Heat Pumps

[MdB]

Heat pumps get mentioned here fairly regularly so I thought some of you might be interested in seeing the insides of mine. I've just reinstalled it so have a few pics of the guts.

As you can see, there isn't an awful lot inside though all the sensors do make it look more complicated than it actually is.

The right hand two grey pipes coming out the bottom go outside and connect to 150m of pipe buried in the garden. The left hand two grey pipes lead to our underfloor heating system. The idea is we are going to transfer or "pump" the heat energy that is in the ground in our garden and put it into the floor of the house.

Part 1 is to get the heat out of the garden. This is achieved by the right hand green pump (just an ordinary central heating pump) which moves water through the pipes in the garden into the first heat exchanger (grey box just visible behind the pump) and then back out into the garden. The water from the garden typical arrives in the machine at 7 deg C, has heat energy removed by the heat exchanger (i.e. it is cooled) and leaves to go back out to the garden at 4 deg C. The pipes in the garden are 1.2m down where the temperature is typically around 10 deg C all year round. This is enough to reheat the water from 4 deg C to 7 deg C as it circulates in the pipe.

Part 2 is to get the gained heat into the floor in the house. The left hand green pump moves the water through the underfloor pipes in the house, through the second heat exchanger (again, grey box behind pump) and back out to the underfloor pipes. The water coming back from the underfloor typically returns at 30 deg C and has heat energy added to it by the heat exchanger (i.e. it gets hotter) and leaves to go back to the underfloor at 40 deg C.

As you can see, both loops are very similar in design, it is just the direction of heat transfer is different as are the temperatures involved.

Part 3 is the final bit and the section which most people have difficulty understanding. This is the compressor (or heat pump - big black lump top left) and its job is to move the heat from the right hand heat exchanger (and hence cool the outside ground loop) and put it into the left hand heat exchanger (and hence heat the inside underfloor loop). It achieves this by simply pumping a refrigerant through the left hand heat exchanger (1), then through a restriction in the pipe (2), then through the right hand heat exchanger (3) before returning to the compressor (4).

The following diagram for a fridge shows this process nicely. A heat pump and a fridge are basically identical in the way that they work.

The compressor does what it says on the can and compresses the refrigerant in the left hand section causing its pressure to rise. To keep Mr Boyle happy, increasing the pressure of a gas causes its temperature to rise, in our case by quite a lot - I don't have figures but it will be considerably higher than the 40 deg C we need and hence energy is transferred by the heat exchanger into the inside underfloor loop. The restriction in the pipe is designed such that only a small amount of gas can travel through it. This causes a large pressure differential and we end up with a very low pressure gas in the right hand loop. Again to keep Mr Boyle happy, the reduction in pressure causes a large temperature drop - this time to significantly below the 4 deg C we need and hence energy is removed via the heat exchanger from the outside ground loop.

As before, this is exactly the same principle as your fridge with the inside of the fridge being the same as the ground loop and the back of the fridge (the hot bit) being the same as the underfloor.

I guess that is probably far more information than you ever really needed to know...

 

[LadyArowana]

I guess that is probably far more information than you ever really needed to know...

Actually I found it quite interesting! Thankyou

But I do have some slight nerdy tendencies, try and keep them well hidden of course.

 

[worms]

Actually I found it quite interesting! Thankyou

But I do have some slight nerdy tendencies, try and keep them well hidden of course.

I believe elephants work on the same principle, which is why elephant dung steams on a cold morning!

 

[DJH]

Yes, very interesting. That's the best description of heat pumps I've heard. I wonder if they work as well on bore holes. I came across a heat pump recently that was situated in the garage of the property and most of the pipes seemed to be covered in ice. Does it make much of a difference as to where the exchangers are situated as ambient temperature must affect the pipework?

 

[Flyfisher]

Interesting and quite timely as we're planning to (finally) install some heating in the main part of our house in the spring.

However, I'm afraid to say that my initial enthusiasm for 'alternative' systems has waned during the time I've been looking at all the fine details.

The two main things I've wrestled with are:

Heat pumps are about 300% efficient but run on electricity, which is about 3x more expensive than mains gas (on a per kWh basis). So the operating cost is expected to be about the same.

A wood stove + back boiler + heat store (the currently favoured option) is not really an 'all year' solution, leaving us without DHW during the warmer months. This is not a problem per se as along as we have a supplementary form of DHW heating. Options could be electricity (perhaps on an 'Economy 7' type cheap rate for heating the heatstore overnight) or mains gas. Even a cheap rate electricity tariff will work out more expensive than mains gas (plus it increases the cost of 'day rate' electricity), so a gas boiler seems the best option. But if we install a gas boiler for summer DHW then why not also use it for winter heating as well?

So I'm now considering a standard mains gas CH & DHW system supplemented with wood stoves and open fires, with the CH used to keep the chill off the house (say 10-12C ?) with a wood stove/open fire to warm the occupied rooms.

I'm still thinking that a wood stove/backboiler/heatstore could be a good way of supplementing the CH and also for driving the underfloor heating in the new kitchen extension, but it involves a quite high capital cost (£7-8k) and the payback time is likely to be measured in decades.

Hmm.

 

[CliffordPope]

Many thanks MdB, fascinating and easy to follow.

Put so simply it's tempting to think one could make one from an old fridge. Perhaps that would work - just build the fridge into an outside wall so that its radiator panel was on the inside, and then route the underground pipework through a coil sealed in the icebox on the outside?

 

[AMc]

Great explanation MdB!

Yes, very interesting. That's the best description of heat pumps I've heard. I wonder if they work as well on bore holes. I came across a heat pump recently that was situated in the garage of the property and most of the pipes seemed to be covered in ice. Does it make much of a difference as to where the exchangers are situated as ambient temperature must affect the pipework?

AFAIK Ground Source Heat Pumps shouldn't ever have sub zero temperatures on either the ground or heating loops?
There was a fashion amongst some installers for putting Air Source Heat Pumps in garages, perhaps that's what you saw? It doesn't make a lot of sense and results in them icing up and generally not working properly. I have also seen the heat exchangers installed in loft spaces by installers who didn't understand the basic principals involved. They should be installed in open air. This type of problem was one of the things that the MCS program was supposed to stop IIRC.


We have air source heat pumps (ASHP) which work on similar principals to ground source but blow ambient air across the heat exchangers instead of circulating water through ground loops or bore hole loops.
Essentially an ASHP is an airconditioner running in reverse - indeed one of ours can be run that way.
They are cheaper to install but are generally less efficient. We haven't got enough land for a ground loop and bore holes were prohibitively expensive.
Bore hole based ground source works in exactly the same way as a ground loop (described in MdB's post). The temperature remains stable as you go down. Once you get to the water table you can extract a lot of energy from a comparatively small collector.
I have a friend who has a lake :shock:, he installed a water loop for his heat pump which also works very, very efficiently.

So I'm now considering a standard mains gas CH & DHW system supplemented with wood stoves and open fires, with the CH used to keep the chill off the house (say 10-12C ?) with a wood stove/open fire to warm the occupied rooms.

If you have access to mains gas then you will struggle to find any alternative that is cheaper, more efficient or more green than converting fossil fuel to heat at source in an efficient boiler. That is accepted wisdom even amongst the renewable industry, we had no gas so our choices were oil, LPG or biomass (a.k.a wood). In practice the heat pumps are about as cost effective as oil and maginally cheaper than LPG but have the advantage that your energy arrives via a national grid based on a mixed fuel source so as renewable generation increases the carbon footprint will drop. There was some talk of a subsidy for "renewable" heating technology, but that seems to have been nixed. I think probably for the best given the impact of the feed in tarrifs in creating a boom/bust solar PV industry, the last thing we need is an "industry" of installers convincing people to rip out gas boilers and install electric heat pumps to receive a subsidy based on dubious claims of cost savings.

 

[MdB]

I wonder if they work as well on bore holes.

What matters on the ground loop side is how good the heat conductivity is between the pipes and the ground. A wet bore hole as AMc says will work better than a dry ground array even though it is collecting from a smaller area. A lake is perfect.

I came across a heat pump recently that was situated in the garage of the property and most of the pipes seemed to be covered in ice.

I've been told that our ground loop will work down to -5 deg C though it will be less efficient within the heat pump and heat transfer within the ground will drop also as the water around the pipes turns to ice. I can't see it as being a good thing.

Does it make much of a difference as to where the exchangers are situated as ambient temperature must affect the pipework?

Condensation on the pipes is something to worry about. It doesn't affect the heat pump but it will make a puddle on the floor....

Heat pumps are about 300% efficient but run on electricity, which is about 3x more expensive than mains gas (on a per kWh basis). So the operating cost is expected to be about the same.

I was told by the heat pump people I bought ours off that if you have mains gas, stick with mains gas on cost grounds

Put so simply it's tempting to think one could make one from an old fridge. Perhaps that would work - just build the fridge into an outside wall so that its radiator panel was on the inside, and then route the underground pipework through a coil sealed in the icebox on the outside?

...
It would work - just probably not very well. Getting the heat transfer from your ground pipes to the fridge cooling section would be the difficult bit.

If you have access to mains gas then you will struggle to find any alternative that is cheaper, more efficient or more green than converting fossil fuel to heat at source in an efficient boiler.

Cheaper - yes. More efficient / more green is debatable. Our heat pump is 350% efficient (measured approximately) and if you take the generation of electricity to be 50% efficient gives you an overall efficiency of 175% which is roughly double that of gas. Greeness is a difficult thing to define but if you take greeness is proportional to cost (again debatable) then it comes out in gases favour.

that your energy arrives via a national grid based on a mixed fuel source so as renewable generation increases the carbon footprint will drop.

Agreed.

 

[biffvernon]

Nice explanation, MdB.

One way to look at the question of ecological footprint is to consider whether the electricity used in the heat pump is used more usefully there or somewhere else instead or not used at all and something else used to keep you warm at home.

Or one might look at the cost. Is the cost of the electricity you buy for the heat pump greater or less than the cost of keeping yourself warm at home by other means.

Either way, heat pumps don't come out too well.

 

[88v8]

Very interesting, thankyou.
A couple of neighbours have installed ground-source in their fields, this is their first winter so awaiting the result with interest.
We haven't the space. A borehole might be an option, leaving aside the cost.

Woodburners are fine as a supplement, but I wouldn't size a ch system on the basis that you have to use the woodburner, because I can tell you that as an alternative to gas, the relentless sourcing of tons of seasoned wood, stacking of wood, chopping of wood, chopping and storage of kindling, carrying in of wood, lighting of wood, keeping the stove(s) going, too hot too cold, removal of ash... well, I can tell you that although I wouldn't be without a wood fire, the novelty of having to light up every evening wears off...

Ivor

 

[lou]

This post is mainly to get this off my chest so not sure it's helpful but might serve as a cautionary note to anyone considering the options of log boilers or heat pumps. I have returned home after two days to spend the last 3 hours trying to get the blighter lit. The logs are seasoned - or should I say 'were'. Despite being kept in a more or less rainproof garage the damp has got to them and they just won't fire up properly. I second Ivor's opinion about the diminishing novelty of chopping wood (smaller is better than what's delivered so chopping is essential to get kindling of the right size to get it going).

AND.... We were assured that the very very expensive limecrete insulation would ensure that the heat generated by the boiler would not only adequately warm the huge thick flagstones to a comfortable temperature but that the boiler would heat the upstairs rads and some DHW. It doesn't. Maybe it's undersized or there's another problem but it would seem that heating the floor absorbs the whole 3000 litre tank of hot water. When we move onto the barn I'm going for ground source and wood burners. And just to add to the annoyance, I planted 50 ash trees with a view to achieving some measure of self sufficiency in 10 years time. :evil:

 

[Flyfisher]

So sorry to read about your ash planting, Lou, but these 'real life' experiences of such systems are very helpful - the more the merrier.

Ivor's point about the necessity of felling/logging/splitting loads of firewood certainly touches a nerve. At present, I thoroughly enjoy the time I spend on such things but I'm very aware that not only will the novelty likely wear off but that age will eventually get the better of me anyway (if the chainsaw, tractor or log splitter doesn't get me first :lol: ). And Mrs FF pointed out recently that if I'm indisposed for any length of time (and we all know how serious man-flu can be), she will not want to be humping logs around the place each day and actually wouldn't be able to prepare them.

So I'm reluctantly coming to the conclusion that such things must be a supplementary form of heating and DHW. Great if we can use it full-time, but we must have something else as well.

Planting trees for fuel is a whole other interesting discussion. We're fortunate to have space to do such things but I'm not sure about what's best to plant. I've read that the energy content of wood is basically the same for any species on a weight-for-weight basis (dry weight, obviously). So perhaps it's best to plant something fairly fast growing such as hazel or willow?

Really, after hours and hours of reading about all the options and tradeoffs, the temptation of a simple 'on-off' switch is very powerful.

 

[lou]

Thanks for the sympathy Flyfisher - boiler now ablaze so the house should be nice and warm in about 6 hours time - yes, really.

There are lots of good websites outlining the relative burning merits of trees. Ash is considered to be among the very best and is relatively fast growing, or used to be. But beware, if disease doesn't get them, the deer will. They have taken the tops off every one of mine. I'm now going to stop sounding like the prophet of doom, put the electric blanket on and head bedwards with a small welsh whiskey. Mmmmmmm!

 

[JoceAndChris]

Great thread, and I'm so grateful to MdB for the excellent explanation of ground source heat pumps as I've often wondered what they are. Hope that's going to be a really warm solution, it sounds like it is. It looks like masses of hard work and all credit to MdB, not sure we'd have the dedication, but it's an ingenious and well thought out technology.

I agree wood should be a supplementary form of heating - open fires are extremely time consuming ( 5 hours a week at least ) and if other things happen there will be times when you can't prepare wood. We harvest all our own wood so this is a year round job, and the benefits of it should be pointed out. I feel the same about it as I do about the chickens, were it not for these things that have to be done outside you'd sit on bottom drinking tea and nibbling biscuits, getting fatter, missing the baby blackbirds hatching out and the sunsets, your joints stiffening, your awareness of the seasons dulled. My Dad works extensively outside and he's getting towards 80 and I think this keeps him young. Use it or lose it! And working outside warms you up if it's warmer than inside, or just the heat generated by moving about, and when you come inside it seems warmer and you're glad not to have the wind lashing in your face.

FF - I'd have thought there was enough wood kicking round your place for you not to have to do too much planting? Even giving the big trees a little prune generates masses. We took a big dangerous low branch off a beech and that's a couple of month's worth. We're knee deep in firewood, the wretched stuff just blows down constantly and we've less land than you??

The more I read the more I think we need a few tweaks not a major overhaul, get better insulation all round ( double glazing on the skylights and casements, hemp on the render, wool in the loft, carpets) and a couple of storage heaters and I think we'd be sorted.

 

[Flyfisher]

You're absolutely right about the benefits of working outdoors, Joce, and I certainly don't underestimate them. You're also so right about quickly the cold can be shrugged off with only a minor amount of activity. Perhaps I shouldn't be so foward looking and stop thinking about life's slow decline!

As for planting trees, you're probably right that it isn't strictly necessary but it's a nice thing to do. We've also had a couple of small trees blown down this autumn, which will provide a lot of firewood when I finally get around to logging them - the ground has been too wet for the tractor and trailer recently and shows no sign of drying out anytime soon. We also have a large walnut tree in bad need of some serious pruning, so that will also yield a lot of good firewood.

So with all this wood available, I can't envisage not having a fire in the house but I also can't envisage it being the only source of heating.

 

[Zebra]

This is a very useful discussion, as I have only wood for heat at the moment, and am looking into the options regarding a central heating system. Interesting that the gas boiler option which I will end up with, isn't seen as the devil's work in terms of environmental impact. Although clearly there are others who will disagree.
I rather enjoy the routine of felling, stacking, cutting, splitting etc, but am dimly aware that this is an enjoyment born out of the anticipation of being warm, and that when the on/off switch becomes available on a daily basis, then I might become rather lazier, which would be a shame.
Totally agree about doing something to warm up!

 

[MdB]

As others have suggested, if we had mains gas available, we would probably have gone down that route - financially it was and I believe still is the sensible option. As mains gas is miles away, we looked at oil, night storage electric and heat pumps. Financially, the night storage was the cheapest as although the running costs were higher, installation was so much cheaper it was many years (15?) before the others became cheaper overall. I hate night storage though as a heat source when running on E7. Hot in the morning and cold in the evening. Oil was only a bit cheaper to install than a heat pump IF you install as much as possible yourself and the running costs were higher. I wouldn't want to pay for someone else to install a heat pump though as the quotes I've heard of were rather large and it wasn't a difficult DIY job. A fair amount of work maybe but nothing too complicated.

If were to do it all again, I would definitely go down the heat pump route again. Very pleased with it. There are a few things to point out to people though who are thinking about it.

1) Make sure you run it off E10 electric rather than E7. This give heat through the day and reduces the variation in temperature.

2) It doesn't work well with radiators due to lower operating temperatures so go for underfloor heating instead. Its lovely...

3) Make sure your house is very well insulated - basically up to current new-build regs. Without good insulation, you can't get enough heat out of the floor to keep the house warm unless you run it at a high temperature. If you do so, you lose the efficiency of the heat pump which defeats the point of it.

4) If you insulate well, you can get away with having no heating upstairs if you don't mind it being a few degrees cooler. This works well for us.

 

[Zebra]

If you insulate well, you can get away with having no heating upstairs if you don't mind it being a few degrees cooler. This works well for us.

This is good to know.
Are you OK, MdB, in the Devon and Cornwall deluge that we're hearing about on the news?!

 

[MdB]

Are you OK, MdB, in the Devon and Cornwall deluge that we're hearing about on the news?!

No. We need help.


The beer has almost run out...

 

[AMc]

Happy New Year!

I'd echo what's been said. Our woodburners were used in the heat loss calculations for our ASHP system which wasn't a wise idea. It's lovely to use them when I have the time and energy but they are strictly supplemental due to the work involved in lighting and maintaining them even if we're at home all day. I don't light fires first thing in the morning except at Christmas. As a supplement they take a load off the main ASHP system which in times of extreme cold - when we're more likely to want a fire - the heating system is more able to cope but don't rely on them unless you're prepared to do the work and source the fuel. My firewood costs have gone up a lot in the last 5 years.
A load of duff logs and spending hours trying to coax a fire into life really spoils my mood. My current wood delivery contains some wet logs which if you happen to try and light all alone just fizz all evening If you get a mixture then you do at least get a fire!

In general there is a tendency to over estimate the output of heating and under estimate the heat loss of the building.
Manufacturers of all heating systems quote ideal outputs and if you press them you may find there are significant things missing - for example my ASHP produce their maximum heat at relatively warm outside air temperatures, by the time you're below 0C they're not able to output the 16KW in the model number, more like 11.
It used to be common practice with oil and gas boilers to apply a margin of oversizing for both the boiler and the radiators but this is much more tricky to do with heat pumps and underfloor - so be wary!
The other issue is heat loss calculators use theoretical values for heat loss from walls, windows, floors and roof. My house is a warren of insulated and uninsulated surfaces, I have two cellars and a long vent under half the house plus a T shaped roof with different tiles. As a result when you put the building into SAP software you have to make approximations and the heat loss calculations here were significantly lower than the actual heat loss when we encountered in the first winter.
Old houses are supposed to be harder to estimate due to the construction methods being unknown and untested but my pal in the trade has told me horror stories about brand new builds that have failed air tightness testing and as a result required significant reworking to meet the standards required by building control and the numbers used to specify their heating! If the builders didn't seal them properly then I'd lay money that they didn't do all the detailing to make the insulation work to its manufacturer ratings.

If you have an existing system them take a good look at fuel bills for as long as possible and work that back to find out how much heat energy you need to be comfortable, then size the system accordingly. There are ready reckoners that will tell you the heat output of various fuels (e.g. 1 ltr of heating oil is 10KWh). Of course bearing in mind you want to take the coldest quarter and account for the coldest couple of weeks. An average heat load over 365 days won't keep you warm in a cold snap! That calculation was complicated for us as we were significantly extending the living space but doing the calculations retrospectively I discovered the original design would have struggled to heat the house before we moved into the bits that were sealed off!