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Renewable Energy: Ground source or LPG boiler for underfloor heating?

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    • CommentAuthorEdF
    • CommentTimeMay 10th 2007 edited
     
    I'm shortly to install underfloor heating in an outbuilding conversion which will make a 2 bedroom bungalow. The house will probably be occupied by only two people and our intention is to rent it for a few years and then move into it ourselves .. We had intended to use a ground source heat pump as we have a large field next to the property. However, I understand the heat pump will use nearly 3KW of electricity all the time it's running, but I do realise that one 'gets out' about 9KW. Underfloor heating has to be left on for long periods, days rather than hours and I fear that our energy consumption may work out more than at present. We certainly don't use 3KW of energy constantly now. My wife and I live in a large house which has LPG wet central heating with radiators, installed before we bought the place. We only have the heating on in three rooms for a couple of hours in the evening and morning in the winter, our main heat source being a large woodburner in the sitting room. As I get into my dotage, however, in about 10+ years time I don't want to have to cut 4 tons of wood a year (we live in the Highlands and are surrounded by managed woodland and there's a ready supply of firewood not suitable to go to the mill). The very high cost of a heat pump and underground loop is also a main concern, and I am concerned that we will never see a return on the extra cost and in the meantime the electricity bills may be much higher than at present.
    I would be interested in opinions as to which way to proceed. At the moment we favour the UFH with an LPG boiler, with the possibility of changing over to a ground source heatpump in a few years time should the cost fall. I cannot afford the extra cost of electricity generation to offset mains useage. I can see UFH with a heatpump would be a boon if one has a large house to heat most of the time and one has very high heating bills, but I am sceptical about it in our case.
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    What about using a log or multi-fuel stove to heat the water for the underfloor and your domestic hot water coupled with a solar panel for summer hot water. Surely you can get logs delivered ready chopped? A cheaper set than up than a GSHP and, in such a heavily wooded area, better fuel security than LPG.
    • CommentAuthorpyrogaz
    • CommentTimeMay 10th 2007
     
    I think the first question you need to consider is, how much heating will the property need? Assuming that the conversion will comply with current Scottish building regs, you might be pleasantly surprised by how little heating is required. I live in Caithness and am now on my third house build, I am a bit of a zealot when it comes to insulation and aim for average floor, wall and loft U-values of 0.15, 0.18 and 0.10 respectively which is about 1/3 better than regs. With an LPG condensing combi, low thermal mass UFH, a gas oven and hob, we use about 900 litres of LPG per year in a 120m2 bungalow. If you stick to the minimum insulation required to comply with regs this would probably rise to 15-1800 litres per year.

    The efficiency of your house also dictates how you might install UFH. The normal solution with underfloor heating is to bury the heating pipes in several tonnes of floor screed, which is then heated to give a kind of thermal store effect. My own experience of this type of set-up is that it takes a long time for the system to react from cold, but then gives out too much heat for several hours after rooms have reached their desired temperatures; this is particularly true if the house is well insulated. However you tend to find that your heat source will run for say 4-8 hours to heat the slab but then stay off for the next 12-24 hours whilst the slab emits heat in an uncontrolled manner.

    My preference is a low thermal mass set-up, i.e.heating pipe directly on top of thick insulation with a floating floor above it, you can do a similar thing with a suspended timber floor; this gives a more controllable heat output. Your heat source will cycle on and off a bit more often but in general you should use slightly less energy as won't have to open windows to dump heat in the middle of the day when a warm slab is emitting more heat than you need.

    Overall I would go for the option you seem to be favouring, LPG to start with and then consider your options later on if the gas bills are higher than expected; you can still add in solar hot water panels or a wood burner at a later date as Chris Wardle suggests; however I'd be amazed if you were able to install a GSHP in a new conversion and ever get a financial payback.
    • CommentAuthorEdF
    • CommentTimeMay 13th 2007
     
    Thanks, guys. I'm against a wood/multifuel stove to power any form of central heating simply because I tried it once in a previous house in Scotland. The stove had to be blazing away to power the few rads (using loads of wood) and the heat went to the hot water tank by gravity first. Once we went to bed then the rads were cold by 1.30 am because the fire had burnt out and in winter we woke up to a freezing house with no heat for half an hour until I had re-lit the stove. I suspect, looking at the above, that it may work better with UFH with the 'thermal store' effect and lower temps required.
    I can buy logs at about £10 a ton but delivery is about the same again. Buying split logs is not easy; if one can find a supplier the price is much higher and the wood tends to be green. Although it sounds like it's an ideal area to obtain wood, getting transport arranged is VERY difficult. I'm physically beyond loading a trailer myself.. Most timber trucks are busy on contracts and are huge artics and most cannot negotiate our narrow lane and drive.
    Pyrogaz - thanks for that, I take your point. I can't install a suspended wooden floor as everything is geared to solid floors, including the plan, spec. and the building, and work kicks off (again) very soon. At the moment I have a typical stone steading 50ft x 15ft with a concrete 'byre' floor and a new large cavity wall extension waiting for it's solid floor base. (We live near Beauly.) We're going for insulation values over the spec.
    What do you mean by a 'floating floor'? Wood? We were planning a concrete subfloor with insulation on top, then UFH then about 2" of screed on top of that. I''m having a small woodburner as a room heater in the sitting room. We tend not to use much hot water as we favour an electric shower. Perhaps with a new combi boiler we may not use one again, though. In the present house I could never see the point of heating 40 gallons of water to have a shower or to do the pots!
    I've found that the most economical way of heating the present house is by using the woodburner in the sitting room (room heater only) which also heats the hall and landing and by having a wall mounted convector in the bedroom, which uses about 200-300 watts/hour in the winter and a radiant electric fire in the shower room. Not particularly 'green', but the radiant is only on for 15 minutes and the convector cuts in and out and it's all far cheaper than running the central heating.
    I've been left in the poo by a builder who was originally doing the entire job but cleared off shortly after getting a staged payment. (awaiting the court hearing...) I've got other tradesmen coming but the planning and materials sourcing is now in my lap and it's a bit daunting! It's also much more expensive to pay a brickie, joiners, tiler, slater, sparkie and a plumber than one man who was going to do the lot.
    • CommentAuthorroger247
    • CommentTimeOct 9th 2007
     
    as an ex UFH company person and now a air to water specialist please let me put you right. UFH is required to be on constant, but you use correct control and thermostats to regulate the room temperatures and link to outside weather compensators. The rooms are zones and only require hot water (35 - 40c) in low demand because they self circulate instaed of rads flowing around the dwelling all the time. The better air/water heat pumps use inverter technology which means power in steps and better efficency, no bang on and dead stop! These once in full operation will mainly run at 45% capacity. The cost half as much as ground source and NO digging or drilling! Please if you want to know more ask me as we all need to learn more because time and energy is running out - rogeratkins@tiscali.co.uk

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