use the 10% as a rule of thumb but it is surprising how much heat can be lost through pipework. Why do you say quirky?

Just weird, but not in a good way.

 

[Rory]

Well, as the OP I should report that the system is in now - 3-4 days work for 2 guys was done in 2 days, complete new CH system with WB 28i Junior, the WB magnetic thing, and Wave controller.

As requested, the installer has tended on the large size for the rads and the large living room has a couple of double rads with single fins, rather than the singles with single fins originally offered. Although it's a smallish house, there's actually quite a bit of radiator acreage now.

Chilly here tonight but arriving at the house it was certainly very warm.

As 'promised' by the installer, flow and return seemed very similar temperatures. I think it's going to need a bit of playing with over the next few days. A query about condensing mode was responded to with an explanation of modulating.

Wave seems like a bit of fun - got the app and linked in remotely with no issues. Turned the temp down a bit as we're not there! Setting the heating parameters for the weather compensation has me completely baffled - it says to ask your installer.

 

[doitmyself]

As 'promised' by the installer, flow and return seemed very similar temperatures.

What are the flow and return temperatures?

 

[Rory]

What are the flow and return temperatures?

I can't say with any precision at the moment, but by touch both very hot with little difference between them. In fact it wasn't obvious which was the flow and return and for a moment I thought they'd put the magnetic filter on the flow, but I looked at the installation book and it's on the return.

 

[doitmyself]

I can't say with any precision at the moment, but by touch both very hot with little difference between them.

That doesn't sound right. There should be a noticeable difference between the flow and return - about 20C - despite what the installer told you. If you can, borrow an infrared thermometer and measure the flow and return pipes near the boiler. Do the same for a few of the radiators. As the pipes are all brand new and probably unpainted, you will need to wrap some insulating tape round the pipe and measure the temperature off the tape, holding the thermometer almost in contact with the pipe.

 

[Rory]

That doesn't sound right.

I know! I'm just a bit dismayed that the installer doesn't get it.

I've got an IR thermometer so no problem to set it all up when I get time.

What I don't know (and much searching hasn't helped) is what effect the Worcester Wave unit has on the flow temp when using weather compensation mode. I'll have to communicate with WB.

 

[newcastle phill]

Try opening the rad valves more to help the system give up more heat and turn the boiler temp down (sorry can't remember boiler type and too lazy to read back haha)

 

[Rory]

Try opening the rad valves more to help the system give up more heat and turn the boiler temp down (sorry can't remember boiler type and too lazy to read back haha)

Oh - that's the opposite of what I'd have thought. My instinct would be to close them down to slow the flow. Worcester 28i Junior, by the way.

 

[chris watkins]

Oh - that's the opposite of what I'd have thought. My instinct would be to close them down to slow the flow. Worcester 28i Junior, by the way.

And you would be right Rory.
You need to balance them yourself, closing them down allows the rad to loose more of the heat energy in the water resulting in a lower return temp back to the boiler.
The boiler electronic controls are looking to maintain a 20deg drop across the system by modulating but if the return is close to the flow temp it thinks the house is up to temp & shuts down often not coming back on for 10 - 15mins.

The trouble with balancing is that to do it correctly you need a steady design flow temp (of say 70) with the design room air temp (say 21) with a design outside air temp (at say -3) you could then adjust the flow rate to give that 20 deg drop.
As you can now see it is not an exact science on domestic system but that doesn't mean you should not try, play around when the system is warm & the outside is cold.

 

[newcastle phill]

Sorry my bad.
Misunderstood what he said and typed back to front.

But yes needs balancing.
Still think hes gonna struggle tho by sounds of it.
Have a good play and let us know

 

[Worcester]

Heat given up (energy) = flow rate (litres /min) x temp drop (Δt) x time x SpecificHeatCapacity of Water(Constant)

We design ASHP systems with only a 5° Δt
So need 2 x the flow rate as a 10° Δt to achieve the same power / energy during a given period

ASHP's have minimum design flow rates and also a target Δt.
As heating engineers we design our systems accordingly. To high a Δt and the HP's don't like it, same with too low a flow rate, and we put devices in to measure both during both installation and in use.
- Surely everyone designs systems like this don't they?
( I guess that's the difference between a heating engineer and a plumber / gas safe fitter... you may be competent to install a boiler, it doesn't make you competent to design and install a heating system)

It's a case of design and balance.. what is the optimum Δt specified for your boiler?

Use a decent electronic thermometer to set that across each of your radiators by adjusting the lockshield, (assuming the radiators are correctly sized of course) and then you'll end up with a properly balanced system.

 

[Ermintrude]

K type thermocouple on anton