Boiler Sizing and Modulation

JamieB3009 · 2 Feb 2021

Hello,

I am after some advice.

I am having my central heating system upgraded to facilitate bathroom upgrades and better hot water pressure. The current system is a gravity fed system with a conventional boiler and a open vent HW cylinder. I am having the system changed to an unvented system with System boiler and unvented cylinder.

My property has 4 bedrooms, 2 bathrooms (2 showers, 1 bathtub), downstairs toilet and has 15 radiators at present. My future plans (2-3 years) anticipate extending somewhat and possibly adding 4 - 5 more radiators and some water fed underfloor heating. I have been looking at options regarding boilers and I am stuck between a 26kw and 32kw system boiler. The difference in price between the two is negligible. The 32kw has a modulation ratio of 5:1 max, equating to a min output of 6.4kW. Would this be the better option to provide some level of future proofing for any future extensions? If the boiler can modulate down to 6.4kW, does this mean that once up to temperature, the gas consumption would drop to this level to keep it steady? I assume that the 32kW would just get to temp quicker in this instance?

thanks

rbranco · 3 Feb 2021

You need to calculate requirements by getting the KW of all the combined radiators, this will give baseline CH requirement.
The HW requirement will be based on your required flow rate, ie only one HW tap/shower running at a time etc etc.
Remember that combi boilers are higher KW due to the HW requirements, most houses really only need 8-18kw (depends on solid brick/cavity etc) heating requirement and if you have WC fitted it will be lower on average as it modulates with low flow temps, if your boiler does not go low enough when WC scales it back (based on calculations etc) because you oversized it then the boiler will start to cycle as it can not maintain a constant flow temp.

If you think you will need more HW requirement, then maybe a storage combi is better or just get a system boiler and a HW tank on pressurised system.

So many variables.

muggles · 3 Feb 2021

Do a proper heat loss calculation to find what you need now https://www.heat-engineer.com/heat-loss-estimator

5:1 modulation is fairly poor in modern terms. There are boilers out there which can achieve much better ratios

JamieB3009 · 3 Feb 2021

rbranco said:
You need to calculate requirements by getting the KW of all the combined radiators, this will give baseline CH requirement.
The HW requirement will be based on your required flow rate, ie only one HW tap/shower running at a time etc etc.
Remember that combi boilers are higher KW due to the HW requirements, most houses really only need 8-18kw (depends on solid brick/cavity etc) heating requirement and if you have WC fitted it will be lower on average as it modulates with low flow temps, if your boiler does not go low enough when WC scales it back (based on calculations etc) because you oversized it then the boiler will start to cycle as it can not maintain a constant flow temp.

If you think you will need more HW requirement, then maybe a storage combi is better or just get a system boiler and a HW tank on pressurised system.

So many variables.

Thanks. It is a system boiler and pressurised tank i am looking at. What i want to consider is if the property post modification (20+ radiators and potentially a large sqm of water fed underfloor heating) would better with a 26kW or 32kW boiler...

rbranco · 3 Feb 2021

There is no exact way of knowing unless you have calculated your losses, if you have an 18kw now and plan on another 10 radiators you could get away with 26kw but if the radiators are huge, rooms huge etc large bifold doors etc etc could push to 32kw. Who knows, your calculations are needed to save your pain later

ScottishGasMan · 3 Feb 2021

As said, you need to do a heatloss calculation on your house (not based on the radiators you have, on the actual house) then add a reasonable amount on for an extension (bearing in mind extensions should be better built and insulated than any older existing house, so often don't add a lot on to the boilers load)

Again as said, 6 to 7kw is pretty poor modulation, id expect that from a worster or baxi, but there are much better modulation ratios to be had, although its not necciserally the main factor in a boiler running efficiently

vulcancontinental · 3 Feb 2021

15 radiators is a fairly substantial system but so much depends on the total heat loss from the house (now and later) and the hot water demand. In properties with regular hot water demand of quantity, baths for example, it's becoming fashionable to put a very fast heat up cylinder in which can absorb most of the boiler output and reheat quickly and assign it priority so that once reheat is ended the boiler can revert to space heating but at low condensing temperatures squeezing a little more efficiency out of the boiler (generally hot water reheat is at a high temp so the boiler doesn't condense. Speed is important in order the property does not cool during DHW reheat). The DHW reheat may total more than the space heating.

You need to determine how much water you need and how much you use (monitoring the present installation might give you a clue if occupancy doesn't change).

Use the heat engineer software for the heat loss or an installer who does, each time I investigate it I am surprised at the capabilities. It's a tenner for a one off calculation (do two, one at the lowest external temperature you expect in the area and one at the highest that you would anticipate requiring space heating, that'll give you the minimum output). There is a free version but obviously very limited. Or you can guess but a few hours of your time and £20 is good value)

These will give you the boiler output. As for modulation that's becoming the flavour of the month, reducing cycling as much as possible. I don't put as much store on it as others, recent independent tests came to the conclusion that if the burn time exceeds 3 minutes then the loss of efficiency over a boiler that cycles less will be 1%-1.5%. To buy a 30kW boiler that will hardly ever cycle (modulation 1:17) will cost £1600 and spares are dear. A boiler that will cycle 1:8 for example will cost half that amount as will the spares. (BTW if the boiler cycles every 10 seconds the efficiency hit is 12% but that is an unrealistic scenario)

If the property does actually need say 15kW after the extension that will only be at extremes of weather which is a comparatively infrequent occurrence, typically 2 to 3 weeks of a 20 month heating cycle. At times of minimum demand when space heating is still required a typical house of that size, if there is such a thing, might only require 2-4kW so a boiler of 30kW with modulation of 1:8 would drop to just under 4 kW and if 1:5 6kW.

Therefore if your annual space heating bill is £1000 and buying a boiler for an additional £800 extra saving say 1.5% that equates to £15/annum. There have to be other reasons to pay the extra as payback would be 53 years.

As for cycling reducing wear and tear, a decent spec boiler should not wear out through cycling, after all they used to cycle continuously in the old SE days. Choose one from an established brand, not a franchise, with a good service network and a long warranty.

Razor900 · 3 Feb 2021

I like boilers with a good modulation ratio. However I'm getting more and more concerned about the longterm effects of very low outputs/flow temps with aluminium heat exchangers.

I've seen more than a few that have excessive wear after only a few years - enough that I'm actively looking at boilers with stainless hex's...

Perhaps it's the modern form of back end rot the older guys will be familiar with. The cure for that was to raise the boilers internal temp

vulcancontinental · 3 Feb 2021

Razor900 said:
I like boilers with a good modulation ratio. However I'm getting more and more concerned about the longterm effects of very low outputs/flow temps with aluminium heat exchangers.

I've seen more than a few that have excessive wear after only a few years - enough that I'm actively looking at boilers with stainless hex's...

Perhaps it's the modern form of back end rot the older guys will be familiar with. The cure for that was to raise the boilers internal temp

I believe with the UK having so many high temp rad systems Ali was king. Stainless cannot live with Ali for that, ss strength is its strength and less servicing. Why do you think it's the stainless manufacturers with poorer efficiency have been pushing low temps, it's because they can then equal Ali and say this is better. It's why we now have a stainless domestic boiler. Run Ali hot there are no more problems than any other boiler.

Razor900 · 3 Feb 2021

vulcancontinental said:
Run Ali hot there are no more problems than any other boiler.

Thats pretty much what I meant. Trouble is it doesn't fit in with modern thinking...

JamieB3009 · 5 Feb 2021

vulcancontinental said:
15 radiators is a fairly substantial system but so much depends on the total heat loss from the house (now and later) and the hot water demand. In properties with regular hot water demand of quantity, baths for example, it's becoming fashionable to put a very fast heat up cylinder in which can absorb most of the boiler output and reheat quickly and assign it priority so that once reheat is ended the boiler can revert to space heating but at low condensing temperatures squeezing a little more efficiency out of the boiler (generally hot water reheat is at a high temp so the boiler doesn't condense. Speed is important in order the property does not cool during DHW reheat). The DHW reheat may total more than the space heating.

You need to determine how much water you need and how much you use (monitoring the present installation might give you a clue if occupancy doesn't change).

Use the heat engineer software for the heat loss or an installer who does, each time I investigate it I am surprised at the capabilities. It's a tenner for a one off calculation (do two, one at the lowest external temperature you expect in the area and one at the highest that you would anticipate requiring space heating, that'll give you the minimum output). There is a free version but obviously very limited. Or you can guess but a few hours of your time and £20 is good value)

These will give you the boiler output. As for modulation that's becoming the flavour of the month, reducing cycling as much as possible. I don't put as much store on it as others, recent independent tests came to the conclusion that if the burn time exceeds 3 minutes then the loss of efficiency over a boiler that cycles less will be 1%-1.5%. To buy a 30kW boiler that will hardly ever cycle (modulation 1:17) will cost £1600 and spares are dear. A boiler that will cycle 1:8 for example will cost half that amount as will the spares. (BTW if the boiler cycles every 10 seconds the efficiency hit is 12% but that is an unrealistic scenario)

If the property does actually need say 15kW after the extension that will only be at extremes of weather which is a comparatively infrequent occurrence, typically 2 to 3 weeks of a 20 month heating cycle. At times of minimum demand when space heating is still required a typical house of that size, if there is such a thing, might only require 2-4kW so a boiler of 30kW with modulation of 1:8 would drop to just under 4 kW and if 1:5 6kW.

Therefore if your annual space heating bill is £1000 and buying a boiler for an additional £800 extra saving say 1.5% that equates to £15/annum. There have to be other reasons to pay the extra as payback would be 53 years.

As for cycling reducing wear and tear, a decent spec boiler should not wear out through cycling, after all they used to cycle continuously in the old SE days. Choose one from an established brand, not a franchise, with a good service network and a long warranty.

Thanks for the comment. I have run the calculator knowing the detail i have and it comes out with a heating demand of about 16

JamieB3009 · 6 Feb 2021

Hello, Just to reply on this.

Thank you for your replies and information!

I have done the free calcs on the heat engineer site using the data as best I know about the house and it outputs a heat loss of 8.9kW, total energy demand of 32,006kWH, hot water being 8490kWh & heating 23510kWh. I'm not sure how accurate the U values for some elements, as they are based on best endeavours without knowing the design info for elements such as windows etc. Additionally, the size of the radiators added together (as it is now pre extension) gives a BTU output of about 55k BTU, which equates to just over 16kW. My extension plans would add (potentially) probably another couple of large double rads and a single or two, so maybe another 5-6kW. Hot water demand is usually a couple of showers per day, plus a bath (not every day) and then water for washing up etc.

With regards the modulation and boiler sizing, the question has come really from the fact that I am planning to replace the boiler anyway, as the current unit is 20 years old and i am performing mods the the HW system to upgrade to an unvented system and a system boiler. The 26kW boiler initially quoted is only £89 less than the 32kW, so it comes down to;

Fit the 26kW and hope it is sufficiently large enough to support an extension with 3-4 rads and approx 35sqm of underfloor heating (water) in a few years. If not then i have to pay another £1k+ to change the boiler again.
Upgrade to the 32kW and hope that in the event of my plans not happening, it will be able to suitably operate the current system without costing me a fortune.

Given my heat loss is above the minimum modulation of both boilers, would this then work? Heat exchangers are stainless steel not aluminium as per the previous comments.

Sorry if i am being daft here, but every source i read shows a different opinion. There is an element of future proofing i am trying to consider.

MJN · 6 Feb 2021

JamieB3009 said:
Sorry if i am being daft here, but every source i read shows a different opinion. There is an element of future proofing i am trying to consider.

I'll leave it to the pros to continue advising on the specifics of your setup/approach, but one comment I would make is to be cautious about taking heat loss calculations too literally. As you said there are components of your construction (eg windows ) that might not have been accurately represented but perhaps more to the point is that thermal performance is as much to do with construction quality and detailing than materials used eg if there are gaps around insulation, thermal bridges etc then the heat loss in practice will be greater than what the theoretical figures on paper might suggest. I suppose my point in a nutshell is that if you're any doubt about 'going up or down' then I'd suggest 'going up' would be the wiser of the two as you don't want to end up cold after all this effort and expense.

sxturbo · 6 Feb 2021

Get the 32 with decent modulation and heating controls,

Get a boiler that has an anti cycling feature, and one that has adjustable range rating.

The Atag is a good option, as is the vokera vision

vulcancontinental · 6 Feb 2021

sxturbo said:
Get the 32 with decent modulation and heating controls,

Get a boiler that has an anti cycling feature, and one that has adjustable range rating.

The Atag is a good option, as is the vokera vision

Vision Plus, 20, 25 and 30kW. The old Vision is Ali.

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Boiler Sizing and Modulation

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