Heating home with a Heat Pump - any thoughts ???

[Mikefromlondon]

I am responsible for maintenance of an office building which is approximately the size of a large 5 bed detached house. It is heated with a central 'air source' heat pump. (Mitsubishi City Multi) I can advise as follows:

Heat pumps do not reach the same temperature as a boiler, so they only work in really well insulated properties. If you connected a HP to your existing radiator system they would become tepid only. That is why heat pumps are used with either underfloor heating which gives a lower surface temperature but over a much larger surface area, or to a ducted blown air system. The advantage with the latter as you say, is cooling in the summer and this is what they use in the USA.

The system has two parts, one indoor and one outdoor. In simple terms one part gets warm the other cool, so during the summer the outdoor unit gets warm and the indoor unit cold. In the winter this is reversed so that the indoor unit is warm and the outdoor unit cold, so cold in fact, that when the outdoor air temperature falls below 4 degrees C ours freezes up. This needs defrosting. So for up to 20 minutes in every hour, all of the heat it generates is used to thaw the outdoor unit. None is available to heat the property.

Also the colder it gets outside the less heat it generates, when it's below freezing ours has to be left on 24 hours a day 7 days a week to keep 21 degrees indoors and even then it can still drop to 19 during the night, so much of any efficiency saving is lost. I should add that the property is 7 years old and won an award for its high levels of insulation.

A ground source heat pump overcomes this in some measure as the ground is usually a few degrees warmer than the air, but the installation ground works are very disruptive, expensive and can require a lot of land.

I believe that the installation cost of our air source system, which includes a ventilation system with heat exchanger was just under £30,000 a gound source system would be considerably more

I can't comment on the running costs of the system, because the same electric meters also supply power to a small industrial unit with a standard electric heating system.

A very useful post truly deserving my thanks, I had not been aware that ice could form on the outside , requiring prolonged defrosting.

As for the system efficiency, it would suffer as the outside temperature falls, since a compressor pump has nothing to do with the temperatures and its job is to pump the gas and compress it, the gas is in a closed loop, so the pump draws a steady amount of electrical power, therefore the amount of heat extracted then depends on the temperature of the outside air and the extraction rate of the system, the extraction rate varies as the temperature falls the extraction rate also falls, whilst the pump continues to use the same amount of electrical energy.

So I can see there can be many many pitfalls already.

Looking at various neighbour's bills, most are about the same between £300 to £375, so may be I am under some disillusion that mine are very high. I will be checking a few more neighbours bills to find out what is the average street consumption, our houses are very large, 3 bedrooms with 3 receptions and a big cellar, high ceilings, bay windows and rear extension, built during 1930's and so do not have cavity walls.

 

[stem]

Interesting info on ASHPs.

Can I ask what rating the HP is and the floor area in m2?

I'm thinking about kW/m2.

Thanks

I reckon the floor area is about 350m2 in total, spread over two levels.

Unfortunately I don't know the rating of the HP, the only information written on it is that it is a Mitsubishi City Multi and requires 38.44 Kg of refrigerant. Here's a photo. Perhaps someone else may be able to size it from that. It's only gone green and mouldy in the last 6 months, probably because of the continual rain!

Untitled

• stem
• 28 Jan 2014
• 1

Maybe my original post was a touch negative, however on a positive note, the system has been 100% reliable over the past 7 years, although it is serviced every 6 months. Our system uses ducted air, and so also cools in the summer, so we have a very consistent and comfortable working environment all year around. As long as I remember to leave it on overnight and at the weekend in very cold weather.

 

[Mikefromlondon]

Stem, are these things anything to do with cooling towers as we heard in papers how they can breed legionire's disease?

I also noted something very intersting, I have been avoiding taking a shower everyday now as I found I was using around 45liters of hot water going down the pan, 45 liters of water requires a considerable amount of gas to heat it up to around 35-40 C, but strangely now on two occasions I noticed after taking a hot water shower, and after dressing up, I come downstairs and the temeperature is 18C and I am not feeling cold at all, today my heating is off, and I am feeling as though I don't need heating and I am wearing a vest and a long sleeved shirt and a pair of jeans!

I found this phemnomenon on two occasions so may be I should do this instead of not taking regular baths and instead wasting more gas on heating the house. Like I said one needs to change habbits.

So if I don't feel cold at 18C then I simply don't need heating , if that makes sense. (Outside temp is 9 C)

 

[stem]

It's the very large systems that supply schools, hotels, hospitals etc., that use water cooling that can spread legionaries.

During the summer the cooling water/ponds can warm up to the point where legionella bacterium can breed (between 25-45 degrees C) then the cooling towers can cause it to become airborne and inhaled.

Air source heat pumps as the name suggests use air not water, and are pretty much what you have in your cars aircon system, so don't present any more risk than that. You can get mould and spores developing when condensation forms on the cold parts of the system, but at most, that may give you a musty smell and possibly cold like symptoms.

As a sort of related issue, when used to heat domestic hot water, heat pumps often don't get the water above the 60 degrees necessary to kill legionella, so the final heating to a safe temperature has to be done by another method.

 

[ALEC1]

I think we have to change our views as to how to heat things efficiently, both for gas and heat pumps...heating at lower temperatures and at higher efficiency requires longer heating times at lower temperatures and by extension less kilowatts.

Best described with experience. I heat my 87m2 flat which has a calculated heat loss of 12 kws, with rads that have about 7kws emitter capacity with a rated down vitodens 242 gas boiler to 30% or 5.7kws.

the heating is on 16.5 hours a day at 20c, and the rest with a set back of 12c

I can tell you in six weeks the burner has run for about 700 hours and started over 1400 times... thats an average burn of 30minutes.

All these figures imply that a heat pump could have done the job.

The boiler is weather compensated with viessmanns own controls which are transportable to their heat pumps.

all this tells me the problem with adoption of efficient technology is what customers take to the heating equation. Low kilowat ratings (and hence heat pumps) can be used, but only if we use them in such a way to avoid peaks and troughs in the ambient temperature, something we can't do if we are wedded to hot radiators....)

(ps my gas consumption is considered slightly above average by british gas for this size home, that I know... the place is a ground floor flat in a victorian house in london with a lot of external walls...)

 

[Tipper]

We visited many stands at a Grand Designs exhibition looking for GSHP info for our new renovation project.

Almost to a man they said if we had gas then don't bother!

There seems to be no incentive to use less gas and reduce carbon footprint, it all seems to be driven by cost comparison 'off grid' heating sources which seems a little naive to me.

What I intend now is to install a small ASHP to provide the base load heat to our UFH which will suffice for 75% of the year with the gas combi boiler providing DHW and topping off the heating requirement during the coldest periods. Sad that the RHI won't apply even though we will use less gas. Shouldn't use much more electricity assuming a high COP.

 

[ALEC1]

gas is actually far cleaner than electricity, except nuclear, and distribution losses much lower..

 

[Norcon]

A2W are the biggest con in the refrigeration industry at the moment.

You could add a plate heat exchanger to a supermarket condenser and have an A2W system for about £1200.

For some reason the manufacturers add a £150 plate heat exchanger and then ask about £3500 for much the same product.
Except they add a tin surround to make it look nicer.
Daylight robbery.