Boiler Sizing Too Big?

[flappy8]

Hi All,
I’ve had a couple of people in to quote for boiler install with weather compensation in a well insulated refurbishment. It will be a system boiler on a 250L unvented tank with rads upstairs and UFH downstairs. I’ve done the heatloss calcs from the CIBSE book and run through the whole house boiler sizing calculator from the BRE data.

I get 13kw (inc 2kW hot water allowance) from the whole boiler method and 13.5kW from the CIBSE calcs. Similar result from the BAXI website calculator too.

Both people have said I should have a 30kW system boiler as ”that’s what you use on this size house” and that the hot water tank can use a lot of power. I’m concerned that as the boiler will only modulate down to 5kW, when it’s not deep midwinter it will end up cycling a lot and then not running efficiently. I also see a lot of stuff on the web about boilers being oversized. Has the 2kW heating allowance that gets mentioned a lot not caught up with modern cylinders?

All comments welcome.

 

[lame plumber]

get some other quotes, cant comment without seeing system layout, rad sizes used etc

 

[AlexGas]

Both people have said I should have a 30kW system boiler...

You are saying "people". Are they Gas Safe people?
Oversizing may increase running costs.

 

[flappy8]

You are saying "people". Are they Gas Safe people?
Oversizing may increase running costs.

Yes Gas Safe. I was just concerned that there appeared to be no science in the sizing.

 

[AlexGas]

Yes Gas Safe. I was just concerned that there appeared to be no science in the sizing.

Get more quotes.

 

[hammers4spanner]

Yes Gas Safe. I was just concerned that there appeared to be no science in the sizing.

Depends whom you get in to quote as many a muppet out their

However without being rude someone who tells you your job is a pain in the urse

 

[flappy8]

............However without being rude someone who tells you your job is a pain in the urse

Understand that entirely - I guess I have to ask them if they will do the calcs properly.

On the hot water - how many kW do most people add as an allowance?

 

[Rory]

I’m concerned that as the boiler will only modulate down to 5kW, when it’s not deep midwinter it will end up cycling a lot and then not running efficiently. .

Even the 15kw boilers still only seem to go down to 5kw.

Having just had a complete new system installed in my daughter's house it's clear there's a lot of overkill going on - the last thing they want is you calling them back as it's cold so you end up with a system significantly OTT.

 

[hammers4spanner]

depends on reheat times

 

[chris watkins]

Depends whom you get in to quote as many a muppet out their

However without being rude someone who tells you your job is a pain in the urse

Only if you are wrong & they are right !!

Otherwise it is great cos it takes away all the design responsibly from you.

 

[chris watkins]

Understand that entirely - I guess I have to ask them if they will do the calcs properly.

On the hot water - how many kW do most people add as an allowance?

None, as you will have full boiler power to heat the cylinder if you have weather comp (you can't have heating on at the same time as HWS)

Don't let them try to talk you into having oversized boiler on the grounds it will modulate down to what it needs.
Size is important !!! to big & the electronic brain is set up to expect a larger system which will lead to problems.

 

[hammers4spanner]

Only if you are wrong & they are right !!

Otherwise it is great cos it takes away all the design responsibly from you.

i would still want to check myself as i would imagine if you install on a custs say so and its not quite right you as the professional trained installer would probably end up liable in todays world

 

[happyflyer]

Are we saying that GSR engineers all genned up on system sizing and are all spot on??

Also Flappy, 2.5 kw isn't enough to run the coil in a 250 l Mega flow, look at what the manufactures say about the coil load I think it's nearer to 12 Kws, it's OK looking all the BRE and CIBSE tosh but you need a light touch and a bit of experience, like some says get some more lads into quote and then make your mind up who sounds who sounds the most convincing, don't let him see your calculations and look at his, he won't even have heard of CIBSE or BRE.............SCREWFLUX maybe.

BTW who is doing you UFH, make sure you don't get someone who has only done it once before at his mums house, get someone in proper and a proper design, watch the pipe spacings in the screed too.

I surveyed a house 3 years back £3.5M it cost and they had laid the pipes in at 500 mm centres, they had to use electric fires it was that cold, they moved out into a hotel whist the builder removed everything from the house, floors doors skirting boards,kitchen bathrooms everything out, took 6 months, so don't get in wrong. BTW a second consultant tried to trash my report and said add a few radiators, they did what I said, ripped it all out and start again.

Not trying to put the wind up you Flappy wouldn't want you in a Flap later

 

[Ermintrude]

Depends what cylinder you have mate. If you have a £7.99 bargin or a ACV. Know of a few cylinders that will munch 78kw.

Why not box clever? Fit ACV SLME and have a say 40kw CDI regular on to cylinder, your rads zoned room by room off of at base of cylinder - basically LLH on cylinder - prevents cycling but means boiler can kick bum when it's required.

But if your uber talented set it up with two diverters so when rads are on the return water is supplied back to the UFH as a flow and then direct back to cylinder. Basically a double VT.

Fit weather comp but fit it properly. Modulate ufh temp and rad temp so to avoid overshoot. Basically u blend flow from store up or down electronically VT Rad circuit is basically ufh with proportional temp control from thermal store CT Cost u ££££ but your boiler won't every cycle. Your pumps won't run 24/7 and you won't get overshoot on temp.

 

[flappy8]

i would still want to check myself as i would imagine if you install on a custs say so and its not quite right you as the professional trained installer would probably end up liable in todays world

If a customer says please install a 20kW boiler and it's too small it's their fault - not yours!

 

[SimonG]

If a customer says please install a 20kW boiler and it's too small it's their fault - not yours!

That is complete bull. Being a professional is about doing the job professionally. If a cust said to me I want a 20kw boiler and I new it was far too small, then they would get the correct sized boiler or told to foxtrot oscar.

Well Mr Trading Standards, I knew that the 20kw boiler wasnt fit for purpose, but its what the cust wanted.

 

[Ermintrude]

Well simon is 100% correct.
If custard supplies it well you just try not to laugh as your being paid. 24kw main fitted in a moderate dwelling in winyard park supplied by a landlord lasts about 3 months from experience .

 

[happyflyer]

This will end in tears

 

[SimonG]

Of laughter?

 

[goz666]

Had a customer once that told me I wasn't putting enough clips on the pipework then rang me weeks later to say the system is noisy when it's heating up, I went and broke a few clips off and lo and behold no ticking noises. Nobody likes a know it all.

 

[flappy8]

........ Nobody likes a know it all.

Indeed - same as in my line of work - electrician -

Its a pity if that means that all customer's thoughts are just rubbished. I'm surprised that boiler manufacturers produce anything under 30kW!

 

[SimonG]

That's why the cust employs a professional. Most of the time a custs thoughts are exactly that.

 

[happyflyer]

I have forgot what this post was all about, it's mi age you see. Was it about boilers or electrics

Baldy old Cantankerous Git

 

[stevetheplumber]

god save us from customers i got a call last night bathroom all first fixed and ready to tile now she says basin is to low its on a pedestal so hiefght fixed so she is now going to get a semi ped which means i have to change all the pipework

 

[gpbeck]

A boiler sized to match heat loss will slowly heat the house to temperature, might take 3 hours. A bigger boiler heats the house quickly, allowing temperatures to fall when the house is empty but getting up there quickly when required (mobile phone control for example). This save energy.

 

[chris watkins]

A boiler sized to match heat loss will slowly heat the house to temperature, might take 3 hours. A bigger boiler heats the house quickly, allowing temperatures to fall when the house is empty but getting up there quickly when required (mobile phone control for example). This save energy.

"A boiler sized to match heat loss will slowly heat the house to temperature" No it won't you know, as heat is leaving as fast as it is being put in, so it would not heat up but just stay the same.

Installing big boilers to heat things up quickly does not save energy it wastes it, why do you think all modern boilers modulate ?? i.e. match there out put to only what is required?

 

[gpbeck]

"A boiler sized to match heat loss will slowly heat the house to temperature" No it won't you know, as heat is leaving as fast as it is being put in, so it would not heat up but just stay the same.

Installing big boilers to heat things up quickly does not save energy it wastes it, why do you think all modern boilers modulate ?? i.e. match there out put to only what is required?

Yes it will you know.

Heat loss is proportional to temperature drop across the walls.

At 6 am the house might be 12 degrees, 0 outside so gradient 12. Boiler is sized to heat gradient of 20, so initially is 66% oversize. So house heats up ever more slowly until it gets to 20, when it is balanced out.

Of course the boiler will modulate. It's not always freezing out there. But high insulation and rapid warming for occupied times is the future.

 

[oz-plumber]

Yes it will you know.

Heat loss is proportional to temperature drop across the walls.

At 6 am the house might be 12 degrees, 0 outside so gradient 12. Boiler is sized to heat gradient of 20, so initially is 66% oversize. So house heats up slowly until it gets to 20, when it is balanced out.

Of course the boiler will modulate. It's not always freezing out there. But high insulation and rapid warming for occupied times is the future.

I'm lost - on many angles.
Decipher?

 

[gpbeck]

Trying again. This idea that a boiler sized to heat loss at 20 room temperature won't heat up the house is plain wrong because heat losses are less when the house is cooler. So the boiler heating exceeds heat losses and the house warms. It is a decelerating curve and a powerful boiler makes it get there quicker.

This is why small boilers attract customer complaints, particularly in a poorly insulated home. Still cold when they leave for work!

 

[gpbeck]

One of the many problems with education nowadays is that facts are simplified and then presented And accepted as truth.

 

[oz-plumber]

Who sizes heating systems to heat up from temperature 0 C to 20 C in 3 hours.

It doesn't matter what temperatures you use but I'm yet to find anyone who will calculate a system heat up time of 3 hours.
Most would calculate a system heat up time of an hour.

 

[gpbeck]

I'm not suggesting sizing a boiler like that! Would be crazy. Just pointing out what would happen if OP did.

 

[gpbeck]

And in my post that confused you the house heated from 12 to 20 in 3 hours. Very poor, but I know a house that does :-(

 

[chris watkins]

One of the many problems with education nowadays is that facts are simplified and then presented And accepted as truth.

Tell me about it gpbeck, like post 25, sweeping statements lead to confusion & miss understanding. Which could lead others to think it is OK to install oversized boilers for the odd times that a dwelling would get that cold. :wink5:

 

[doitmyself]

A boiler sized to match heat loss will slowly heat the house to temperature

No it won't you know, as heat is leaving as fast as it is being put in, so it would not heat up but just stay the same.

But a boiler is sized to raise the temperature from, say, -1C to 21C, taking into account the rate of heat loss, plus 10-20% extra for a quicker warm up.

If we ignore the time it takes for the boiler to reach working temperature, initially the boiler will be producing heat much faster than it is disappearing through the walls. Now, rad output depends on delta-T between rad and the air. (It also depend on flow and return temperatures, but we will assume they are constant. )So when the room is at 0C a rad can give out about 50% more heat than when the room is at 20C; but this will depend on the output of the boiler . As the room temperature rises, the rad output reduces, until equilibrium is reached at the required room temperature.

When this happens the room stat turns the boiler off and the temperature start to drop. If you left the boiler off until the house has cooled down to -1C, you would need the full output of the boiler to raise the temperature to 21C again. But this doesn't happen as the thermostat turns the boiler back on when the temperature has dropped by a degree, or even less. This means the boiler only has to provide enough heat to raise the temperature by 1 degree, not 22. So the boiler only has to provide about 5% of its output to raise the temp by 1C.