Combi Boiler Size Advice

mgns

Hi,

After getting some boiler quotes, I have a choice between a Vaillant Combi 831 or 837. The house is a 3 bed semi with 10 rads/ 1 bath + 2 toilets and mains supply at approx 16.2 l/m and 3.5 bar pressure.

I was wondering if the 837 would cost any more/lots more to run than the 831? - as long as the boiler heating output was appropriately down rated.

I've been advised that the 837 may be the better option as I'm also looking into a loft conversion + en suite shower but I'm not sure how soon a loft conversion might happen - so I'm unclear about the running costs in the meantime.

After doing a few online heating calculations, I think the current house heating is inbetween the 12-28kw range of the 837 but that may fall if we fully insulate the loft.

Thanks for your help.

macman

With a combi you need to choose the rating by the required flow rate, not the output. So your combi will probably be over-rated in term of it's ability to heat the property.

gas4you

Apart from always sizing a combi for hot water usage, you also need to consider the physical size. The 837 stands an extra 75mm off the wall and also don't forget the gas pipe size will more than likely have to be bigger for the 837.

mgns

Cheers for the advice. My main concern is whether the running costs of the 837 will be the same or a lot higher than the 831 whilst the house layout/general usuage remains unchanged?

Plus, would the existing heating requirements cause the 837 to function incorrectly?

macman

A higher rated boiler shouldn't cost any more to run. It'll simply heat the house to the required temp more quickly, within the constraints of the rest of the system. The amount of energy need to raise the temp from A to B is the same, so the cost should be too.

gas4you

If, eg, your home needs 15Kw of heat for the radiators, then the 831 and the 837 will be range rated to 15Kw, so both will only operate at that maximum.

The 837 will use more gas on initial firing up for HW use though, but will then modulate down to match the flow rate and temperature set.

mgns

This is my first experience of dealing with a combi so thanks again to both of you for answering my questions.

The 837 will use more gas on initial firing up for HW use though, but will then modulate down to match the flow rate and temperature set.

Would this add up to be a big waste of gas & money compared to the 831? If I'm future proofing against layout changes, I guess I don't have a choice.

gas4you

In reality I don't think you would notice any difference, and as you say you are future proofing as well.

But you must also bear in mind that if you run a single cold tap and get 16 l/m, you wont get that at each tap when running 2 taps, be it 2 colds or a hot and cold.

mgns

But you must also bear in mind that if you run a single cold tap and get 16 l/m, you wont get that at each tap when running 2 taps, be it 2 colds or a hot and cold.

Is this the general warning about combis i.e. 16 l/m would be split between two taps or are you saying there would be a drop off and that two outlets would add up to less than the 16 l/m incoming mains?

gas4you

General warning about flow rates in general.