Air Source Heat Pumps

Cardew

jeepjunkie wrote: »

Cardew - Am I right in thinking you have solar pv? I'm finding this technology harder to grasp than ASHPs. The more I look into it I'm finding that the relationship between the inverter and panels is important especially being in Scotland where light levels are possibly lower? All the quotes received so far are not like for like execpt size, 4kw or just under making comparisons dificult... Only one supplier so far doen not bother to much SAP calcs as they do their own inverter/panel tests so they know what works best...

Zeupater in the 'Green' forum is excellent at all the detailed calculations and there are a couple of threads on this isue.

In simple terms you might expect around 800kWh pa per kWp installed - assuming you face roughly toward Hadrian's wall!

You are unlikely to generate 4kW - perhaps on very rare occasions.

Some installers advance the theory, that as smaller inverters are more efficient, overall you will gain from having a slightly smaller inverter than the theoretical maximum output of your panels. i.e. slightly higher efficiency for the majority of the time will outweigh the rare occasion that the inverter cannot handle all your output.

No idea if this is factual, or a ruse to fit a cheaper inverter.

You might be interested to watch Watchdog on BBC on Thurs - assuming Scotlands TV shedule is the same as England.

giscool

jeepjunkie wrote: »

Here are a few pointers...

Our requirement was 11kw ...

£17-£20k for a rad based system is robbery, Considering fan convectors can cost £400ish each plus an electrical circuit and we needed 13/14 plus ufh plus bathroom towel rail your quote is £6k north of us...

Some useful pointers thanks Jeepunkie not sure if you realise that these quotes are for replacing an electric storage heater system wth wet raiators and all the plumbing as well as the HP, tank and controls. If you can get that done for £6K please give me a name, it would be worth putting them up in a 5* hotel while they do the work if they are not local to me. A quote for connecting an Ecodan 14kw and 250l tanks to installed radiator system is £11.5k

I have considered fan assisted radiators but accordign to your price that would add £2-3K to the quotes depending on how many you use.

I take your point about oversized radiators. It is quite often quoted that this should be 30-50%, but if the correction factor of 0.4 is correct for low temperature flow rates surely they should be 150% bigger?

Two of the qotes are forecasting seasonal performance factors of 1.75 and 1.8. Not much of case for inveting £6k let alone £17-20k

Cardew

giscool wrote: »

Two of the qotes are forecasting seasonal performance factors of 1.75 and 1.8. Not much of case for inveting £6k let alone £17-20k

Could you eleborate on those seemingly realistic forecasts please.

I have never seen any blurb that forecasts less than 3.0(and often much higher) including Mitsubishi's Ecodan website.

giscool

Cardew wrote: »

Could you eleborate on those seemingly realistic forecasts please.

I have never seen any blurb that forecasts less than 3.0(and often much higher) including Mitsubishi's Ecodan website.

In one case the installer is offering an Ecodan 14Kw unit and in his letter he states "estmated annual kWh demand is 8,287 based on a COP of 1.75:1, a COP of 3:1 the demand would be 4,834 kWh" then in bold "the predicted energy performance per annum 8,287 kWh pa" followed by some disclaimers. So that is the 1.75. (COP of 4.24 is claimed by Mitsubishi at 7/35)

The other installer does not state perfomance explicitly, he is offering a Grant 15.5kW unit and he has provided me with the calculation sheet done by Grant which include performance comparisons with other fuels. The predicted energy demand for the HP is 12,300 kWh pa and for electric 22,400 kWh pa which is where I get my 1.8 (Grant claim COP of 5.8 for this unit at 7/35). My total electricity is 23,000kwh pa so I know he has overestimated the heating demand if I heat the house to the same level as before, and I m guessing the introducing of radiators into 3 bedrooms and two bathrooms where there are non now could account for this anomaly.

With such different figures it is a leap of faith if I am to proceed with an ASHP in the hope that the performance is better than 1.8. If not although there is an enegy reduction, there is no cost saving because of a higher unit cost for the electric.

Cardew

Thank You.

Another possible reason for the anomaly(heating demand) is that heat pumps require to be running for longer periods , regardless of you being in the house or not, as the output is relatively low.

It is a total nonsense to be talking of £17k to £20k for an ASHP. Just the lost interest on that capital invested long term is £680 to £800 after tax and a lot more if you borrow the money.

You could install a first class Oil CH system for £4k to £5k and still be saving £500pa in interest. Oil costs today are around 6p/kwh which compares favourably with an ASHP with a COP of 1.8.

The great advantage of gas or oil CH is that you can have water in your radiators at 80C+ so you can set your heating to come on 20 mins before you get up, or return from work, and have a warm house. That is not so with an ASHP as it need to be running for much longer periods as stated above.

albyota

Hi Giscool, I have the 8.5kW Ecodan which has been installed since October 08, has been through 3 of our coldest winters, I run my system with UFH so I run my system at 36 degrees from around late Sept/ Oct depending on the weather, although it is not needed yet, it does the domestic hot water daily, the actual formula for sizing rads with a heat pump is 0.634 so about 30 - 35%. Stick with Mitsubishi, Daikin or Panasonic, all inverter kit. Unless Grant has recently got an inverter driven heat pump their startup current is 70 Amps.

If you get the design done for 45 degree water temp, then this will work down to minus 15 at COP of 1.61, at minus 7 COP of 1.95, at plus 2 COP of 2.27 and at plus 7 COP of 3.2, bear in mind the output at minus 15 is 11kW, at minus 7 its 13kW. so if your house needs 14kW at minus 15 you will need aditional heating.

14kW Ecodan with 250L DHW cylinder average cost £8,660 + 5% vat fully fitted, 15 radiators c/w TRV's between £2,500 - £3,000 depending on quantity and size.

giscool

Cardew wrote: »

Thank You.


You could install a first class Oil CH system for £4k to £5k and still be saving £500pa in interest. Oil costs today are around 6p/kwh which compares favourably with an ASHP with a COP of 1.8.

Thanks Cardew I have taken off my green tinted spectacles and green blinkers now.

If you think that COP of 1.8 is normal for these systems and not just suppliers learning to cover themselves then ASHP must be a dead duck for retrofit. But I cant bring myself to even contemplate a new oil CH system.

You raise the issue of return on investment but dont you have to factor in that interest rates are 3-4%, inflation about the same but electricity price increases are 15%.

A few years ago it was put out that electric prices were set to double in the next five years. Doubting this claim I looked at my electricity bills for the previous five years and to my surprise they had doubled in that time = 15% pa. There is no reason to suppose they will not continue to rise if not even faster with someone having to pay for the 43p/ kw FIT. So isnt any investment in energy saving better than sticking it in a bank - by about 10% pa?

In an earlier post I said that my electricity tarif was 4.5/9.9p. I am not so sure now as I discovered they have changed my "product" and increased my DD by 19% (even though I am in credit) without letting me know and I cant see what the new tarif is!!! That is another story. Cheers

Cardew

jeepjunkie wrote: »

For even massively oversized rads to work you are going to drive your ashp very hard so your cop will be rubbish therefore expensive to run and in winter as your ashp is going to have quite a job to lift the temp to something useable if at all which could cause the unit to ice up frequently.

Cheers

Sorry that statement makes no sense to me.

All fan assisted radiators achieve is a better distribution of heat from a smaller radiator.

If your room needs xxxx BTu, you size radiators to achieve that output. That can be with big radiators, UFH or Fan assisted radiators.

To achieve that heat output, at any specific water temperature, your ASHP will be producing exactly the same xxkW output.

So why would your ASHP's COP be 'rubbish' and ice up more frequently when it is producing the same output?

You make it sound like fan assisted radiators improve the efficiency of an ASHP. They don't! All they do is, in effect, increase heat dissipation such that a smaller radiator will produce the same output(heat/BTu) as a larger radiator.

albyota

Urmm, if using UFH at lower temps means a heat pump is at its best efficiencies, it is because the low temperature water flowing through the pipes 70mm below the screed is a very very large radiator....placed where your feet and body are closest to....on the floor, unlike a standard radiator which due to heat rising....heats from the ceiling down, hot head......cold feet, fan convector type radiators blow the low grade heat across the floor to where you are..... before rising to the ceiling.....so using the correct type of fan convectors means.....you can run the heat pump at lower temperatures, thus increasing the COP.

richardc1983

Cardew wrote: »

All they do is, in effect, increase heat dissipation such that a smaller radiator will produce the same output(heat/BTu) as a larger radiator.

They do greatly increase heat up times at lower water temperatures as you say the heat dissipation is much better from them.