Combi Boiler Sizing

[JohnBoyII]

Can someone remind me what the rule is for estimating the size of a combi boiler?

I seem to remember a rule of thumb: total up all the rad ratings and add a bit for the hot water, but I can't remember how much to add for the hot water.

 

[swbjackson]

Combis are rated for hot water output and modulate the heating output to a maximum (24Kw for most). You need to ensure that the heating output from the combi is larger than the house requirement and then look at what sort of flow rate you want.

 

[JohnBoyII]

When I last sized a boiler about 7 years ago I remember a post on the Forum that said you had to allow a certain Kw for the DHW. Its not exact but a rough figure, thats all I need to know.

I tried Google but most of calculators now talk about a "whole house" figure where you need to enter things like floor space, walls etc.

 

[Dan Robinson]

Its a combi... why make an allowance?

Are you sure you should be doing this?

 

[JohnBoyII]

Many thanks for the helpful responses, but I thought I'd asked a simple question? I'm sure it can't be a trade secret.

Does anyone remember what the rule of thumb was? If its all done different these days, whats the new rule.

 

[landyandy]

Do a bit of research on this. There are plenty of threads on here re. boiler sizing.
I suggest you are thinking about this the wrong way round. The question you are asking seems to be for a boiler to serve rads and to add a bit for a cylinder which you don't need to do.
A good start would be to look at the hot water demand first as this will usually be the highest Kw demand from your combi.

Andy.

 

[JohnD]

The rule of thumb you mention applies to boilers which heat stored water in advance of use (mostly HW cylinders) and is still valid for such systems.

However you specifically said you were choosing a combi, which must heat all HW immediatly on demand (though a few may incorporate a small storage buffer)

This requires far greater HW power, and it would be abnormal for the CH demand to be anywhere near the HW demand on a combi.

 

[mysteryman]

A combi heats either the water or the radiators, never both at once. Usually the output required for a satisfactory water flowrate is more than the heating load, so that's all you need to match. This also applies to modern water-priority system boilers, so the old days of thinking of a number, adding twenty percent, plus three kilowatts for hot water, are gone. Basically, a modern system should be sized to suit the central heating load or the hot water load, whichever is the greater, not the sum of both. This is almost always the hot water load for a combi.

I do not know why others on here chose not to answer your question!

 

[JohnBoyII]

Aaa, thanks for setting me straight on that. I didn't realise it applied only to cylinder type systems.

The system has 17 rads which total up to 62000 BTU (18KW). The current boiler is a Baxi 133HE Plus with a second pump fitted to get it to output 100000 BTU. We mostly have showers and there are three thermostatic showers, although only one ever gets used at a time.

Will the next boiler still need to be that big or can I get away with a smaller one?

 

[JohnD]

If as you say, you are changing to a Combi, then it will need to be more powerful. 30kW or more is usual, dependent on available water flow.

Measur how many litres per minute your incoming water main supplies, by running the cold tap into a bucket and timing it. When you change to a combi, this is the total amount of water you will be able to draw at any time, regardless of the number of taps or showers used, and regardless of whether they are hot or cold.

 

[mysteryman]

Also, please tell us how adding a second pump increases the boiler output by nearly fifty percent?

 

[JohnBoyII]

Thanks for that.

Not sure how the second pump increases the boiler output. The installation instructions said that the boiler was factory readied for 80000BTU, but to raise it to 100000BTU you had to fit a second pump. I remember the fitter being a bit confused by it too, eventually he spoke with the Baxi people and said it was OK but never told me the exact reason. Its mentioned a number of times in the installation manual, there might still be a copy available in Google.

 

[JohnD]

100k BTU is near enough 30kW, if you buy a new combi it will have an output of round about that, but it will modulate itself down so it does not run at full power much of the time unless you are running a bath or shower.

This does give you the opportunity, if you wish, to have oversized radiators so the house will warm up faster from cold. I did, because I used to be away a lot on business.

 

[JohnBoyII]

Yes I was told at the time to just fit the biggest one, especially if you want a good shower.

The biggest ones I can find are the Baxi Platinum Combi HE, the Worcester Bosch 42CDi and the Vaillant EcoTEC Plus 837. All are rated around 40KW but the max hot water flowrate (around 16 l/min) is almost the same as mine which is supposed to be only around 30KW. Is that extra 10KW just for the CH then?

 

[JohnD]

have you measured your water flow yet? You need to, it's important.