Air to water with Rads?

ELBAOS

Hi Guys
Just looking for any feedback from people who have air to water with rads? how is it? I know its more efficient with UFH but just want feedback from it with rads.
Thanks

zulutango

I don't have this set-up, but my understanding is that the temperature of the water generated by air to water heat pumps is low (something like 45 or 50 degrees) so it's really only suitable for use with underfloor heating systems. Typically for radiators you need about 80 degrees. In order for the radiators to work with the lower water temperature you'd have to get very big ones, which isn't very practical.

Metric Tensor

An Air to Water unit as you said would be very inefficient at the temperatures required for standard rads - it would essentially be like using an immersion to heat the water you then pump through the rads. If you want to go with standard rads then, in my opinion, you need to go for a different heat source.

There is another option though - there are specific "low surface temperature" radiators. I have never seen it done but I'd imagine it might be possible to pair them with an AtW Heat Pump. Of course you'll need much bigger rads.

Calahonda52

As above: in reality the UFH slab is just one big horizontal rad, or if it is well zoned, then a series of large horizontal rads.

The science is simple: rate of heat transfer is a function of both
the temperature differential between the emitter surface (rad) and the medium being heated(air) via conduction, convention and radiation.
and
the surface area of the emitter.

So if you want an emitter at 50 to heat a room from say 16 to 21 anytime soon you need a much bigger surface area than for an 80 degree emitter.

A similar issue arises when old style, non condensing boilers are replaced with super efficient self condensing boilers, the rads are sometimes found to be too small. when the boiler is run to condense.

KCross

Calahonda52 wrote: »

A similar issue arises when old style, non condensing boilers are replaced with super efficient self condensing boilers, the rads are sometimes found to be too small. when the boiler is run to condense.

@Calahonda52, can you explain that to me. I have a general understanding of what a condensing boiler does (i.e. extract heat from the exhaust gases) and thus it is more efficient.

What I dont understand is how does that affect the rads that were already in place? Isnt the boiler still sending the same temp water to them just using less fuel to do so?

zulutango

Condensing boilers have lower flow and return temperatures than standard non-condensing boilers I think. I don't quite know why though.

ELBAOS

Ok thanks guys, yes know it wouldn't be as efficient as when used with UFH, had went with oil and solar, now I am told that where house is solar is fine but wouldn't be highly efficient so was looking at other options, was told air to water is doable still considering I have plumbed now for rads but that system wont be so efficient either so might just stick to the oil and solar, or may look at PV.

Metric Tensor

If it was me I'd stick with oil and solar thermal. Different people will have different opinions on that though!

Calahonda52

KCross wrote: »

@Calahonda52, can you explain that to me. I have a general understanding of what a condensing boiler does (i.e. extract heat from the exhaust gases) and thus it is more efficient.

What I dont understand is how does that affect the rads that were already in place? Isnt the boiler still sending the same temp water to them just using less fuel to do so?

Kc,
AFAIR, the return has to be 45c or less for it to condense and thus give up the latent heat in the flue gas, so "typical" self condensing boilers, to condense, send the hot out at about 65 or maybe 70.

Older boilers might have output at 80, which will make a big difference, as you probably know from the rad sizing tables.

There is another thread that alludes to it, let me find it and link it.

http://www.boards.ie/vbulletin/showthread.php?t=2057683791 second post

This is the general idea so don't put the above in your Phd thesis.

I have this issue in my own house,with a replacement Viesmann 100 gas boiler, the normal not cold weather setting of 4 is grand and it condenses at that, the flow is 70 or so however once it gets cold, I need to go to 6, which is the max or even beyond to get max temp, I get to 85 but its not condensing.
Hope this helps.

ps
see here
http://www.castironradiators.ltd.uk/blog/2014/09/10/delta-t-and-heat-output-explained/

murphaph

Metric Tensor wrote: »

If it was me I'd stick with oil and solar thermal. Different people will have different opinions on that though!

I agree and we have A2W with UFH throughout. Originally we'd planned low temp rads in the basement but when I was informed just how large they would need to be to reach 20°C I had a quick change of heart and went for UFH there too. I was also concerned about rust in the system, which IIRC is worse in low temp systems than high temp ones.

zulutango

Seems like madness to put in an oil boiler in this day and age, I think. Whatever you do, go with very high spec insulation and windows and doors, and make the house as air tight as possible.

Metric Tensor

zulutango wrote: »

Seems like madness to put in an oil boiler in this day and age, I think.

I have to say I disagree on this one. There is definitely a place for condensing oil boilers. Particularly for houses with radiator systems.

A lot depends on how you plan to live your life too - bear in mind that the heating profile and design behaviour of a house with UFH is very different to a house with rads.

Some of the cost benefit analyses that have been carried out by A2W or ground source heat pump suppliers use "interesting" future costs for oil. It depends on what your crystal ball says about future oil prices.

ELBAOS

Yes have it very well insulated,triple glazed,mhrv, 2 stoves. wanted to keep it simple with the oil it's what I'm used too!!☺ thanks for replys.

zulutango

ELBAOS wrote:

Yes have it very well insulated,triple glazed,mhrv, 2 stoves. wanted to keep it simple with the oil it's what I'm used too!!☺ thanks for replys.

Cool. With all that you won't use much energy anyway. Well done! Do you know the BER of the house?

ELBAOS

Yes I have been told with all that I wouldn't need a big/expensive heating system, but obviously want it to be efficent. won't have ber result for another while until alot more done but I'd be happy with A3

Water John

If you want to use A2W and rads, use aluminium rads.
Quite easy also to mix down, to 40C, oil burner water for UFH.

With good insulation and sealed envelope as detailed above, your heating bill, what ever source, will be quite low.