What size boiler do I need !?

[WillM]

Hi all, I have got a completely renovated and now well insulated 4 bed detatched house with wet ufh throughout , 1 bathroom, 1 shower, no radiators. Original build date was 1970, filled cavity external walls. 1600sqft over 2 floors. The main living area is open plan with double height ceilings and a lot of glazing. 
 

My plumber has said “min 30kw LPG system boiler and min 210 litre indirect hot water cylinder” My concern is that this a bit too vague for my liking and he hasn’t done the proper maths! Maybe it’s fine, but would be good to know if anyone here knows better ?? 
 

thanks in advance 

[SteamyTea]

40 minutes ago, WillM said:

he hasn’t done the proper maths

Going to be up to you to do it then.

 

 

 

[IanR]

1 hour ago, WillM said:

I have got a completely renovated and now well insulated 4 bed detatched house with wet ufh throughout , 

 

Insulation under the UFH?

 

1 hour ago, WillM said:

filled cavity external walls.

 

Have you considered ASHP?

Off-grid fossil fuel boilers are planned to be banned for new installs from 2026. The next boiler your property has is unlikely to be LPG, and the economics of existing LPG's could well shift once all the companies currently doing new and replacement installs no longer have that business.

At the moment there's a £5K BUS grant available to persuade you to go ASHP rather than LPG. Once the ban comes in there's unlikely to be grants available.

Edited January 11, 2023 by IanR

[jayc89]

1 hour ago, WillM said:

Hi all, I have got a completely renovated and now well insulated 4 bed detatched house with wet ufh throughout , 1 bathroom, 1 shower, no radiators. Original build date was 1970, filled cavity external walls. 1600sqft over 2 floors. The main living area is open plan with double height ceilings and a lot of glazing. 
 

My plumber has said “min 30kw LPG system boiler and min 210 litre indirect hot water cylinder” My concern is that this a bit too vague for my liking and he hasn’t done the proper maths! Maybe it’s fine, but would be good to know if anyone here knows better ?? 
 

thanks in advance 

 

We were in a similar situation, much older house though. Before I came across my second home (this forum), we just took the plumbers advice, we ended up with a 32kw system boiler. Massive overkill for our actual demands, and that's before adding insulation! I think he was still working in the days of 70c flow temps and massive radiators everywhere. It's looking like we're going to be needing some sort of buffer tank by the time we're finished to avoid any short cycling, so it pays to make the right decision early on.

 

The heat loss calc has been incredibly useful to me, I'd certainly plug your numbers into it and add 3kw for your DWH.

If you don't already have a mains gas supply, I'd also seriously consider a heat pump.

Good luck!

Edited January 11, 2023 by jayc89

[Temp]

If you change your mind and go for oil beware they don't modulate down like a gas boiler - there is an argument for using a thermal store instead of just a DHW tank with oil.

 

 

 

 

 

 

 

 

 

 

[WillM]

2 hours ago, SteamyTea said:

Going to be up to you to do it then.

 

 

 

Thats great, thank you. I'll have a play later!

[WillM]

2 hours ago, IanR said:

 

Insulation under the UFH?

 

 

Have you considered ASHP?

Off-grid fossil fuel boilers are planned to be banned for new installs from 2026. The next boiler your property has is unlikely to be LPG, and the economics of existing LPG's could well shift once all the companies currently doing new and replacement installs no longer have that business.

At the moment there's a £5K BUS grant available to persuade you to go ASHP rather than LPG. Once the ban comes in there's unlikely to be grants available.

Yes insulation under the UFH but total house U value still higher than is ideal for ASHP. I have run the new internal pipework in the size that will fit with an ASHP if we want to go that route in future when prices for pump and electric hopefully reduce. For now though the LPG made the most sense overall. I dont have enough south facing roof to make a big enough solar install viable either. 

[WillM]

1 hour ago, jayc89 said:

 

We were in a similar situation, much older house though. Before I came across my second home (this forum), we just took the plumbers advice, we ended up with a 32kw system boiler. Massive overkill for our actual demands, and that's before adding insulation! I think he was still working in the days of 70c flow temps and massive radiators everywhere. It's looking like we're going to be needing some sort of buffer tank by the time we're finished to avoid any short cycling, so it pays to make the right decision early on.

 

The heat loss calc has been incredibly useful to me, I'd certainly plug your numbers into it and add 3kw for your DWH.

If you don't already have a mains gas supply, I'd also seriously consider a heat pump.

Good luck!

This is exactly my gut feeling - thanks for confirming all points.

[Nickfromwales]

5 minutes ago, WillM said:

I dont have enough south facing roof to make a big enough solar install viable either. 

Even a couple of kWp will put a huge dent in your annual electricity bills. 
A mate of mine has 4 second hand 250w panels that he rescued from a skip. Routinely belting out around 700-800w. Almost all of his bare loads whilst in work are dealt with by them. 
Reconsider maybe? With a PV diversion controller a lot could be stored as hot water during the summer  

 

 

[Adsibob]

Your plumber's recommendation sounds way overkill. By way of comparison our house is a 1930s semi, originally a three bed, but as part of our complete gutting, extension and refurb we added a loft floor which has two bedrooms. It is fairly well insulated now, with around 45mm of EWI on all old walls, and cavity walls for the extensions on ground and first floor. We also have UFH, but on the ground floor were only able to put 100mm of PIR under it.

We upgraded all the windows to fairly decent 2G ones.

We have MVHR and very good airtightness, having spent a lot of money on tape and supervised its install closely. Do you have MVHR and an airtight house, as that is a big factor.

 

i did a heat loss calculation based on the spreadsheet linked above got to about 21kw added a bit to give me a margin of safety. Told my plumber I was buying a 24kw boiler for him to install. He thought I was mad. He literally said that I shouldn’t be taking the risk with a five bed house and that anything less than 32kw was madness and that really I should get at least 35kw. I started to doubt my calculations and got a 32kw boiler. I've posted about this elsewhere, but about a month ago, we had a situation which caused us to run all the heating in the house plus heat the hot water tank. I checked the display of my Viessmann Vitodens 200W 32kW boiler. The gas burner was modulating at 54%. My boiler is massively oversized. I think even 24kW was an excessive heat loss calculation and then I remembered that in my calcs I erred on the side of caution and rounded up most uncertainties. I should have stuck with 21kw.

 

So do the calcs, add a small amount for safety, and ignore your plumber.

 

I also recommend Viessman (at least for gas if you are considering gas) as their modulation ratios are market leading. We have a low loss header but no buffer tank and have no short cycling issues, even when we draw less than 10% output.

[ToughButterCup]

13 minutes ago, Adsibob said:

Your plumber's recommendation sounds way overkill. ...

....

So do the calcs, add a small amount for safety, and ignore your plumber....

 

The times we've heard that sensible bit of advice here....😶

[PeterW]

250 litre minimum on the UVC, 210 litres is too small for a 4 bed where you could end up with multiple sequential showers etc.

 

Boiler size is by heat loss calculation - if you’ve gone down the UFH route already and have sized it to meet the demand (ie pipe spacing gives correct W/m² for the space) then now you get heat in becomes a moot point - ASHP could also be used. 

[JohnMo]

Gas, oil, heat pump, all provide heat, no one would install a heat pump rated at 30kW, so why would you need a 30kW system gas or oil boiler.  Treat your heating system the same as you would if installing a heat pump, you will surprised how small your boiler could be.

 

If you are getting a new cylinder get one with a heat pump coil, that way you can operate domestic hot water heating while the boiler is condensing at all times.

 

You also need to go down X plan, not S or Y plan.  X plan will allow the boiler run at two different temps, one for heating and the other for heating the cylinder.

[WillM]

Can anyone direct me to a good U Value table of materials please? 

[WillM]

56 minutes ago, JohnMo said:

Gas, oil, heat pump, all provide heat, no one would install a heat pump rated at 30kW, so why would you need a 30kW system gas or oil boiler.  Treat your heating system the same as you would if installing a heat pump, you will surprised how small your boiler could be.

 

If you are getting a new cylinder get one with a heat pump coil, that way you can operate domestic hot water heating while the boiler is condensing at all times.

 

You also need to go down X plan, not S or Y plan.  X plan will allow the boiler run at two different temps, one for heating and the other for heating the cylinder.

Thanks for the advice. Is an "X plan" common language? Ie will my fitter know what I am on about if I say this to him?

[WillM]

4 hours ago, Adsibob said:

Your plumber's recommendation sounds way overkill. By way of comparison our house is a 1930s semi, originally a three bed, but as part of our complete gutting, extension and refurb we added a loft floor which has two bedrooms. It is fairly well insulated now, with around 45mm of EWI on all old walls, and cavity walls for the extensions on ground and first floor. We also have UFH, but on the ground floor were only able to put 100mm of PIR under it.

We upgraded all the windows to fairly decent 2G ones.

We have MVHR and very good airtightness, having spent a lot of money on tape and supervised its install closely. Do you have MVHR and an airtight house, as that is a big factor.

 

i did a heat loss calculation based on the spreadsheet linked above got to about 21kw added a bit to give me a margin of safety. Told my plumber I was buying a 24kw boiler for him to install. He thought I was mad. He literally said that I shouldn’t be taking the risk with a five bed house and that anything less than 32kw was madness and that really I should get at least 35kw. I started to doubt my calculations and got a 32kw boiler. I've posted about this elsewhere, but about a month ago, we had a situation which caused us to run all the heating in the house plus heat the hot water tank. I checked the display of my Viessmann Vitodens 200W 32kW boiler. The gas burner was modulating at 54%. My boiler is massively oversized. I think even 24kW was an excessive heat loss calculation and then I remembered that in my calcs I erred on the side of caution and rounded up most uncertainties. I should have stuck with 21kw.

 

So do the calcs, add a small amount for safety, and ignore your plumber.

 

I also recommend Viessman (at least for gas if you are considering gas) as their modulation ratios are market leading. We have a low loss header but no buffer tank and have no short cycling issues, even when we draw less than 10% output.

Thanks for the detailed reply. In answer to your Q's I have not got MVHR and it has not been renovated to PH levels but the builders have worked on PH houses before and have taken care everywhere to minimise gaps. This is entirely anecdotal but I was very surprised how quickly it warmed up in the main open plan living area from just one small electric radiator and 1 tiny blower heater having been on to get some of the plasterwork drying out so I am hopeful on the actual efficiency levels in real life.

 

I assume because no MVHR I cant accurately calculate the air change per hour ? If so, any advice what value I should put in cells B5 and B6 please?!

 

Regarding your comment about your boiler being massively oversized, if its modulating and providing the heat you needed at 54% of its potential max then is it burning more gas than a boiler that is correctly sized but running at higher % of its max potential output? Apologies if these are elementary questions - this is all pretty new to me! 

[Adsibob]

19 minutes ago, WillM said:

if its modulating and providing the heat you needed at 54% of its potential max then is it burning more gas than a boiler that is correctly sized but running at higher % of its max potential output? Apologies if these are elementary questions

My understanding is that because I have a pretty sophisticated boiler that modulates well, I’m not losing money in terms of any inefficiency. However I have lost money (and storage space in my utility room) by buying a bigger boiler than I needed to. 
 

Instead of a 32kw boiler, I could have gone 46% smaller, ie 17.3kw (or 18kw or so). So my initial heat loss calculation was actually very close to accurate.

 

 On the plus side, presumably if my boiler rarely works at even half its total capacity, it should hopefully have longer durability (or maybe I’m dreaming!).

[SteamyTea]

14 minutes ago, Adsibob said:

 On the plus side, presumably if my boiler rarely works at even half its total capacity, it should hopefully have longer durability (or maybe I’m dreaming!).

I don't think it will make any noticeable difference to aging or reliability. It is usually the electronics that let them down, and that is either poor design or age.

 

What is going to happen in the late spring when your boiler needs to modulate down even further, and then starts to short cycle?

[JohnMo]

1 hour ago, WillM said:

"X plan" common language

Here's a write up, if your plumber doesn't know what it is, he needs to get back to school.

 

https://heatingacademynorthampton.co.uk/s_and_y_plans_should_be_banned/

 

Boiler efficiency is nothing really to do with where is runs within the available min and max allowable kW.  The biggest effect on efficiency is return temperature. 

 

All new boilers in the UK have to be condensing. What this means, if your return temperature is below 53 degC, the steam within the exhaust flue is converted back to water, during this process the latent heat energy is captured and put back into the boiler. The lower the return temp the higher the efficiency. You get efficiency of around 110% with return in the high 20s low 30s.

 

An oversized boiler or too many small zones can lead to the boiler short cycling, this will drop about 20 to 30% of efficiency as the boiler spends at lot of time doing in-effectively.

[Mattg4321]

My house currently has an 18kW gas boiler fitted. 1970 house. Uninsulated floor, half the windows original single glazed. Original radiators from 1970 - no fins. Minimal loft insulation. 120m2
 

All this will soon change, but my point is that the boiler easily copes. Even in -8 temps recently no problem, plenty in hand. It wasn’t running continuously by any means. 

[Radian]

39 minutes ago, JohnMo said:

Here's a write up, if your plumber doesn't know what it is, he needs to get back to school.

 

Interesting reading. From the article you linked:

 

"This means much smaller boilers (in this case 12Kw) are needed so no gas pipe upgrades."

 

I'm looking at a possible boiler replacement for a 30kW condensing heat-only one. I've already tried range rating it down to 15kW which is more in line with my heat loss calculations and barely notice any difference so I'm looking to replace with something having half the capacity. My current boiler has a 15mm gas feed and is only 2m from the meter connection point and I'd rather like to retain the feed pipe as it's neatly hidden behind tiles. I had been anticipating the imposition of a new 22mm gas feed which seems to be automatically installed with any new boiler these days (judging by experience of other recent installations I've seen) but this article makes it seem as though it might be possible to retain the 15mm feed?

[SteamyTea]

Generally, in physics, the larger a 'machine' is, the more efficient it becomes.  This certainly holds true for heat engines/power generation.

So while a smaller boiler may deliver the desired power needed at any time, it may not be as efficient as a larger one, this may become doubly true when the complete heat delivery system is taken into account.

It is a bit like doing the same journey, in the same type of car, one of 1.8 lt and one of 1.4 lt.  Having actually done this 1.8 S Escort and a 1.4 GL Escort, the 1.8 S was more economical.

Hard to find the specific fuel consumption numbers for boilers, so all a bit speculative.

[MJNewton]

46 minutes ago, SteamyTea said:

Generally, in physics, the larger a 'machine' is, the more efficient it becomes.  This certainly holds true for heat engines/power generation.

 

Not if you end up running the large machine at an output way below its sweet spot. Efficiency tends not to be constant across the range of operational output.

 

Quote

It is a bit like doing the same journey, in the same type of car, one of 1.8 lt and one of 1.4 lt.

 

Try doing the same journey in a 5L V8 - at the same (desired/required) speed - and the inefficiencies of using such an oversized beast for the task would become quite apparent.

[SteamyTea]

4 minutes ago, MJNewton said:

Not if you end up running the large machine at an output way below its sweet spot. Efficiency tends not to be constant across the range of operational output.

It is the whole, non linearity, that is the unknown.

SFC can often improve at fixed output extremes compared to smaller units.

 

6 minutes ago, MJNewton said:

Try doing the same journey in a 5L V8 - at the same (desired/required) speed - and the inefficiencies of using such an oversized beast for the task would become quite apparent.

Is there such a vehicles that has a mass of 1100 kg, a drag coefficient of 3.2 and a frontal area of 1.95 m², with the rolling resistance being the same.

 

It is all about taking out the variables and running them in the same conditions to find the SFC.