Anybody got any experience of Air Water heat exchangers
I have a man coming around to give us a quote to take out our oil fired boiler
and replace it.
Good Idea ?
Cheap to run ?
Give out enough heat?
What about fitting PV cells on the roof to power the pump, does that make it more viable ?

Are you meaning heat pumps?

Air-water meaning which way around (there's a few different possibilities - air outside water inside, water outside air inside etc etc.

Heat pumps are very effective but as far as I'm aware you can't use them like normal heating - you get best results with low temperature differences with heat pumps (highest coefficient of performance).

Be wary, they rarely get their quoted COP.

PV cells are very pricey, what power does the pump take? IF you don't have a battery bank and inverter setup you won't be able to power your pump during the evening/night/morning without mains use. Most folk turn their heating down during the day when it's bright.

[Edited on 12/9/12 by coyoteboy]

Unfortunately most people who sell them quote massively ambitious figures regarding heat output and COP they usually quote figures at an ambient outside temp of 7 degrees at a water temp of 35

For them to work effectively in the uk they have to operate at ambient temps of minus 4 or lower with water temps of 50 degrees if using radiators so their actual efficiency is much less, they usually depend upon internal electric elements to do the heating at low external temps because the heat pumps can't give out enough heat.

They are a heating appliance which looses capacity the colder it gets outside.

You would be much much better going for a Biomass boiler.

The heat pumps do work if designed correctly but operate at much poorer efficiencies than advertised

It would be Air outside to water inside
then heating my existing radiator set up
taking the place of combi oil fired boiler
There are large grants £850 to install
then £850 per year for 11 years to use
Makes it all sound very atractive !!

From the small amount I've read the Renewable Heat Initiative grants/payments make what would not be worth the money worthwhile again through subsidy. It's more likely to be worthwhile as you're not on the gas grid - oil is expensive.

Interested to know which way you go as I have 3kW solar PV and thought that if I can get the RHI payments an ASHP could be a viable replacement for my old boiler. Not ready to leap just yet though.

Lurch. I did an enormous amount of work on ASHP about 6 years ago.

My response is going to be long...

My grandmother had had one fitted about 22 years before, it had worn its self out so I specced up and installed a new one.

The unit I put in is a 12Kw ASHP (requires 2.5Kw of electricity). This just about warms the house via 17 conventional CH radiators. Its a standard Y plan with pumped CH and HW.

The system is not ideal in any way shape or form but as the origional system 22 years ago was designed like that... My grandmother being set in her ways would have nothing different...Consequently the house is never warm!

I have actually had to add in an electric 9Kw heater in series with the ASHP to supplement the heat during the coldest time of the year.


Ok so the ASHP provides all the hot water during the summer and contributes to the heating during the mild parts of the year. In the winter however it provides very little. With the 9Kw boost heater running we can just about heat the house above 18c. (the actual heatloss of the house is around 22Kw via 17 rads!)

On an average mild day the heatpump alone will (eventually) heat the water in the loop up to around 50c. (with a temperature differential across the pump of 8c)

So from the above, yes, you can use an ASHP in replacement of a gas boiler...but its not a good idea atall.

The system I wanted her to have installed was a bit more complex.

It involved a 1000 litre thermal store, solar panels, ASHP and a gas boiler (or immersion type boost).

Solar would heat the very bottom of the thermal store, the ASHP (or ground source) would heat the middle of the tank and the gas boiler could be used to top up the heat if ever needed. From that thermal store you can draw off water via heat exchangers for underfloor heating (35c) or space heating (50c). Plus have direct hot water on tap via an additional heat exchanger.

Ok, so that little lot is not cheap but over about 20 years it would have paid for its self....Perhaps Ok on a new build but on a retrofit, not so ideal.


For your situation..... Dont even think about PV to power the ASHP... without enormous battery banks you would never be able to run the compressors.

I would be looking to fit the largest solar hot water array you can (ideally in two loops, one high gain and one low gain). Feed that hot water into a thermal store. If you size it correctly you would get 90% of your hot water from that. You would need something else however to top up the heat when the sun fails to shine.... If you have the ground (or can put in a borehole) ground source is more consistant than air source... Or fit a biomass boiler, or if your oil boiler still works, keep it to just "top up" the heat if the renewable solutions cant quite cope during the coldest spells.

Either way on a DIY basis for thermal store, solar, piping etc you would be looking at around £3000+.... Plus maybe another £2-3K for a decent heat pump be it ground or air source.

Back when I installed the ASHP there were no grants for air source, only ground... and even with the grants you need an installer signed up to the scheme which was going to cost more than I would ever make back from the grants....


If you have any questions regarding my above ramblings... drop me a U2U...

They talked about these on Rip off Britain program, UK houses are not insulated enough and the electric backup heater fires up a lot costing you a fortune.
Gas boiler is still more controlable and cheaper IMHO

quote:

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Gas boiler is still more controlable and cheaper IMHO

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In the short term I'm probably going to do nothing with mine (I don't happen to live there anyway at the moment!)

When I return I may build / buy and then install one of these proportional controllers than uses the existing 3kW immersion in the storage tank to mop up all *excess* solar PV generated (i.e. it won't ever use more than what is spare). Savings aren't massive at the moment (probably <£100 per year as gas is still quite cheap) but you don't lose anything by doing it at the moment as the export amount isn't measured just estimated at 50% so you might as well do something with it

Tempted perhaps to upscale this to either a second storage tank / thermal store and perhaps try and get it to work at the other house that has a combi but would require quite a few valves to pre-heat incoming water etc.

I haven't done the maths yet around the RHI to see if it could be profitable for the aggro but I take your point that an ASHP as the sole replacement for a conventional boiler is probably a bad idea! And trying to power it off solar PV alone is an even worse idea. I keep forgetting that the power needed to run the compressor is massive so unless you've got a big PV array at max power it won't be enough. Whereas dumping excess power to a thermal store or electric under floor heating or whatever (as a simple resistive load) is easy.

Using electricity to heat water is Laughably inefficient. Even If gas cost more than electricity, it would still be cheaper.
Some bespoke Boilers are Very Efficient and get every last bit of value out of the heating flame.
But like anything of quality worth owning ; Not free.

I suspect all the technology and gizmo's needed will drop massively in price once more people buy them. ie they become viable. Just look at what happened to the price of condensing boilers, halved overnight when they had to be fitted. I expect the same to happen to heat pumps and so on in the next 10-20 years. Meanwhile, buy a jumper. Insulate everything.

wow
fantastic responce from the forum again !!
You guys never cease to amaze me

Thank You

Lurch