Boiler size confusion

[deuce22]

Hi.

 

Is there a way I can check what the heating engineer is specifying for my boiler size. He is stating a 25kw heat only boiler, but a friend (who is also a HE, but lives a few hours away, so didn't do the work here), has said it only needs around 15kw.

 

Thanks.

[SteamyTea]

That is a difficult one to answer without knowing what your space heating loads are and your domestic hot water usage is going to be.

 

It is not unusual for the heat source to be sized for DHW rather than space heating.  This is more true for combi boilers.

[deuce22]

It's a heat only boiler with a 300l cylinder. Underfloor heating on the ground floor, radiators on floor 1 and 2. It's an ICF construction and my friend said straight away that 25kw was overkill. My friend said that oversizing can cause more wear and tear and will waste energy. I don't mind buying the bigger boiler, but will it just cause problems?

[jayc89]

Only way you'll know is by doing a heat loss calc - 

 

[JohnMo]

18 minutes ago, deuce22 said:

25kw was overkill

What really matters is modulation range, if it can go down to 2kW it's suitable for just about any house. If it only returns down to 15kW it maybe too big for most properties.

 

Do a heat loss calculation as linked above. Or read your sap report (full version) and it will have max demand hidden there also.

 

I'm currently running a 32kW boiler in a 3.5kW (at -10) heat demand house. But it turns down to 6kW and is running into a thick screed UFH. It doesn't cycle, runs at 37 degs for hours on end, when asked to do so. I am running simulated weather compensation (boiler thinks it's running WC) but delivers a fixed flow temp that modulates power.

 

If your doing a new boiler, set it up correctly - priority demand hot water (PDHW), and weather compensation. 

 

I would do a system boiler not a heat only and get one that manages pump flow rate also. Go Intergas, Atag, Viessmann or a new series of Vaillant which modulate better.

[deuce22]

11 hours ago, jayc89 said:

Only way you'll know is by doing a heat loss calc - 

 

Thanks for that. I'll input the details and see what it states.

[deuce22]

11 hours ago, JohnMo said:

What really matters is modulation range, if it can go down to 2kW it's suitable for just about any house. If it only returns down to 15kW it maybe too big for most properties.

 

Do a heat loss calculation as linked above. Or read your sap report (full version) and it will have max demand hidden there also.

 

I'm currently running a 32kW boiler in a 3.5kW (at -10) heat demand house. But it turns down to 6kW and is running into a thick screed UFH. It doesn't cycle, runs at 37 degs for hours on end, when asked to do so. I am running simulated weather compensation (boiler thinks it's running WC) but delivers a fixed flow temp that modulates power.

 

If your doing a new boiler, set it up correctly - priority demand hot water (PDHW), and weather compensation. 

 

I would do a system boiler not a heat only and get one that manages pump flow rate also. Go Intergas, Atag, Viessmann or a new series of Vaillant which modulate better.

I have had a quick look at my SAP report. Where would I find the info I need?

 

I'll pass some of your advice on to the HE.

 

Thanks.

[JohnMo]

54 minutes ago, deuce22 said:

I have had a quick look at my SAP report. Where would I find the info I need?

 

I'll pass some of your advice on to the HE.

 

Thanks.

Look at as built or as designed space heat required section. Depending on how accurate your sap is, depends how good the figure is. 4th line down - heat loss rate W. Look for the most onerous month. So that is your ball park heat loss rate. 

 

 

 

[deuce22]

12 minutes ago, JohnMo said:

Look at as built or as designed space heat required section. Depending on how accurate your sap is, depends how good the figure is. 4th line down - heat loss rate W. Look for the most onerous month. So that is your ball park heat loss rate. 

 

 

 

This is what I've got. I've just contacted them, but they don't know how to calculated the boiler size from the figures shown.

 

 

[JohnMo]

So you just need a dinky gas boiler.

 

7 minutes ago, deuce22 said:

I've just contacted them, but they don't know how to calculated the boiler size from the figures shown.

Find a new plumber. They are just sticking a finger in air and having guess.

 

The difference in a good install and poor one is using twice the gas.

[deuce22]

Sorry, I didn't mean I contacted the plumber, I contacted the company that did the SAP calculations.

 

Is there not a way for me to work out what boiler size I need from these monthly/yearly heat loss figures. I just want to know that the correct boiler is being installed. I know I should trust the HE, but I feel like I need to keep on top of every trade, with the eperience I've had since starting this project.

[JohnMo]

Heat loss rate W is the heat loss. You need a boiler that can provide that to heat the house, worst case. Allow some time for DHW heating you are still only needing less than 7kW.

 

But realistically you will be in the 11 to 20kW range of boilers. It's just important that you get one with a good modulation, if it can go down to about 3kW you will have a good stable operation all year, running a simple WC curve.

 

So you just need the following 

 

Boiler (that does PDHW and weather compensation)

Get a heat pump cylinder with 3m² heating coil

Diverter valve.

Ideally size radiators for the same temperature as the UFH (you can then run the whole system from the system boiler circulation pump all at the same temperature. (no UFH mixer or pump needed)

A controller from the boiler manufacturer.

 

Run all as a single zone, with trvs on the bedroom radiators only.

 

Your system is then heat pump ready also. But will make the most out of the gas boiler.

 

 

[JohnMo]

I would look at something like an Atag i15S, with PDHW kit. 18 year warranty.

[deuce22]

Thanks very much John, for your help.

 

A 25kw boiler will be too much then, the 15kw that a friend suggested looks right. I may find another HE as it looks as if he doesn't know what to do, he hasn't mentioned any of the other things you've suggested.

[marshian]

7 minutes ago, deuce22 said:

Thanks very much John, for your help.

 

A 25kw boiler will be too much then, the 15kw that a friend suggested looks right. I may find another HE as it looks as if he doesn't know what to do, he hasn't mentioned any of the other things you've suggested.

 

 

Unfortunately many of them don't - they are "conditioned" to throw an oversize white box on the wall and set it up with S Plan

 

No heat loss calcs to determine the optimum boiler size

No weather compensation and HWP resulting in an elevated flow temp to cope with both HW and CH

No optimisation of boiler settings to ensure the HW and CH requirements are matched to the boiler settings

 

UK is miles behind Europe where this is concerned in so many aspects - even down to constant BBOE set up of Rads which loses 5% heat output.

 

If you choose a Viessmann 100-W "Heat Only" my only recommendation would be not to spec/purchase a 11kW version (they can't be range rated like the 16kW or larger ones)

[JohnMo]

36 minutes ago, marshian said:

"Heat Only" my only recommendation would be not to spec/purchase a 11kW version (they can't be range rated like the 16kW or larger ones

That's what I like about Atag boilers there is none of this range rating to do, you can limit max power output, but you don't really need to, and no faffing with boiler flow rates. Everything in fully integrated - pump flow automatically modulates to suit flow and return temp, and output is automatically modulating.

 

They also start differently from other boilers, start up is at minimum output and they slowly ramp up (depending on how you set the gradient setting) settling at min output and suitable dT to support demand. Pretty much the same as my heat pump.