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

[Rory]

Had a quote for complete new CH system including Worcester Greenstar 28i Junior.

Not sure I totally understand this, hence asking here: I was pretty surprised at how small the rads spec'd were as I thought condensing boilers needed as low a return temp as possible? The installer (WB accredited) is telling me that once the boiler gets going there will be hardly any difference between flow and return temp, and the rads are rated assuming flow temp of 75C.

Am I mis-understanding or does this seems far too high? Indeed the boiler's user guide suggests running the CH at between 1 & 2 on the dial - around 50C - and it also says "the radiators may need upgrading", which I assumes means "replaced with bigger ones!".

 

[diamondgas]

Hi Rory - Do you have concerns about the installer? If not then trust his professional decisions; if you do then get other quotes IMHO

 

[LPC]

Had a quote for complete new CH system including Worcester Greenstar 28i Junior.

Not sure I totally understand this, hence asking here: I was pretty surprised at how small the rads spec'd were as I thought condensing boilers needed as low a return temp as possible? The installer (WB accredited) is telling me that once the boiler gets going there will be hardly any difference between flow and return temp, and the rads are rated assuming flow temp of 75C.

Am I mis-understanding or does this seems far too high? Indeed the boiler's user guide suggests running the CH at between 1 & 2 on the dial - around 50C - and it also says "the radiators may need upgrading", which I assumes means "replaced with bigger ones!".

My opinions:

28i Juniors aren't very good. I'd recommend going for a Worcester CDi (if you want Worcester) unless it is a flat/rental property. Personally in that price range a Vaillant Pro 28 installed by a Vaillant Advance installer (5 year warranty instead of 2) is a much better buy.

Unless you live in a very large property I wouldn't worry to much about F+R temps. You can size the radiators yourself by googling 'radiator sizing' and measuring your rooms.

Worcester Accredited means absolutely precisely nothing. Just means he installs a fair few boilers. It is no indication of quality, and in most cases you are paying for that extra warranty indirectly. Just so you know.

 

[newcastle phill]

Condensing boilers operate at maximum efficiency with a low return temp.
Most radiator manufacturers quote a delta which is based on temps for non condensing boilers and need calculating but with some this is changing.
Typically a lower delta will in effect increases the size of the panel.

Ill see if I can find a link that explains in more detail a little about the general idea

 

[newcastle phill]

http://blog.tradeplumbing.co.uk/the-mystery-of-delta-t-revealed/

Sort of a bit here.
But to be frank I think delta 50 is even too high.
Delta 40 is getting there more where condensing boilers need to be.
And some heating engineers even calculate more towards a delta t of 30-35 ish.

Simply put if the return temp is too high the boiler wont condense fully and the efficiencies will not be realised

 

[Rory]

Thanks for the responses.

[FONT=&quot]Do I have concerns about the installer? Hmmm….he’s very local, recommended etc. I’m confident he can do the job, I’m just not sure he “got” the more technical aspects. I did get other quotes but central heating installers seem to have very strong and fixed opinions![/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]Re the boiler – I understand the 28i Junior was updated last year. The installer says they fit a lot of them and will be guaranteed for 8yrs (although if it fails even at that point I’ll be a bit dismayed). Other installers said they wouldn’t touch WB with a barge-pole. One said he only fits Glow-worm – not a name that comes up much in discussions these days – take it or leave it.[/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]Thanks for the link about Delta-T. I used to be an industrial electrician, although I’m well out of trade now, but I’m used to having to calculate stuff like this – you can’t just throw a long bit of cable in and hope for the best! My reading is that you really need to substantially oversize rads to be able to run at lower flow temps.[/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]One of the issues with the current installation (which might be single pipe) is the return gets very hot, very rapidly, so the boiler shuts off for 3 mins and everything goes cold. Rinse and repeat. This means the house gets warm in series of very brief steps. I explained that to the installer and I really want to make sure that doesn’t happen in the new system.[/FONT]

 

[Ermintrude]

We use 35! And yes it should be less than 63c !

 

[Rory]

We use 35! And yes it should be less than 63c !

So for Delta-T 35 that would be a flow of say 60C and return 50C?

And sorry, but what should be less than 63C?

 

[doitmyself]

Had a quote for complete new CH system including Worcester Greenstar 28i Junior.

Not sure I totally understand this, hence asking here: I was pretty surprised at how small the rads spec'd were

How old is the house, how large and how well insulated?

Did the installer do a detailed heat loss calculation? Has he provided this info? If not ask for it.

I thought condensing boilers needed as low a return temp as possible? The installer (WB accredited) is telling me that once the boiler gets going there will be hardly any difference between flow and return temp, and the rads are rated assuming flow temp of 75C.

1. The return needs to be below 55C for any condensing to occur.
2. The lower the return, the more condensing occurs.
3. If a flow of 75C has been assumed and a return of 55, the rads will be producing about 85% of the output given in the catalogue, so the have to be "oversized" by 17%, say 20% for safety.

 

[wattsie1987]

The boiler should modulate and adjust pump speed according to the heat demand and keep itself condensing. Trust your installer. Your property will be loosing more heat than you will save through flue gases.

 

[Ermintrude]

The boiler should modulate and adjust pump speed according to the heat demand and keep itself condensing. Trust your installer. Your property will be loosing more heat than you will save through flue gases.

But the idea is to recover latent heat as it will save money. If my can had a hole in the fuel tank I wouldn't drive fast just because the losses from the leak exceeded what I could save by driving sensibly? What an odd argument?

 

[doitmyself]

The boiler should modulate and adjust pump speed according to the heat demand and keep itself condensing.

In most cases the boiler will modulate to keep the flow temperature constant at whatever value the installer/user has set. Most systems with modulating boilers do not have variable speed pumps.

 

[Rory]

Thanks again all.

The house is a decent size early 1970’s 3 bed end-terrace. It’s double glazed. Gable wall is cavity insulated. Has a lobby rather than a hall so the front door basically opens straight into the living room. However, like many houses of this age and older, they weren’t designed to be sealed boxes so it’s kept well ventilated.

The installer did measure the rooms and used a rad size calculator but came up with that seem to me to be very small sized rads, especially in the bedrooms. At the moment I’ve convinced him to use larger ones more based on appearance than anything else.
The Junior doesn’t have a modulating pump but other WB models do. I did wonder about this – like all aspects of CH systems, there seem to be very polarised opinions on whether modulating pumps are a good thing or not!

 

[Ermintrude]

I'd say 30% of new installs do. ( ststems and Heat only)

 

[wattsie1987]

Maybe its because I only fit the high end boilers. Some will learn the system size and modulate accordingly thus keeping it at optimum dew point for longer. Oversizing rads is fine but will cause intense heat up periods. Balancing the system will help a lot with better condensing. I would personally go for the best boiler and controls you can afford.

 

[LPC]

The installer did measure the rooms and used a rad size calculator but came up with that seem to me to be very small sized rads, especially in the bedrooms. At the moment I’ve convinced him to use larger ones more based on appearance than anything else.
The Junior doesn’t have a modulating pump but other WB models do. I did wonder about this – like all aspects of CH systems, there seem to be very polarised opinions on whether modulating pumps are a good thing or not!

Bear in mind bedrooms have a lower design temperature of around 18 celsius. No doubt he will have sized the rads correctly but of course you can always ask for larger.

Honestly, i'd steer clear of the Worcester Juniors.

 

[wetdrip]

Very interesting thread.
Can someone, given the time, write a Master Class thread explaining to people like myself exactly what's what when it comes to sizing and the various problems that may occur and be thrown up with this type of install.

 

[newcastle phill]

I agree to a degree that some boilers will modulate down to a point.
But there are limits to this.

Like ermys car fuel economy analogy...there is a sweet spot and this needs to be calculated.

2 scenarios of many...


Radiators too small so to achieve flow return heat loss you start to close trvs but then room not hot enough.
Then you shift the problem elsewhere - I.e. to the boiler cycling and not operating at a continuous low gas rate and condensing.

Radiators too small - so to keep room warm you turn rads up and the boiler doesnt condense as the flow return not enough differential and operating at non condensing.

Building fabric Heat loss, boiler size. Radiator size. Pipe size..these need to be in harmony.
Radiators too small (or stupidly too large) and you will discover the limits of your boilers modulation.
Instead of running at its lowest rate and in continual condensing mode is will literally cycle on and off and not condense as it should.

So would i be lax and presumptuous with rad sizes falling back on boilers modulation to take care of poor heating design - nope.

 

[Worcester]

@wetdrip here you go:

Step 1) Do the heat loss calcs - that is a master class in itself taking into account design temperatures, u-values (insulation) (walls, windows, doors, ceilings, floors) Air change factors, degree days, constant / intermittent heating...... -

That will define both the POWER required from any heating source (aka boiler) for the heating and the approximate annual energy bill (on a room by room and total basis)

You'll be suprised how low that will be for the house in question compared to the boiler sizes most "PLUMBERS" would fit.

Why not go and do the three day logic course on Domestic Heating Design... - see details below.

Step 2) You now need to add in the power requirements for the domestic hot water requirements, taking into account reheat speed, design the DHW cylinder to match.

Step 3) Now based on your heat source and emitters (boilers and rads / underfloor) design your flow and return temperatures.

Step 4) Now you know your average flow temperature, taking into account the room temperature you've got your delta T for the emitter design

Step 5) Download the Stelrad book: http://www.stelrad.com/wp-content/uploads/2014/08/stelrad_radiator_book.pdf from there you can look up the power required for a given delta T and choose your rads.

Step 6) Design the hydraulics (pipe sizing and layout) to meet the flow volumes required to meet the heat loads in each of the rads

Step 7) Design the control systems to meet the customers needs and various temperature zones.

Step 8) Produce a quote.

and then lose the job to the cheapest bidder who just walked around with a notebook and does a *** packet calc that sounds good and who installs a system that will never work properly.

We have yet to visit ANY property that has the existing rads properly sized for their heating system.

Rads too large cause problems, rads too small cause problems, oversized and undersized boilers cause problems, badly design controls cause problems.

But then I only wanted you to replace my (old inefficient, broken) boiler...


Here's you starter for 10:

Logic : Plumbing & Heating: Design and Specify Foundation Course (5 Day inc. WRAS, G3 Unvented and Energy Efficiency
Course Outline
• U Values: understanding what they are and their use
• Whole-building design heat loss calculations
• Comfort conditions and basic heating and hot water principles
• Basic heating & hot water system types
• Ventilation & air movement: principles and basics in practice
• Domestic system design
• Understanding TACMA and Part L regulations
• Storage units: comprehensive detail on specifying
• System specification including emitters, pipework, boilers and pump sizing
• Sizing and selection of hot water systems
• WRAS Water Regulations

The course is designed to give you a good level of understanding which will allow you to design and specify basic domestic heating systems and size hot water storage cylinders.

This 8-10 day course has built into it the Logic Certification Foundation Course in Heating & Hot Water Design and WRAS Water Regulations and G3 Unvented.

Certification
Logic Certification - 3 Qualifications
Plumbing & Heating design Foundation
G3 Unvented Hot water
Water Regulations

 

[Worcester]

p.s. also have a look at the Stelrad Stars calculator: Stars App | Heat loss calculator for any room

 

[newcastle phill]

Ive got the big orange book.
Quality reading

 

[wattsie1987]

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!

 

[chris watkins]

@wetdrip here you go:

Step 1) Do the heat loss calcs - that is a master class in itself taking into account design temperatures, u-values (insulation) (walls, windows, doors, ceilings, floors) Air change factors, degree days, constant / intermittent heating...... -

That will define both the POWER required from any heating source (aka boiler) for the heating and the approximate annual energy bill (on a room by room and total basis)

You'll be suprised how low that will be for the house in question compared to the boiler sizes most "PLUMBERS" would fit.

Why not go and do the three day logic course on Domestic Heating Design... - see details below.

Step 2) You now need to add in the power requirements for the domestic hot water requirements, taking into account reheat speed, design the DHW cylinder to match.

Step 3) Now based on your heat source and emitters (boilers and rads / underfloor) design your flow and return temperatures.

Step 4) Now you know your average flow temperature, taking into account the room temperature you've got your delta T for the emitter design

Step 5) Download the Stelrad book: http://www.stelrad.com/wp-content/uploads/2014/08/stelrad_radiator_book.pdf from there you can look up the power required for a given delta T and choose your rads.

Step 6) Design the hydraulics (pipe sizing and layout) to meet the flow volumes required to meet the heat loads in each of the rads

Step 7) Design the control systems to meet the customers needs and various temperature zones.

Step 8) Produce a quote.

and then lose the job to the cheapest bidder who just walked around with a notebook and does a *** packet calc that sounds good and who installs a system that will never work properly.

We have yet to visit ANY property that has the existing rads properly sized for their heating system.

Rads too large cause problems, rads too small cause problems, oversized and undersized boilers cause problems, badly design controls cause problems.

But then I only wanted you to replace my (old inefficient, broken) boiler...


Here's you starter for 10:

Or take the C&G 6089 Level 3 which includes all of the above, at a good training establishment of course.

 

[Ollieg]

80% of customers only want a working heating system and a powerful shower.
10% want A 937 or megaflow because their friend has one... in his 1 bed flat in Chelsea.
The final 10% want the best and are like you Rory.
Hats off to you for asking the right questions and doing your own research. Ask your installer these questions and see what he thinks. His answers will let you know if he is the type of installer you need.
If not be prepared to pay a bit more for someone who is.

 

[Rory]

80% of customers only want a working heating system and a powerful shower.
10% want A 937 or megaflow because their friend has one... in his 1 bed flat in Chelsea.
The final 10% want the best and are like you Rory.

Thanks for the compliments! I wouldn't say I wanted the best, but optimising the rad sizing doesn't cost much and seems quite important with a condensing boiler. As best I can gather, it would be better to err on the side of over-sizing.

I think I'm paying a very fair price for the job - certainly it's not being done cheap.

 

[AWheating]

My opinions:

28i Juniors aren't very good. I'd recommend going for a Worcester CDi (if you want Worcester) unless it is a flat/rental property. Personally in that price range a Vaillant Pro 28 installed by a Vaillant Advance installer (5 year warranty instead of 2) is a much better buy.

Unless you live in a very large property I wouldn't worry to much about F+R temps. You can size the radiators yourself by googling 'radiator sizing' and measuring your rooms.

Worcester Accredited means absolutely precisely nothing. Just means he installs a fair few boilers. It is no indication of quality, and in most cases you are paying for that extra warranty indirectly. Just so you know.

i would not fit a vaillant myself, we all have our own ideas what makes a good boiler and manufacturer.

what makes a vaillant advanced installer better than a worcester accredited installer out of interest?

 

[AWheating]

As for rad sizing, you can do this yourself on the internet at loads of places, if you want to do it long hand get a book and have a go. If your old rads have no convectors/fins then a new rads of the same size will give more ouput if your were comparing apples to apples (with fins).

Your the customer at the end of the day, you can take the advise of your engineer but if you want a larger rad as your the person paying and lives in the property then that what you should have. You can make it as complicated or as simple as you want, your paying you have what you want.

 

[LPC]

what makes a vaillant advanced installer better than a worcester accredited installer out of interest?

Didn't say it was 'better'. WA installers give longer warranties, which in many cases they (aka you) are paying for. VA installers can give longer warranties for free.

 

[kay-jay]

Didn't say it was 'better'. WA installers give longer warranties, which in many cases they (aka you) are paying for. VA installers can give longer warranties for free.

yes but they steal your service work...

 

[LPC]

yes but they steal your service work...

If you sign up for Vaillant Advance there is the option for Vaillant never to contact your customers. They have never once contacted any of my customers.

 

[chris watkins]

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 would recommend this as it will work nicely with your new system, it basically varies the water temp to suit the outside temp thus keeping the boiler in condensing mode for longer periods without effecting the comfort levels. (why is it in there? cos in a most of europe it has to be installed & working).

Can I say, it would be nice to have a more customers who took such an interest in there heating systems, as it might start to raise the standards of design in the domestic industry let along the installation quality.

 

[doitmyself]

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.

Assuming that delta X is the difference between room temperature and the mean rad temp [(flow+return)/2], then unfortunately a 20% drop in the delta does not give a 20% drop in the rad output; it's nearer to 25%.

I set flow temp to 65 allowing room temp to be 15...... Delta 40!

That assumes the return is 45.

But what if the customer doesn't want his house at 15? Answer: your rads will be undersized by 20%.

Output of a "1 kW" rad at different temps:

75/65/15 = 1.14 kW
75/65/20 = 1.00 kW
65/45/15 = 0.73 kW
65/45/20 = 0.60 kW

 

[chris watkins]

Assuming that delta X is the difference between room temperature and the mean rad temp [(flow+return)/2], then unfortunately a 20% drop in the delta does not give a 20% drop in the rad output; it's nearer to 25%.



That assumes the return is 45.

But what if the customer doesn't want his house at 15? Answer: your rads will be undersized by 20%.

Output of a "1 kW" rad at different temps:

75/65/15 = 1.14 kW
75/65/20 = 1.00 kW
65/45/15 = 0.73 kW
65/45/20 = 0.60 kW

Modern design recommendations on design room temps Rad's = 21deg C U/F = 20deg C
I use 21 (for rads) in all rooms, easer that way.

 

[townfanjon]

As Chris has mentioned wether comp, IMO it's an absolute must , the sensor for my boiler at home was approx £100 (viessman 200) it's nicked plenty off my gas bill, the big thing if you go for it your house must be WELL insulated.
To use it you strangely have to rethink the way your heating system works, ie her indoors constantly telling me the rads aren't hot, my reply , are you warm , answer yes, is the house warm , answer yes, = low gas bill .

 

[wattsie1987]

Assuming that delta X is the difference between room temperature and the mean rad temp [(flow+return)/2], then unfortunately a 20% drop in the delta does not give a 20% drop in the rad output; it's nearer to 25%.



That assumes the return is 45.

But what if the customer doesn't want his house at 15? Answer: your rads will be undersized by 20%.

Output of a "1 kW" rad at different temps:

75/65/15 = 1.14 kW
75/65/20 = 1.00 kW
65/45/15 = 0.73 kW
65/45/20 = 0.60 kW

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

 

[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?