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Discuss Can a 12kw boiler ever heat a 4/5 bedroom house with 3 bathrooms? in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

bradymd

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Hi - I'm a homeowner, not a DIY person.
My 20 year old Gloworm boiler needs replacing.
British Gas quote is ok at £6100 for a 24kw replacement -valiant ecofit 625 system boiler. The tank/heat exchanger and copper pipework bring the price up.
OK so I object (rightly or wrongly) for the extra copper pipework ruining the look of the house outside so shop around. The gas safe rules demand it.
So a new company offer to charge £7000 to put a 12kw valiant pure system boiler (no additional pipework). But I don't feel I'm on top of their smooth explanation of how they can manage to do that. (Smart heating, condensing boiler etc). So, more expensive and less work really. This is not my world but Mr Google suggests this doesn't add up. The company seem OK .. website, livery, spiel, vans, google reviews. Not happy they didn't itemize and not happy they went £1000 over what I thought was already a high quote. Is there some magic they are offering? Or are they magicking away my money?
 

EvilDrPorkChop

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Depends what size of 4/5 bedroom house it is, in addition to how well insulated it is. What is the current size of your existing boiler? Most modern houses 4 bed new build houses will have a 12kw boiler for instance.

In terms of heating load, 12kw can be quite a lot of radiators. You could do it with HW priority thus you relieve some of the load.

I'd stay away from the EcoFit Pure too. Had a few issues with them. You want the EcoTec Plus system instead. Solid boiler.
 

ShaunCorbs

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Any of them done a Heatloss eg measured all of your rooms and windows ?
 

bradymd

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Depends what size of 4/5 bedroom house it is, in addition to how well insulated it is. What is the current size of your existing boiler? Most modern houses 4 bed new build houses will have a 12kw boiler for instance.

In terms of heating load, 12kw can be quite a lot of radiators. You could do it with HW priority thus you relieve some of the load.

I'd stay away from the EcoFit Pure too. Had a few issues with them. You want the EcoTec Plus system instead. Solid boiler.
OK I didn't appreciate new builds for large houses would be 12kw. So thats interesting.
The House is a Town House (3 floors, built 2004) end of terrace. Can get cold on the ground floor.
The Glowworm Micron 70FF is 20kw I think.
There is 17 radiators and 3 bathrooms, so most sizing tables say more like 24kw.
I will read up now about EcoTec Plus.
 

bradymd

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Any of them done a Heatloss eg measured all of your rooms and windows ?
Both british gas and the second plumber measured up to get the overall sq footage of the house. But "heatloss" I don't know about that really. There is some kind of heat loss on the ground floor but the house is not too bad otherwise for insulation - if thats what you mean.
 

EvilDrPorkChop

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OK I didn't appreciate new builds for large houses would be 12kw. So thats interesting.
The House is a Town House (3 floors, built 2004) end of terrace. Can get cold on the ground floor.
The Glowworm Micron 70FF is 20kw I think.
There is 17 radiators and 3 bathrooms, so most sizing tables say more like 24kw.
I will read up now about EcoTec Plus.
I suppose the question is why aren't they putting the same output back? As I assume the gas pipe is big enough for your current boiler.

As @ShaunCorbs They should do a heat loss calc if the output is in question. The online calculators are just generic.
 

ShaunCorbs

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The cold spots might be down to either too small of a rad in that room /sludged up one or an overall over-demand on the boiler

Tbh I would do a Heatloss Calc and see where you end up
 

bradymd

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The cold spots might be down to either too small of a rad in that room /sludged up one or an overall over-demand on the boiler

Tbh I would do a Heatloss Calc and see where you end up
Using an online calculator the 13 rooms totals up to about 12 kw.
It's end of terrace town house and can be cold, doesn't catch the sun, there are 17 radiators.
So why would some sizing tables (and most plumbers including British Gas) says 24kw and other say 12kw? Madness...
 

Chuck

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So why would some sizing tables (and most plumbers including British Gas) says 24kw and other say 12kw? Madness...
12kW seems low for the house you are describing unless it has had a lot of external insulation applied, which would be unusual and you haven't mentioned.
 

ShaunCorbs

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Using an online calculator the 13 rooms totals up to about 12 kw.
It's end of terrace town house and can be cold, doesn't catch the sun, there are 17 radiators.
So why would some sizing tables (and most plumbers including British Gas) says 24kw and other say 12kw? Madness...

Some just use rules of thump and oversize which doesn’t help did you work out if your rads are big enough?
 

bradymd

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12kW seems low for the house you are describing unless it has had a lot of external insulation applied, which would be unusual and you haven't mentioned.
No special insulation. In fact, there is a overhang at the front and the rear of the house were kitchen and bathroom vents dispense air and I think THIS is the source of the cold loss on the ground floor.
 

bradymd

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Some just use rules of thump and oversize which doesn’t help did you work out if your rads are big enough?
I used an online calculator which never asked me about rads or even how many showers.
OK so with a 12kw condensing boiler with Modulation Controls (ability to control output) and PDHW (Priority Hot Water) and a low flow temperature (<55c) through the rads (am I speaking anyones lingo here?) with reference to the website "heating hub" which espouses low wattage boilers - that is how the plumber intends to achieve heating this house and supplying hot water to three showers rooms on a 12kw boiler. So a Vailant Vsmart for PDHW with outside temperature sensor.

This versus the traditional quote - actually from two other plumbers now - who both said 24kw boiler and 22m copper pipes to replace 15mm pipes.

So are they revolutionaries or is there a hidden snag here?
 

EvilDrPorkChop

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I used an online calculator which never asked me about rads or even how many showers.
OK so with a 12kw condensing boiler with Modulation Controls (ability to control output) and PDHW (Priority Hot Water) and a low flow temperature (<55c) through the rads (am I speaking anyones lingo here?) with reference to the website "heating hub" which espouses low wattage boilers - that is how the plumber intends to achieve heating this house and supplying hot water to three showers rooms on a 12kw boiler. So a Vailant Vsmart for PDHW with outside temperature sensor.

This versus the traditional quote - actually from two other plumbers now - who both said 24kw boiler and 22m copper pipes to replace 15mm pipes.

So are they revolutionaries or is there a hidden snag here?
You've still not answered why the Same size output isn't being put back? Why not replace like for like. You're jumping from 24kw to 12. 18kw would be fine.

Also it's more than likely that if it's a 15mm gas pipe running down, it'll be 22mm in the ceiling above. So may not need replacing, or will just want dropping down in 22mm.
 

Chuck

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So are they revolutionaries or is there a hidden snag here?
The snag is you'll be cold in the winter and have a limited supply of hot water.

If a qualified plumber specifies an undersized boiler it's their problem. If you choose your own using a free website and it's wrong it's your problem.

To answer your next question, expect a used 12kW boilers have a negative resale value. (No one will want it and you'll have to pay to have it taken out.)
 

ShaunCorbs

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The snag is you'll be cold in the winter and have a limited supply of hot water.

If a qualified plumber specifies an undersized boiler it's their problem. If you choose your own using a free website and it's wrong it's your problem.

To answer your next question, expect a used 12kW boilers have a negative resale value. (No one will want it and you'll have to pay to have it taken out.)

You should be staggering your hot water heat up times anyway so this shouldn’t be a problem

Also you don’t want to have an oversized boiler either
 

bradymd

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You've still not answered why the Same size output isn't being put back? Why not replace like for like. You're jumping from 24kw to 12. 18kw would be fine.

Also it's more than likely that if it's a 15mm gas pipe running down, it'll be 22mm in the ceiling above. So may not need replacing, or will just want dropping down in 22mm.
Hi - the plumber is saying 12kw will suffice and referring me to heating hub website too which espouses low wattage boilers.
I was fully expecting a like-for-like but other plumbers insist on increasing the size of the pipe from the mains - to draw more gas.
(I am listening/reading but I might not always understand the significance.) I think I would feel more comfortable with 18kw as a compromise.
 

bradymd

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11
The snag is you'll be cold in the winter and have a limited supply of hot water.

If a qualified plumber specifies an undersized boiler it's their problem. If you choose your own using a free website and it's wrong it's your problem.

To answer your next question, expect a used 12kW boilers have a negative resale value. (No one will want it and you'll have to pay to have it taken out.)
Yes its all on the plumbers recommendation to go for 12kw with the Vaiilant Vsmart, not me. I'm merely trying to see why its contrasts so sharply with a lot of other recommendations. The free website just backs up what they are saying.

Yes, I could be lumbered with a buying a new boiler, I guess thats the worse case.
 

Enthalpy

Messages
9
I was in a similar position last year. I’m a homeower not a gas engineer.

I tried several free heat loss calculators and preferred the MYSON Heat Loss Manager because it lists the heat loss in each room for each element: Glazing, External Wall, Floor, Roof Glazing, Roof and Ventilation.

I also measured all 13 radiators and looked up the heat output of similar radiators to determine the total heat output of my radiators with a 50 K Δt.

A 25 kW boiler was replaced by a 15 kW boiler. It was more than enough last winter.

Behind the front panel the Vaillant ecoFIT pure 625 appears the same as the Glow Worm Energy7 25S. The installation and maintenance instructions are largely identical.
 

ShaunCorbs

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Just a tip a condensing heat only should be sized at a delta t of 35 eg

65 + 45 / 2 - 21
 

Chuck

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Yes its all on the plumbers recommendation to go for 12kw with the Vaiilant Vsmart, not me.
My (mis?)reading of your earlier posts was that plumbers including BG had recommended 24kW and only your DIY calculation was 12kW and my earlier comments should be considered with that in mind.

Nonetheless, you did say that the lower floor of your house was 'cold' with your current 20kW boiler so in your position I'd want a credible explanation of how the 12kW system is going to fix this problem.

Another useful sanity-check in this sort of situation is to look at your winter gas bills for the last few years and see how many kWhr per day you have historically used on average to keep warm. Remember that the peak needed during the coldest few days will be significantly more than the average.
 

bradymd

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I was in a similar position last year. I’m a homeower not a gas engineer.

I tried several free heat loss calculators and preferred the MYSON Heat Loss Manager because it lists the heat loss in each room for each element: Glazing, External Wall, Floor, Roof Glazing, Roof and Ventilation.

I also measured all 13 radiators and looked up the heat output of similar radiators to determine the total heat output of my radiators with a 50 K Δt.

A 25 kW boiler was replaced by a 15 kW boiler. It was more than enough last winter.

Behind the front panel the Vaillant ecoFIT pure 625 appears the same as the Glow Worm Energy7 25S. The installation and maintenance instructions are largely identical.
Thanks for introducing the idea of Delta ... wondered what the triangle sign mean't for a while there. I will pursue this. We have 15 radiators.
I think they are suggesting a Vaillant Pure but now I have realised they didn't say 412 or 612 so I need to follow that up with them.
Not even clear what the difference is from my perusal just now.

Other plumbers reckoned big boilers need the fatter pipe as I said so I need to keep lower than 20kw ( think my Glowworm 70ff is 20 kw)
 

bradymd

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My (mis?)reading of your earlier posts was that plumbers including BG had recommended 24kW and only your DIY calculation was 12kW and my earlier comments should be considered with that in mind.

Nonetheless, you did say that the lower floor of your house was 'cold' with your current 20kW boiler so in your position I'd want a credible explanation of how the 12kW system is going to fix this problem.

Another useful sanity-check in this sort of situation is to look at your winter gas bills for the last few years and see how many kWhr per day you have historically used on average to keep warm. Remember that the peak needed during the coldest few days will be significantly more than the average.

We have high bills ... in winter its 84 kwh a day for gas. But might that mean our current system is both inefficient and the ground floor cold. I need to insist on a bigger boiler don't I ...
 

king of pipes

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You should be staggering your hot water heat up times anyway so this shouldn’t be a problem

Also you don’t want to have an oversized boiler either
Agreed mate but you don't want a undersized one either I'd be splitting the difference 18 kw if you go Valliant the eco Tec plus would be a better choice if you're Tec savy V Smart is great but many a end user can stuggle ?
Also address the heat loss in your property it's often easier than you think . Kop
 

ShaunCorbs

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Agreed mate but you don't want a undersized one either I'd be splitting the difference 18 kw if you go Valliant the eco Tec plus would be a better choice if you're Tec savy V Smart is great but many a end user can stuggle ?
Also address the heat loss in your property it's often easier than you think . Kop

And cheaper in the long term with gas prices not going down
 

king of pipes

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I was in a similar position last year. I’m a homeower not a gas engineer.

I tried several free heat loss calculators and preferred the MYSON Heat Loss Manager because it lists the heat loss in each room for each element: Glazing, External Wall, Floor, Roof Glazing, Roof and Ventilation.

I also measured all 13 radiators and looked up the heat output of similar radiators to determine the total heat output of my radiators with a 50 K Δt.

A 25 kW boiler was replaced by a 15 kW boiler. It was more than enough last winter.

Behind the front panel the Vaillant ecoFIT pure 625 appears the same as the Glow Worm Energy7 25S. The installation and maintenance instructions are largely identical.
Basically they are the same 😉
 

snowhead

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Here's the factory in Derbyshire.
I wonder why the Vaillant and Glow worm are the same,, 😃

You don't have permission to view attachments. Attachments are hidden.
 

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Chuck

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We have high bills ... in winter its 84 kwh a day for gas. But might that mean our current system is both inefficient and the ground floor cold.

Assume an average winter temperature of, say, 9°C and that you want your house at at average of 19°C that's 3.5 kW average for 10°C of heating. For the coldest weather (-10°C) you need more like 30°C of heating, which takes you to 10.5kW. So, maybe 12kW would be okay but, that doesn't leave much margin for error. It's also big step down in size from your current boiler. In your position, I'd want at least 15kW and probably 18kW just in case, which is the compromise others have already suggested.

Don't forget that if you have a terraced house, if your neighbours decide to leave it unoccupied in the winter for some reason your gas consumption will rise with the increased losses through the party wall. So you need a bit in reserve in case this happens.
 
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bradymd

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Assume an average winter temperature of, say, 9°C and that you want your house at at average of 19°C that's 3.5 kW average for 10°C of heating. For the coldest weather (-10°C) you need more like 30°C of heating, which takes you to 10.5kW. So, maybe 12kW would be okay but, that doesn't leave much margin for error. It's also big step down in size from your current boiler. In your position, I'd want at least 15kW and probably 18kW just in case, which is the compromise others have already suggested.

Don't forget that if you have a terraced house, if your neighbours decide to leave it unoccupied in the winter for some reason your gas consumption will rise with the increased losses through the party wall. So you need a bit in reserve in case this happens.

Thanks for this summary advice ... I have been losing a bit of sleep over this ... plus the Mrs is from a tropical country so our house average temperature is more like 21.5c 😅 hopefully (probably) a 15kw or 18kw won't mean I need the 22 mm pipes being drawn from the front of the house so ... I'd like thank everyone on plumbersforums for reading and commenting ... it is much appreciated
 

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