Combi boiler with underfloor heating?

Question

My existing convential boiler and rad system needs replacing and I want to do it this Summer while we can cope with heating down time in the house.

House is 3 bed semi. 1 bath, 1 shower, 11 rads. Has a floor area of

100 sq.m and has an existing hot water flow rate of 14 l/min.

I want to go the combi route and also want to replace the rad system with underfloor heating (UFH). I have made initial enquiries about boilers and the range of prices is mind boggling.

My main questions are:

1) Will UFH work OK with combi boilers? I have noticed even some of the cheaper boilers have functionality to be able to turn the central heating water temp down to 40C. Would I still need a mixing valve in this case?

2) I was thinking of going for a 30KW combi but initial quotes I have got (just for boiler) range from =A3700 - =A32000. If I paid more for the higher end boiler I realise that I would be getting a higher quality, more efficient boiler with a higher flow rate, but I would be keen to find out from others about how their combi boilers perform, particularly anyone out there who is using theirs with Underfloor heating.

Thanks in advance.

The Natural Philosopher · Accepted Answer

Yes, realistically because you want hot water for bathing.The peak heat requirements are for baths and showers, not CH.I heat a huge area on a 12KW boiler.

John Rumm · Answer

No, because on a combi the CH and HW usually have independant temperature control.

David Hansen · Answer

On 9 Apr 2006 01:17:04 -0700 someone who may be "jks" wrote this:-

Why?

If you want mains pressure hot water what is wrong with a thermal store?

On the ground floor, or both floors?

Yes, it is just a boiler with added multi-point heater.

Possibly, possibly not. Depends on the system and what you want to get out of it.

jks · Answer

Hi David,Excuse my ignorance but by a thermal store do you mean a mains pressurecylinder?  We have not got a great deal of room in the house so mypreference would be to not have a cylinder.I want to do the UFH on both floors (both are suspended timber floors).Ideally I want a combi boiler that will give me at least 12l/min on DHWand is able to heat my house using UFH with the minimum of extraplumbing and valves etc.....   Perhaps this ideal is not achievable???Stuart

David Hansen · Answer

On 10 Apr 2006 01:24:55 -0700 someone who may be "jks" wrote this:-

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and many other links about thermal stores. You might also like to look at

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in particular the Conus 502 which is optimised for underfloor heating (and solar input).

They can be located in the loft, if space really is tight. However, they are just a suggestion of something to consider. Too many people rush into combination boilers and then repent at their leisure. You may be prepared to live with the less than attractive hot water performance of a combination boiler when running your bath, shower and a basin.

You will probably find that heating is reduced to zero when the combination boiler is heating domestic hot water, which it could be doing for some time while filling a bath. Underfloor heating is an advantage in these circumstances due to its inertia, assuming a suitable thermal mass which may be difficult to fit to the first floor.

I take it the house is properly insulated, which should always be the first step.

Combi boiler with underfloor heating?

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