Air Source Heat Pumps

stevehead

UK field trials of 100 Heat Pump installations, both ground and air, are currently underway by the Energy Saving Trust. These trials are due to run Oct 2008 to Oct 2009, and the results will be published at the end of the year.
Ref: http://www.eeph.org.uk/uploads/documents/partnership/JB_Renewable%20Heat_v2.pdf

Condensing boilers are expected to show an actual efficiency of only 80% according to an ongoing EST field trial
Ref: http://www.micropower.co.uk/conference/presentations/Workshop-3-Tariffs-%20David-Matthews.ppt

Andy_WSM

stevehead wrote: »

UK field trials of 100 Heat Pump installations, both ground and air, are currently underway by the Energy Saving Trust. These trials are due to run Oct 2008 to Oct 2009, and the results will be published at the end of the year.
Ref: http://www.eeph.org.uk/uploads/documents/partnership/JB_Renewable%20Heat_v2.pdf

Are they trying to put people off these technologies? Idiots.

Installed cost £7000-£10,000 for an ASHP? What planet are they on?

- And the fact that you theoretically need planning permission is simply laughable.

mech_2

stevehead wrote: »

Nice, well put post Mech.

Regarding the stated & seasonal COP.
This comes down to 2 things

It comes down to a whole raft of things. No static test is going to represent a real-world figure. Things that affect seasonal performance might include (but not be limited to) cycling losses, ice formation, defrost cycles, humidity, part-load performance, siting, off-cycle parasitic loads, aging of the electronics & fans, dirt buildup on coils (inside and outside).

Firstly location. I'm sure the Germans, Swedes and Scots struggle to obtain an impressively high COP given the cold winter climate. I can't help where I live - it's in the South West which is very very mild. So mild, that it approaches the conditions under which the manufacturers COP is stated. (5.45 @ 7C ambient)

The temperature varies day-to-day just like it does anywhere else. Not sure what Swedes have got to do with it, but I think you might be surprised how little difference the Scottish climate is to Plymouth. It's certainly less humid in Edinburgh (less ice buildup?).

Here's the temp chart for Plymouth. Note that the average 'low' will be recorded just before dawn when my ASHP's aren't in use.

They are month averages. They tell you very little about seasonal heat pump performance because COP is non-linear and the most energy will be used on the days of worst COP.

Which brings me to the 2nd and unreconcilable thing - do you accept Mitsubishi's figures or not?

I am sure they can make their heat pumps produce impressive COPs under test conditions. So I can accept that they found a way of justifying those numbers for their marketing material. Establishing real-world performance needs a formal case study. In the absence of test results it would be most sensible to limit the likely performance to the range achieved by units that have been tested. If there was something extraordinary about your specific model of heat pump, I think it would have been noticed by now.

Would it be fair to accept their claim that the Ecodan COP is 3.7 @ 7C, and 3.1 at 2C? If so, then why reject the same company's claim for a COP of 5.45 for a different unit? Or at least 'seasonally adjust' them both by the same factor.

I wouldn't take the figures in any marketing material at face value. There is no evidence to suggest that a seasonal adjustment would be equally valid for both sets of figures. There's just not enough information here.

Is it not the case that an air-to-water ASHP will always have a lower COP than an air-to-air ASHP because it has to heat water to 65C? Air heaters surely have a lower compressor temp?

Maybe. But I have no idea and neither do you. Underfloor heating can run at 35-40 degrees. I have no idea what temperature is necessary for the condenser in an air-to-air unit, but it can't be hugely different. Think how much smaller the heat exchanger is for a start.

As for DHW, I use an instant gas heater. It's rugged, old and non condensing. My understanding is that modern condenser boilers don't have the incoming cold water supply passing through the condensing unit, only the closed loop heating circuit. This means that DHW is always obtained in non condensing mode. There's nothing to be gained by me using a SEDBUK A boiler at all.

You are quick to criticise condensing boilers while clearly having little understanding of them and yet will use wildly optimistic figures when extolling the virtues of heat pumps and still call it 'on the low side'. This does not make for a balanced debate. Condensing boilers are nothing mysterious. They're functionally identical to a non-condensing fan-flued boiler except they run at a low temperature and drain the resulting flu condensate. I fail to see why you think there would be no condensation in DHW mode at a similar primary heat exchanger temperature as for space heating. In my case it's probably the other way around if anything as I have my combi set to produce hot water at ~50 degrees and the flow temperature for the heating set to ~60 degrees (the radiators aren't oversized, they're from the 1980s, the boiler is a retrofit).

richardc1983

Air off temps on an air to air system range between 35-50C depending on load conditions etc.

Cardew

Mech is taking up the cudgel that I held at the beginning of this debate.

There is no doubt that ASHPs are very good, but like so many subjects on MSE, the enthusiasm of those who have taken the plunge and fitted the systems, seems to overcome objectivity.

For a start, a lot of people will not find Air to Air systems suitable. They want to keep bedroom and bathroom doors closed, and the cost of heating water will be very expensive.
I know Steve has a gas boiler to heat his water, but not many people will fit an ASHP if they already have gas CH.

So I suggest the majority of people will want underfloor heating or large radiators to cope with the low water temperatures.

Is it not the case that an air-to-water ASHP will always have a lower COP than an air-to-air ASHP because it has to heat water to 65C?

The systems I have seen generally do not attempt to heat water to 65C. 40C to 50C is the generally accepted figure and they boost the tank temperature(for safety) with an immersion.

The couple of systems I have seen that procuce water at 60C are very careful not to quote a COP at that temperature.

One of the penalties of CH with very low water temperatures is that you need the heating on for far longer periods to reach an acceptable room temperature. If you occupy the house all day, this isn't so much of a problem.

With my gas CH the water temperature is 82C. I can set the heating to come on a short while before I return to the house(or get up in the morning) and the room is quickly up to heat. With a water ASHP the heating has to be on far longer to raise the room temperature.

So if we are comparing costs of gas CH with an ASHP how do you quantify the wasted heat from an ASHP? i.e. heating a room when it is unoccupied or the occupants are in bed and don't require heat until just before they get up.

As for installation costs, is £6,000 too high for an Ecodan system? I thought that was about average.

arty68

Andy_WSM wrote: »

Are they trying to put people off these technologies? Idiots.

Installed cost £7000-£10,000 for an ASHP? What planet are they on?

- And the fact that you theoretically need planning permission is simply laughable.

I've been getting quotes for ASHP lately and I can tell you most of them come in within that range if not above.

Out of interest what is the cheapest quote for decent ASHP around 9KW(ebay and trianco not included) including the water tank ?

Andy_WSM what would you think was a good quote?

The cheapest on I got was about £5000(or £4100 if you include the grant) for the ecodan and supply only tank.

I'm not quite sure about the pp being laughable my council are insisting on a whole new application if I want to add an ASHP on my new build which i'm not finding particularly amusing. Especially as my nearest neighbour is about 300mtrs away so noise would not be an issue!

stevehead

mech wrote: »

You are quick to criticise condensing boilers while clearly having little understanding of them and yet will use wildly optimistic figures when extolling the virtues of heat pumps and still call it 'on the low side'. This does not make for a balanced debate. Condensing boilers are nothing mysterious. They're functionally identical to a non-condensing fan-flued boiler except they run at a low temperature and drain the resulting flu condensate. I fail to see why you think there would be no condensation in DHW mode at a similar primary heat exchanger temperature as for space heating. In my case it's probably the other way around if anything as I have my combi set to produce hot water at ~50 degrees and the flow temperature for the heating set to ~60 degrees (the radiators aren't oversized, they're from the 1980s, the boiler is a retrofit).

Actually your beloved combi will only have a DHW efficiency of 75% - the average for current combi's. It's only the primary heating circuit that passes thru the additional condensing HE. You do know that the UK SEDBUK ratings only apply to the boilers’ space heating performance and completely ignores hot water efficiency.
Next year that'll change due to new regs, but for now your water heater could be terrible and you'd never know.

stevehead

richardc1983 wrote: »

Air off temps on an air to air system range between 35-50C depending on load conditions etc.

Today,
Incoming Air temp = 23C
Outcoming Air temp = 38.2C
Delta T = 15.2C

Actually, if I use the stated flow rate of 240m3/h (on LOW) and my wattmeter I could probably work out the COP?

stevehead

stevehead wrote: »

Today,
Incoming Air temp = 23C
Outcoming Air temp = 38.2C
Delta T = 15.2C

Actually, if I use the stated flow rate of 240m3/h (on LOW) and my wattmeter I could probably work out the COP?

In reply to myself, I can work out the COP like this:

First, calculate the kW Sensible Heat Gain with the formula
H = (v x d) x (Cp x (ts-tr))
Definitions
v = Volume Flow Rate (m3/s)
d = Density of Air (Kg/m3)
H = Sensible Heat Gain (kW)
Cp = Specific Heat Capacity of Air (KJ/Kg K)
ts = Supply Temperature (C)
tr = Room Temperature (C)

Current Conditions:
Current Outdoor Temp = 14.5C
Temp Indoor Unit Air Input = 25.0C
Temp Indoor Unit Air Output = 41.3C
Unit Power Consumption = 445W (0.445kW)

H = (0.11667 X 1.2) X (1.012 X (41.3 - 25)
H = 2.31kW

Then divide this by the Power Consumption of the unit to get the COP
2.31 / .445kW = COP of 5.19

So it doesn't make the 5.45 COP claimed*, even when the outdoor temp is well above the test temp of 7C.**

I am bothered by my measuring methods, and have ordered a thermometer to better capture the off-compressor temperatures.

Sources:
Specific Heat Capacity of Air = 1.012 J/(g.K)
(194m above sea level / 23C / 40.85% RH) - Wikipedia typical room conditions
Density of Air = 1.2Kg/m3 @ 23C - engineeringtoolbox.com
Air Volume Flow Rate at Lo setting = 420m3/h (0.11667 m3/s) - Mitsubishi spec for unit

*Eurovent Certified
** EN14511 Outdoor Test Temp

stevehead

I've just booked in a service for my 2 units. This will be done tomorrow and will set me back 60 quid.