Heating and hot water solutions

[Minkeycat]

We've been planning getting a Heritage Rangecooker for heating our water and running the central heating. We've done masses of research into it but our plans that enable the purchase of it have had to be put on hold a few times now.

Yesterday a leaflet flopped onto the mat about Heat Pumps (both ground and air source). I like the idea of these and there's some kind of govt scheme to offset some of the cost. The advantage is that we could then lose the boiler side of the Heritage and save oil by installing a heat pump elsewhere. My concern is that any saving we might make from reducing oil usage would be more than swallowed up in purchase and installation costs and then electricity to run the pump. Does anyone have any experience of such things? We'd need to run a central heating system with 10 radiators that covers the entire house - is this feasible? Where would you go for unbiased, sensible information on them? Is there an advantage of ground source over air source?

Any help at this early stage would be welcome.

 

[skier-hughes]

I looked into it, but came to the conclusion for our large, old and drafty house it would not work. The heat tends to be very low volume and best used in modern well insulated properties.

 

[Me!]

When I looked at ground source heat pumps several years ago, they only really seemed usable with underfloor heating and not radiators due to the lower temperature output compared to a boiler.

 

[fernicarry]

Look very carefully at how the COP is quoted for a heatpump. The COP is the amount of 'free' heat you get for a given amount of electrical energy in. To compete with oil you probably need about 4 i.e. every unit of electrical energy gives you 4 units of heat energy -- assuming heating oil very roughly a quarter the price of electricity per kWh (I have this in a spreadsheet somewhere).

However, usually the COP is quoted under absolute best case conditions for a water flow temp in the 30 to 45' range. This is fine for UFH in a well-insulated house with high thermal mass but much too cool for radiators that are sized for a flow temp of 60'. Heat into the room is proportional to the difference between the flow temp and the room temp, so if the room is 20' then you get twice as much heat in for a flow temp of 60 than you do from 40. To compensate you would need to double the size of your radiators.

You can crank the output temp of the HP up higher but the COP drops like a stone and you end up with something akin to a very large immersion heater. Some of them actually do have inline immersion heaters...

We'll be putting an ASHP into our new build holiday house which will of course be a suitable type of construction. Even so I still have doubts about using it for hot water heating where you need high temps again. Because the space heating demand will be quite low that means the water heating demand starts to dominate overall usage so we are back into poor COP territory. I'm considering a hybrid approach with the HP heating the water up part way and then topping it up with immersion heaters. I'll take the edge off that with lots of solar panels diverted to the immersions. The capital outlay will be quite significant however.

Another thing to consider is if you are susceptible to power cuts. Its hard to beat the energy storage density of heating oil. Our oil fired Aga could run for weeks without electricity. For the holiday house I'm looking at battery storage but practically that will give us only a couple of days worth of energy.

-AL

 

[Flyfisher]

I basically agree with the above replies. Our house had nothing but open fires and a very old and very inefficient oil-fired Rayburn (converted from the original solid fuel) for cooking, hot water and two radiators. So I pretty much had a blank canvas to work with and became all enthusiastic about 'green' energy systems and new technologies. However, after much research I basically came to the same conclusion as fernicarry regarding heat pumps.

The figures I found (10 years ago) suggested about 300% efficiency, ie 1kW of electricity IN gave 3kW of heat OUT. Sounds good, but I think we all know that electricity is an expensive form of heating, however it is used, and at the time mains gas was about 3x cheaper than electricity (for the same energy). Fortunately, we also had the option of connecting to mains gas, which I felt was the less risky option. And as fernicarry found, a similar thing applies to comparison with oil.

Another issue was choosing between air and ground source heat pumps. My understanding is that ground source ones are generally better because they tap into a more stable heat source whereas air source ones can sometimes freeze up in cold weather . . . . ie just when you need them most! Fortunately we had plenty of space to lay ground source pipework, but of course all that groundwork comes at a hefty price as well.

Then there is the temperature output issue. If you have solid floors throughout and can install UFH then I think a heat pump would be a good match because the water temperature needed to drive UFH is far lower than for normal radiators. This is simply because the larger the heated area (UFH or radiator) the lower the input temperature can be. In general, people do not like radiators the size of an entire wall so they tend to be small but driven with very hot water - fine if you have some form of boiler, but not so good for a heat pump. But even with a heat pump for space heating, there is still the issue of domestic hot water so something else will be needed as well . . . and if you have a boiler for that then is it really necessary to have the heat pump as well?

Having said all that, I accept that we're fortunate to have mains gas and it was undoubtedly a big factor in our final decision to go with gas CH & DHW. However, we also decided to supplement it with a wood burning boiler stove and when that is running the gas boiler doesn't fire up. This, I think, gives us a versatile mix in that we can wake up to warm rooms and hot water while minimising our gas usage in the evenings by lighting the boiler stove. However, we're fortunate to have a good supply of free wood. Last time I checked, buying firewood was on a par with buying coal (based on energy outputs), making both options more expensive than mains gas if you have to buy fire wood. So much depends on the mix of energy sources readily available in your location.

Periodically, I think about solar PV as our 'heat store' also includes an immersion heater and so it would be very easy to connect PV panels into our heating system. I think we've missed the boat as far as the best Feed-In-Tariffs were concerned but I keep reading about how the basic panel prices keep falling so I keep wondering if a DIY system is feasible and cost-effective these days.

 

[livias]

We have a ground source heat pump which went live last year and our experience is
Our payback from the government over 7 years is virtually the same in cash terms as we paid out so balancing off the opportunity cost v the cost of an oil boiler I think we come out ok
There are not many companies who know how to fit them and many more people who say that they do
The people we used were good and helpful on the plumbing side, distinctly less so on the admin side
It relies on a good electricity supply so we have a woodburner as a backup source of heat
Later this year the rules are changing so you have to have a meter which is another level of beauracracy (sp?) I can do without
Laying the GSHP in the garden is VERY MESSY
We have a fairly well insulated house which is essential. Don't think about it otherwise
Our multiplier is just under 4
Hm thats all I can think of at the moment
I think we would do it again

 

[fernicarry]

I had a quick look at my s/s, based on our most recent oil delivery, oil is about 4.6p per kWh. I pay 12.2p for leccy (100% renewable) and 2.5p for gas.
So if you can run at a COP of 2.7 or better then a HP is cheaper to run. I still very much doubt they will do that for the flow temps required by typical period houses or for DHW but happy to be proven wrong.

Also heating oil has been quite a lot cheaper in previous years so its a bit of a moving target. In theory as renewable generation really takes off the price of leccy should decouple from the price of fossils and tumble downwards...

livias you say you get a COP of just under 4, is that measured or based on manufacturer's literature? Are you driving radiators or UFH? What water temp is your HP producing?

 

[livias]

livias you say you get a COP of just under 4, is that measured or based on manufacturer's literature? Are you driving radiators or UFH? What water temp is your HP producing?

Measured. We have boreholes and they hit water 4m down. Water is a good conductor of heat
Underfloor heating downstairs and radiators upstairs
Water is 54 degrees
The underfloor heating is set to 20 deg throughout the ground floor 24 hours a day because the lag is a lot so there seems little point in varying it
The radiators are on TRVs
Just checked and we appear to have improved to 4.1 today. Why I dont know

The kit seems to work well but the control panel is lousy both to drive and to understand :?

 

[fernicarry]

COP of 4.1 at 54'. Thats much better than I would have imagined.

 

[livias]

Yes me too
There seems to be little objective real life info out there about them so in the end we just went with the preferred brand of a relatively local company that we felt we could communicate with. One useful resource is a book called "Heat pumps for the home" written by someone who was early on the installation scene in this country, has retired to Wales and wrote a book distilling his experience. It is a good overview of the workings and issues. What it is not useful for is in deciding what model / brand / supplier to go for

 

[MikeG]

In pure energy terms, a COP of 4 means you have achieved approximately the same as burning fuel directly in your house. And as has been noted, a COP of 4 is seldom achieved.

Electricity generation and distribution runs at approx 25% efficiency, so multiplying that up by the COP means that all you've achieved in terms of global warming is approx the same as burning timber in a modern efficient woodburner.

 

[livias]

True but

Ongoing expense is one quarter what it would be
And
Heat from a wood burner is not as useful as heating throughout he house

For us it came down to whether it was worth investing themoney up front and then having cheaper oncosts and capital payback over 7 years. We decided that it was for us. YMMV

 

[fernicarry]

MikeG, you mean 25% thermal efficiency? Isn't that ignoring non-thermal generation i.e. solar, wind etc. that makes up an increasing proportion of the fuel mix?

Not quite true to say the ongoing cost is a quarter, that's as compared to 'pure' electric. Its better than oil (65%) but not as good as mains gas (120%). Emissions-wise it ought to be significantly better than either especially with a 'Green' tariff.

If the cost of the kit is free over its lifetime then you are laughing (although its not exactly free because the rest of us are subsiding you through levies on our bills!). Its not for everyone because the difficulty of making a building suitable for this approach -- reducing heat loss etc. -- might just be too high a hurdle.

 

[Flyfisher]

COP of 4.1 at 54'. Thats much better than I would have imagined.

It probably helps that Livias' system is using a borehole as the heat source can affect the overall performance due to heat capacity and transfer issues. So, air source is the least effective because air is not a great heat conductor and thus has a lower capacity around the actual heat exchanger, which can lead to significant temperature drops and sometimes freezing. A water source is more effective because it's a good conductor of heat and also has a high heat capacity (ie can absorb/release more heat for little rise/fall in its own temperature). A moving water source such as a river or stream is an even better source because it would deliver a constant source temperature so the heat exchanger temperature would remain constant no matter how hard the heat pump was working.

I may not have explained all that very well. :?

 

[Flyfisher]

MikeG, you mean 25% thermal efficiency? Isn't that ignoring non-thermal generation i.e. solar, wind etc. that makes up an increasing proportion of the fuel mix?

Not quite true to say the ongoing cost is a quarter, that's as compared to 'pure' electric. Its better than oil (65%) but not as good as mains gas (120%). Emissions-wise it ought to be significantly better than either especially with a 'Green' tariff.

If the cost of the kit is free over its lifetime then you are laughing (although its not exactly free because the rest of us are subsiding you through levies on our bills!). Its not for everyone because the difficulty of making a building suitable for this approach -- reducing heat loss etc. -- might just be too high a hurdle.

It's complicated isn't it, and why it's so difficult to make accurate assessments of how 'green' we really are. However, I have always remembered MdB's assertion on here (I wonder what happened to him?) that energy is proportional to money, i.e. the more you earn and spend, the higher your energy consumption and thus your carbon footprint. I doubt it's a rigid relationship but it's definitely an interesting idea. So, turn down your heating, burn less coal/gas/oil/electricity, consume less energy. Replace all your lighting with LEDs, use less electricity, consume less energy. Buy a more efficient car, burn less petrol/diesel, consume less energy. Don't use the car so much, fewer miles, burn less fuel, consume less energy. The immediate result of all this energy saving will be lower fuel bills, which is generally held to be a good thing.

BUT, what do we then do with all that money we've saved? Yep - we spend it on something else and that something else will almost certainly involve the consumption of energy in one form or another. I can't prove it but my suspicion is that any primary energy savings we make is cancelled out by spending the money saved on other things.