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

[MFPA]

To get a decent hot water flow rate for running a bath I need a boiler rated at about 30kWh, but for central heating my flat only needs about 15kWh (according to the calculator at <http://www.idhee.org.uk/calculator.html>).

Is it possible to get a combi boiler with massively different power outputs for the two functions? I see some Intergas combis with ratings such as 36/30, 28/24, or 24/18 but nothing remotely like 30/18.

 

[bernardgreen]

http://www.johnsonandstarley.co.uk/domestic-boilers/combi-he30c.asp

Designed to provide 30kW of heat to the central heating system 38kW of heat to the hot water circuit.

Fully modulating burner.

• Offering a turn down ratio of 4:1.

 

[vulcancontinental]

28, 32, 36, 38 KW Vokera combi's offers turndown ration of 10:1

 

[Dan Robinson]

http://www.johnsonandstarley.co.uk/domestic-boilers/combi-he30c.asp

Designed to provide 30kW of heat to the central heating system 38kW of heat to the hot water circuit.

Fully modulating burner.

• Offering a turn down ratio of 4:1.

4:1 is pants. As are J&S. But you know that.

Vokera are the only ones to really pay any attention to this problem.

 

[bernardgreen]

I see some Intergas combis with ratings such as 36/30, 28/24, or 24/18 but nothing remotely like 30/18.

4 to 1 would seem to provide 30/18

4:1 is pants. As are J&S. But you know that.

My J&S boiler seems to work as expected. Very compact, practically silent and seemed to impress the plumber who installed it.
But mine is a heat only so far less complicated than a combi.

 

[Dan Robinson]

It is about how low it will go. If you have a calculated Heatloss of 15kw. How many days /hours a year do you think you need that?

 

[bernardgreen]

Cottage with stone walls and a thatched roof. Considerable thermal mass in the walls and very good insulation in the roof.

Very different from a modern house.

 

[Dan Robinson]

Irrelevant.

 

[bernardgreen]

Irrelevant.

Wrong

If heating a building with internally exposed stone walls from cold to comfortable then an initial high heat input is needed until the faces of the stone work have warmed up and are no longer cooling the air. Then a much lower level of heat input is required to maintain the temperature.

You seem to be suggesting that the structure and thermal qualities of a building need not be taken into account when designing ( thats designing.. not installing ) it's heating system.

 

[Dan Robinson]

No. I'm suggesting that the heat required to keep a house comfortable is much lower than the calculated amount..

I know full well how to Heatloss. I do it for a bloody living.

My dad's 450 year old farmhouse has been perfectly warm since I fitted the boiler 11 years ago. Or are you suggesting that's a modern building?

 

[bernardgreen]

My dad's 450 year old farmhouse has been perfectly warm since I fitted the boiler 11 years ago. Or are you suggesting that's a modern building?

It is 50 years younger than mine,

 

[vulcancontinental]

The loss is the loss. You may have 300 mm of insulation below the thatch so how relevant is that? It may be in a sheltered hollow or atop an exposed ridge.

I think what Dan referes to is the maximum loss, coldest expected temp in the area with a cold property and is a reletively rare occurance, generally around 14 days a year at minimum temps although you need the heating for many months more then once the property, any property, is up to temp the loss is only to be combated and is drastically reduced because you'r not heating the structure.

Cottage aside a three bed semi might need 10-12 max but when warn 2-3KW to maintain. Houses are better insulated so a combi sized for hot water flow becomes drastically oversized for the heating requirement which is far longer duration. A 36KW combi modulating 4 to 1 would be 3 times oversized heating a warm house and that leads to cycling and rejected energy.

Not everyone lives in an old cottage, you are the exception not the rule and the 4-1 on your boiler may be fine for you but lack a little in flexibility for the majority of homeowners.

 

[Dan Robinson]

It's obvious that being an alternative thinker is very limiting

 

[vulcancontinental]

Mr Green doesn't think alternatively but alternates thinking.

Back to the original post, most online calculators are oversized anyway.

 

[ScottishGasMan]

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.

Thats pretty much the operation of any modern boiler, its only very old units that have on/off burners in domestic situations and they wernt generally combis so were set to whatever heat loss was required.

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.