Air source heat pump – high temp unit or not?

[aa44]

I am soon going to start building a new house (for me) here in windy Shetland. The house will be a detached 5 bedroom place and will be very well insulated and very airtight so heat losses should be fairly small. The main heating will be wet underfloor heating, powered by an air source heat pump.

Is there any disadvantage in going for a high temperature unit (e.g. Daikin HT) rather than a standard unit (e.g. Mitsubishi Ecodan)?

I want a woodburning stove in the house, mainly for the real fire effect rather than its heat output. I was thinking about connecting this to the top of a thermal store to use the output for the DHW. Would life be a lot simpler if I got rid of the thermal store and simply connected the ASHP directly to the UFH and to a DHW tank? Again, am I better off with a high temperature unit?

Not sure if this is the right section to post in. If not, please move it!

Thanks for your help.

[multisync]

A wood buring stove in a super insulated house will be all you ever need.
However in the Shetlands I would check day/night averages throughout the winter and then compare efficiency against a stable ground source HP.
If you intend to stay forever this may prove more economical in the long -very long- run..

[aa44]

Thanks multisync. The site is almost solid rock so a ground source heat pump would be difficult to install. I agree that the woodburner should cover most of it in the winter but, even up here, I don't think that we will run it in the summer so I need something to do the DHW. Most of the roof faces E/W so I don't think that solar would do a lot.

[fridge doctor]

You can forget solar. I live in Cyprus and solars are perfect for the climate. But don't believe that bull**** about still getting some heating on a cloudy day. It just isn't true. I have a digital thrmometer connected to the tank, and whilst in summer I can get water temperatures of 80C, on a cloudy day it doesn't even move by 1 degree. So, as if I needed to ask, how many cloudy days do you get?

To be honest I would also think very seriously about using a heat pump. GS Heat pumps are VERY long term investment, and although I have looked at them from time to time, I have yet to find a good reason for installing one anywhere. I honestly do not think that the quality of heating you can produce in the Shetlands is worth the investment.

[aa44]

The local housing associations have been installing lots of heat pumps up here. They are a mix of ground source and exhaust air and the tenants rave about them. It is cold up here but it's not like the arctic and the tenants seem to reckon that their electricity bills are really small. Obviously, the houses are new and very well insulated but heat pumps do seem to work effectively, even up here.

[multisync]

The local housing associations have been installing lots of heat pumps up here. They are a mix of ground source and exhaust air and the tenants rave about them. It is cold up here but it's not like the arctic and the tenants seem to reckon that their electricity bills are really small. Obviously, the houses are new and very well insulated but heat pumps do seem to work effectively, even up here.

Recently a Scottish H.A looking to trial HP's in a few houses with the mind to retrofit all their 700 stock.So indeed they are the main drivers of domestic HP's

two points

1 You will struggle to get UK planning permission without some form of renewable.
2 The clue is insulation. If you upped the insulation you will probably find you don't need heating.esp if c/w a heat recovery ventilation system

However as you say the DHW is the killer and if gas isn't available the alternatives are very limited

[aa44]

However as you say the DHW is the killer and if gas isn't available the alternatives are very limited

Which leads me back to the question: Are the high temperature units any good? Assuming that I want to store my DHW at 55 degrees (is that about right?) am I better off going for a high temperature unit or going for a standard unit and running it at the highest temp possible (at least for the DHW)?

[mad fridgie]

A well insulated house still needs heat!
A low temp heat pump if correctly balanced from design will be cheaper to run than a heat pump that always produces high temp. (refrigeration fundmentals)
In the shetlands, your heating season is quite long?
You need to undertake a daily to yearly load profile.
What do I mean by this.
lets say every day you need 10Kwhrs for DHW (3650kwhrs/year) and then you need 150kwhr per day for heating but onlt 200 days (30,000kwhrs/year)
If we say a low temp heat pump had an avergae COP 3, then you would use 10,000kwhr/year of power, if ahigh temp heat pump had a COP of 2, then you would use 15,000kwhr, the difference then would indicate that low temp is better.
I would use low temp, and pre-heat the DHW with the unit, then take up to final temp with element or fire

[stuartwking]

Hi aa44,

the question of using a HT system (80 deg) or a LT system (55 deg) should be easy for the contractor your are talking to to answer,
Being a new house with high level of insulation and a correctly matched wet circuit the LT should satisfy your needs well,

The reasons i would say use the HT system for would be,

High heat loss property
Old properties with wet circuits designed to flow at HT.

As for solar, my office has ASHP with solar ( NE Scotland) and we see 55 deg water from solar on over cast days ( proven).

Ecodan is a good piece of kit also but larger outdoor space required and has had problems with HW tank.

Its all about capital cost and pay back/efficiency.

As you mentioned there are propertyies on the island with equipment that works, if it works for them it should work for you.

Kind Regards

[aa44]

Thanks chaps.

Stuart, does your solar get the DHW hot enough to kill off legionella or do you periodically use an immersion to bring it up to, say, 65 degrees?

AA

[freezetech]

hi
all these systems come with a back up hot water element to make sure you can take the temp up to what regs apply ie 65 degree s this has to be done a minimum of once a week

[stuartwking]

aa44,

you are right in that you have to use the electric heaters to boost the water temp, If you take the boat down anytime soon drop me a line and come visit.

[aa44]

Hi Stuart, Thanks for the offer. AA

[barriecats]

I have a HT 16kw daikin air heat pump being commissioned. It is taking for ages. Have had 3 sessions and still it is not working properly.

[aa44]

What sort of problems are you encountering? (I have no idea about how to fix any of them, I'm just interested.)