Combi boiler - can you have a low power for heating and a high one for water?

[Dan Robinson]

Indeed VC, most over egg my place by a good margin.

 

[D_Hailsham]

Tfor central heating my flat only needs about 15kWh (according to the calculator at <http://www.idhee.org.uk/calculator.html>).

15kW is a lot for a flat - unless it's huge and badly insulated. I don't even need that for a 4 bed detached house.

The unexpected result may be due to the calculator, which isn't always very clear. Try using Baxi Whole House Boiler Size Calculator.

As Dan says, it's how low the boiler can go which is more important. Some boilers, e.g Vaillant, allow you to restrict the max CH output independently of the HW output.

 

[vulcancontinental]

The thing here SGM is the building regs are aiming to get a modern 3 bed semi heatloss to below 5-6 kw shortly and for a 4 bed 7KW. Heat loss in flats are even lower. The problem is while that's all laudable gas is cheap (relatively) most people use it, insulation gives best bang per busk but it's leading to problems with older buildings breathing and damp. My property is 170 and I'm just doing mine up and seriously considering not internally insulating the external walls, just replaster, let them breath and heat with the most efficient means at my disposal.

As to the last post that's not quite how to look at it in my opinion, you can restrict the maximum output to heating but that's not the current issue, it's lengthening the output envelope to reduce the minimum setting to match the property requirement more of the time.

 

[ScottishGasMan]

I totally agree with that last bit VC, ideally if gas is being used they would make something that can modulate to 1 or 2 kW while still keeping an efficient gas/cd air mix, but most manufacturers simply arnt doing that, certainly not fast enough. This might begin to take priority as a side effect of erp and efficiency ratings means that most of Europe will now need to lean towards condensing technology (the continent still uses open flues and standard efficiency boilers when fitting new as the most part of their boiler sales)

I'm insulating my external walls with 50mm celotex internal insulation in most places, damp has been a major concern for me, have 2 chimneys that are vented top and bottom to outside, 1 under the floor and 1 to the room (fire place) and hope that will deal with any moisture in the sand stone it's self, 2 single room heat recovery extractors (bathroom and utility room/kitchen) will hopefully deal with a majority of internal moisture, but it will never come close to the heat loss of a new build, just trying to stop it bleeding heat, oh and secondary glazing all round!

 

[ScottishGasMan]

Also most new builds I see are still sizing the smallest/cheapest rads possible to a 80/70 or 80/60 temp, if the rads in my house were in a new build I could literally run 40/25 or there abouts, with slightly longer burn times and much longer anti cycle times,

 

[vulcancontinental]

I totally agree with that last bit VC, ideally if gas is being used they would make something that can modulate to 1 or 2 kW while still keeping an efficient gas/cd air mix, but most manufacturers simply arnt doing that, certainly not fast enough. This might begin to take priority as a side effect of erp and efficiency ratings means that most of Europe will now need to lean towards condensing technology (the continent still uses open flues and standard efficiency boilers when fitting new as the most part of their boiler sales)!

I'm sure it will not be long SGM but I wonder whether the design of the exchangers might alter. As for the rest of Europe and SE boilers, not any more when stocks have gone. I was surprised to hear even Germany didn't make condensers compulsory, only Holland and the UK I 'm told.

 

[D_Hailsham]

As to the last post that's not quite how to look at it in my opinion, you can restrict the maximum output to heating but that's not the current issue, it's lengthening the output envelope to reduce the minimum setting to match the property requirement more of the time.

I agree that boilers need to modulate much lower but making the maximum output match the maximum required is also a good idea. There are plenty of cases on this forum where a house has never got up to temperature because the boiler is trying to output 30kW into 10Kw of rads. Result: boiler overheats very quickly so it cycles and the flow temperature never reaches the desired level. Solution? Range rate the boiler to 10/12kW. This only happens with boilers which always start at max output and then modulate down.

 

[MFPA]

4 to 1 would seem to provide 30/18

It's a case of the amateur not knowing how to read the information. I guess the Intergas HRE 28/24 spec I just looked at is saying something close to 4 to 1 with its
Domestic hot water (DHW) Nom. power rating (lower value) 7.2 - 28.0 kW
Central Heating (CH) Nom. input rating (lower value) 7.2 - 23.7 kW

15kW is a lot for a flat - unless it's huge and badly insulated. I don't even need that for a 4 bed detached house.

Badly insulated ground floor with one flat above, 3 bedrooms (but should really be two bedrooms and a better-sized kitchen), not huge but irregular shaped and long/narrow. Only about a third of the flat shares a party wall with the one next door. Purpose built as flats in 1980s.

The unexpected result may be due to the calculator, which isn't always very clear. Try using Baxi Whole House Boiler Size Calculator.

Interesting. That calculator gives me 7.5kW if I pick "built 1983 to 2002" but 9.4kW if I pick "unfilled cavity wall".

At the end of the day, any decent combi will give you 30kw output for hot water and range down below 5 or 6kw for heating as said some as low as three, you can manually input 15kw as the maximum heating load, and the boiler will run at that when required, as the system heats it will simply gradually slow down the gas rate to suit until it needs to stop.

I've not seen a boiler that presented such an option to the user. I don't think my current 12-year-old Baxi Instant 105e has it. Is it something the installer would have to set?

As for the property, my virtually new build at only 110 years old has solid 400mm (ish) walls all round, the heat loss in it is calculated like every other building, the only difference is the heat up and cool down times of the building fabric being longer but not higher than calculated.

With my heating usually on for less than an hour on Winter mornings and for an hour or two if needed in the afternoon/evening, I suspect the heat capacity of the walls only comes into play on the coldest few days of the year when heating is needed for most of the day.

 

[Dan Robinson]

You'd be surprised at what can be changed on a boiler's software by someone in the know.

Intergas and Vaillant boilers especially.

Vokera though have the best ratios and are surprisingly flexible as VC and I both know.

 

[vulcancontinental]

You'll be getting a condensing boiler so oversizing rads is not an issue as the boilers are more efficient the cooler they run, just remember if you're too warm turn the boiler heating flow selector down not the radiator thermostats, they're for local comfort only (acknowledging evohome TRV heads which may well be an exception (and I see Tado are looking for people to test theirs now in pre-production)).