UPDATED: Air Source Heat Pumps/Air Con - Full Info & Guide, is it cheaper to run than mains gas?

Silent_Dancer

@teecee90 you want an air to air heat pump. Worcester do one and you can find installers on their website.http://www.worcester-bosch.co.uk/mobile/heat-pumps/air-to-air-heat-pump

dearlouise

How much do these cost to run, per hour?

Robwiz

dearlouise wrote: »

How much do these cost to run, per hour?

It depends on the room temperature you set, the outside air temperature as well as the output rating of the unit and your electricity tariff. When we run our 2 kW unit on low fan setting with outside temperature of 10º with the unit set at 20º it averages around 0.4 kWh. Our electricity cost is 10.99p per kWh so the hourly cost for us is 4.4p.

brianposter

Please could someone explain how the calculations to determine the power of a heat pump to heat a house work.

I am very puzzled by the fact that the power required does not seem to vary with climate - it hardly seems to matter if your minimum outside temperature is 0 degrees or 10 degrees C.

zeupater

brianposter wrote: »

Please could someone explain how the calculations to determine the power of a heat pump to heat a house work.

I am very puzzled by the fact that the power required does not seem to vary with climate - it hardly seems to matter if your minimum outside temperature is 0 degrees or 10 degrees C.

Hi

Most manufacturers will have a performance curve somewhere, usually in an 'engineering data book' or something similar. Air Source Heat-Pumps (ASHP) should be power rated at an outside temperature of 7C and are tuned to perform best at close to this level, above it you'll usually get a higher max heat output and below it less - ground sourced pumps (GSHP) are effectively unaffected by ambient temperatures, just ground temperature at the depth of the loop. As for the energy multiplication effect of the heat pump (COP), well that depends on a combination of ambient exterior temperature, thermostat set point and the current interior temperature. Inverter based heat pumps will also usually have a 'sweet spot' to achieve the best COP dependent on duty (%age of max power), however, this normally has a far smaller overall effect than the ambient temperatures.

If you're looking at a wet pumped heating system, you'll need to ensure that your heat transfer area is large enough to allow a low operating temperature as under sizing will inevitably lead to running the heating loop at a higher than necessary temperature, which significantly decreases efficiency ... you will not be able to run an existing radiator system (designed for a delta50T GCH system) at something like 35C (~delta15T) and expect that the system will perform well, so replacing radiators with much larger ones will normally be necessary - as will planning to run the system for longer periods ....

Now, how to size ... well, you'll need to consider where you are and what other heating is available to you. If you want to use a heat pump as the only source of heat then you'll need to size it to perform at the lowest likely winter temperatures, but if you have another heat source (eg log burner) you could get away with a smaller system ...

If you currently have mains GCH, or even a mains gas supply, it's probably best (and cheaper) to use gas as your main heat source ... you can install a bivalent system, but do check that the investment calculations really stack-up. Also, if you have solar pv you could possibly justify a decent spec air/air unit as a source of some heat to take the chill off during the day in spring & autumn, but when you start talking about a number of units the cost starts to ramp-up and the combined power requirements will likely be higher than the pv generation...

Sorry for the ramblings, but I've tried to address a post which doesn't really contain a lot of detail on what you're trying to achieve ...

HTH
Z

matelodave

Adding to the above, whatever heating system you have you need to be able to provide enough heat inside the house to balance the heat loss through the fabric. A heat pump, boiler or convector heater that's sized to keep you warm when it's 10 degrees outside won't be big enough when it's zero outside.

So you need to do the same heat calculations with a heat pump as you do with any other sort of heating system. If a house requires a 12kw boiler to keep it warm then it will need a 12kw heatpump.

Most ASHP efficiencies (COP) are calculated at 7 degrees outside to 35 degrees water temperature. At lower outside or higher water temperatures the COP falls - most manufacturers have COP curves or tables which show how the COP varies.

If you decrease the outside temperature the pump will have to work harder to maintain 35 degrees likewise it will have to work harder to provide temperatures above 35 degrees. (estimated at about 2.5% per degree) so running it at 45 -55 degrees will cost you 25%-50% more than running it at 35). You therefore need heat emitters or radiators that are large enough to heat your room at 35 degree flow temperatures.

Heating systems are designed to an outside temperature of about -3 to -5 degrees. The output of a heatpump will fall as the temperature drops until it reaches the equilibrium point ie. where the HP won't be able to maintain the house temperature.

Most systems have a back-up immersion heater which cuts in to boost the heating performance at very low temperatures. That's how ASHPs can work down to temperatures as low as -20 degrees by supplementing their heating with an immersion heater.

Ideally the heatpump should be sized to optimise it's performance but to minimise the use of the back-up heater. Even with the backup heater operating a heatpump should have a COP or more than 1.

brianposter

Thanks, guys.

I was puzzled by the calculations of my local installer so I tried the sizing calculator which appears in the first post of this thread. I put in a 100 sq. m. room and as the calculator is for New Zealand, I tried it in the south of the South Island (Southland), which I gather is pretty cold, and the north of the North Island (Northland), which is apparently almost tropical.

You get recommended the same capacity heat pump in both places; a result which seems hardly credible, but also does not seem to be consistent with the comments about ambient temperature.

Cardew

brianposter wrote: »

Please could someone explain how the calculations to determine the power of a heat pump to heat a house work.

I am very puzzled by the fact that the power required does not seem to vary with climate - it hardly seems to matter if your minimum outside temperature is 0 degrees or 10 degrees C.

It is the output that changes with very cold outside temperatures and/or the warmer water temperature.

Below is the specification for the Ecodan range.

https://heating.mitsubishielectric.co.uk/Products/Documents/Ecodan%20PUHZ-%28H%29W50-140VHA%282%29-YHA2%20%28FTC4%29%20-%20AUG%2014%20PI%20Sheet.pdf

Look at the mid range model PUHZ-W112VHA - nominally referred to as an 11kW heat pump. It gives three 'scenarios' with differing input power (3.65/3.35/2.51kW) for a nominal 11kW output.

The important point to note here is all three scenarios quote water output temperature of 35C and an inlet water temp of 30C. see below.

1 Under normal heating conditions at outdoor temp: -3°CDB / -4°CWB, outlet water temp 35°C, inlet water temp 30°C.


*2 Under normal heating conditions at outdoor temp: 2°CDB / 1°CWB, outlet water temp 35°C, inlet water temp 30°C as tested to BS EN14511.


*3 Under normal heating conditions at outdoor temp: 7°CDB / 6°CWB, outlet water temp 35°C, inlet water temp 30°C as tested to BS EN14511.

It is important to note that 35C is really only suitable for Under-floor heating, and would be inadequate for almost any size of radiator.

If you can find the spec for water at even, say, 50C or really cold outside temperatures then the output will be considerably less than 11kW.

It is also important to note that the data given is just for the unit itself. Installed it will be lower for a number of reasons - not least the power taken for defrost.

dweaver

Hi all,
I've been a long time reader of this forum and used it extensively when deciding whether to install an ASHP or not (previously had an old oil fired boiler which cost around £1,600 a year when being careful) .


I've just had a 14kW Ecodan and 250l cylinder with FTC4 installed and a new radiator system (no underfloor) and would like to share my experience so far. The house is a 1960's bungalow which has been completely renovated with cavity wall insulation, around 250mm loft insulation, and all new double glazing plus some extra floor insulation in the lounge and kitchen (the rest is concrete flooring). It's around 190m2.


Cost of heat pump and cylinder was just under £9k (not including the rads). Will have to wait 6 months before applying for RHI as the house was empty for over 6 months due to renovation so would otherwise have to have metering (another expense). The rads were all sized using the Mitsubishi heat loss calculation spreadsheet using a 45degree flow temp.


The heat pump was running for a few weeks before we moved in (on the heat compensation curve mode) and the house was nice and warm (maybe too warm). However, 2 days after we moved in it stopped working and we had no heating and hot water. The installer with the help of Mitsubishi diagnosed a faulty circuit board on the Ecodan. He hot wired the immersion to give us hot water and we used portable electric heaters to warm the house. The new board came next day and was fitted straight away and now all seems fine. In fact, the installer had altered a few incorrect settings and the system now switches off when up to temperature, which it didn't seem to do before. I've upped the hot water temp from 40 to 45 degrees as well as the wife thought it wasn't warm enough.


As for running costs, too early to tell as the I've only been using it a few weeks, but will post info for anyone who's interested.


Cheers,


Dave

EcoTryHard

Hi all,
I'm looking for a highly efficient air source heat pump (air to water) that combines easily with solar thermal.
Does anyone have experience of these? Nibe seem to have the goods, but there doesn't appear to be much info out there about them. Anyone any experience with them?

TIA