Propane heaters

I have avoided doing heating loss calculations - because it is a minefield. But you have shamed me into doing a few figures.

Some time ago I wrote a little program which allowed me to do the heat loss calculations for my house, under different circumstances. My house loses 137 watts per 1C increase in temperature relative to the outside - windows and doors 63, walls 39, ground 24 and roof 11.

The garden room will lose 90 watts per 1C increase in temperature - windows and doors 46, walls 10, roof 28 and ground 6. It sounds quite a lot compared to the bigger house; but the roof loses a lot compared to the house and there are a lot of windows.

That was the easy bit, but converting that into cost is the difficult bit. Last year my house cost £494 for gas (I don't keep it very hot) - excluding the daily standing charge. If I heated my garden room to the same amount it would cost £494 x 90/137 (relative heat losses) x 16.57/5.05 (electric/gas cost) x 50/100 (gas boiler is 46 years old and not much more than 50% efficient - also some house gas is used for cooking). Result £532 to heat the garden room to same temperature as my house, using electricity.

But what is the temperature of the house? The heating is off at night and most of the day - so the average house temperature is lower than any thermostat setting. And if the garden room is set to a lower temperature than the house, as you said Kristen, the heater would be on for a lot shorter time as well as having to maintain a lower temperature. Without statistics on daily temperature profiles its virtually impossible to calculate the actual heating cost.

But one thing does come out. If its +4C inside and -16C outside, the 20C difference will only need 1.8 Kwatts (20 x 90) to keep it frost free.

 

(Although if the power supply will cope with 3kW then I guess buying a heater that size would give you some spare capacity)

Plus on days that are sunny there will be solar gain in the garden room, and the back wall will store some heat for the night.

I reckon you will heat the garden room to 10C less than the house, and that cost will halve for each 5C drop, so the cost will be £532 / 4 ... which is what JWK predicted

But that's for last winter, this winter is anyone's guess

 

Yeas, absolutely. I said its a mine field - there are so many other aspects to take into account. And in houses which are vented under the floorboards, you treat the exterior temperature the same as the air temperature - according to a set of tables. But in a garden room with a solid floor, when its -16C outside, I suspect the ground will actually supply a bit of heat. - a minefield.

I saw one source where a chap had experimented with the heating cost for different thermostat settings in a greenhouse. The figures indicated that the heating cost doubled with an increase of just 2C. I can believe that. With a setting of 4C, a change of just 2C might involve the heater being activated twice as often. Its obviously less sensitive as the thermostat setting increases.

 

You could consider an air-source heat pump. My, naive, view is that when you want heat it will be cold outside, and air-source will be incredibly inefficient at that time (very low COP value). Ground-source would be fine (2M down always being warm), but completely unrealistic to install for a single-area like a garden room.

 

I was interested to see the cost of energy from heat pumps quoted in your cost table link. I note that they were little different from household gas. which is perhaps why we don't hear much about them. When I was at school they were supposed to be the coming thing. But then we didn't have North Sea gas at that time. I think they are supposed to be most efficient when the other end is in a river. You can freeze the ground in a ground base one, and the air may be very cold in an air based one, but a river end is always above freezing and has good thermal transfer. But in every case you have a big capital cost.

You, Pete and others have convinced that electricity is the way. I already have two 3Kwh heaters, and there is an advantage in using two heaters to get the heat spread a bit more evenly. Propane has a capital cost and there would be an additional problem with condensation. Its already bad just from the plants. I think what happens is that pots are left out in autumn when there is heavy rain. Then these very wet pots are taken inside and take weeks to dry out, and all the water that comes out of them has to be deposited on the walls and ceiling - there is nowhere else it can go. Its noticeable that at the end of February, when its at the coldest, the condensation almost stops - there is little water left in the pots.

 

Not heard that. I think unlikely 2M down, but no personal experience. It is also possible to install ground-source in a borehole (where space is tight presumably), but that must cost-a-bundle (although perhaps handy for anyone who also wants an irrigation supply)

No kidding! Particularly if you put ground-source in, and I think air-sourced (for a decent sized house) is likely to need 3-phase juice, and Electricity Monopoly Sharks charge an outrageous connection fee, even when the pole is only a few hundred yards away.

I think heap-pump is only any good for under floor (i.e. low temperature) heating too. I considered underfloor for our Orangery, but decided that for frost-free the response time was too slow (e.g. on the occasional -15C night) and unlike a house, where always-on is a viable option, frost-free heating would be usually-off, so I've gone for Rads there.

For the conservatory, which we think we might use as a "livable room", we have opted for low capacity underfloor and small radiators for top up. Thus on sunny days in Winter the floor slab won't have a huge reservoir of heat pumping out. Time will tell if they are good decisions ...

 

Kristen - I can certainly believe that a heat pump can freeze the ground round it. I am not saying that the ground itself would normally be frozen 6 feet down. But if you keep pumping heat out in winter then you are lowering the temperature of the ground. Heat will flow back in from the surrounding soil - but only slowly and perhaps not fast enough. Thats why pipes in a river are much better - there is a much faster replenishment of the heat that has been pumped away.

I have never been a fan of underfloor heating because it is so slow to react. The benefit being that you have no radiators. I can understand the arguement for base heat in a house. But then you still need to top it up using radiators - and that defeats the original objective.

I watched underfloor heating being installed in an extension in the house over the road. They buried pipes under a mass of concrete. What happens if there is a leak?

I went round the northern headquarters of the Electricity Board many years ago. They had a superb set up - the sort of set up that you can only have when you are spending other peoples money! They had a virtually zero energy building. I don't remember the exact figures, but when it was about 38F outside they could maintain 70F inside with no heating. There was very good insulation, and all the lights and photocopiers etc produced some heat. On sunny days, the sun heated the building up and they had a heat pump that extracted the surplus heat and stored it in a huge 'swimming pool' under the building. On dull days in winter the heat pump reversed the process.

The indoor temperature was constant all day and all year, and I saw pineapples growing and bearing fruit as houseplants.

 

We are installing it in the extension; its a first for me. We have it deep in the concrete (which si not common, more usually it is in the screed). The idea is that the whole of the concrete slab becomes a heat sink / source. The extension is being built to passive house standards (in essence much like the Electricity Board building you saw), and as such should need no heat at all (a passive house equivalent to a 3 bedroom detached house size needs 1kW of heat when outside temperature is -10C. Not worth installing central heating for that, for the few occasions it happens you run a 1-bar electric fire, so the money you save not installing a boiler and plumbing central heating you spend on insulation instead. However, we already have a boiler in the main part of the house and what we are building, attached to the house, isn't "ideal" Passive House, so we are installing heating "Just in case". I'll tell you next year if it lives up to that, or not!)

With good insulation the idea is that you provide heat [underfloor] at ground level, where the people are sitting etc., and there are absolutely no draughts from cold air "falling" on the glass surfaces (inside glass must be no colder than 4C below room's temperature) that it feels much more comfortable, and as a consequence the room can be maintained 2C lower than a room with normal double glazing / draughts.

Yes, underfloor heating means no radiators on the wall, and you can put the furniture anywhere. But ... there is no underfloor heating under the units in the kitchen, or the fridge. And there is none under where the dividing walls are ... so ... if we want to rearrange the kitchen, or the internal walls, in the lifetime of the building ... then there will be problems. We also won't have individual room thermostats, but rather balance the heat into the floor on the supply manifold. The technology is well understood, and people I trust say that it will be excellent, but its all First Time to me. Being able to walk around barefoot on a stone floor will be nice though

I have insisted that we have a radiator installed in the (new) kitchen, which has horrified the heating designer. My argument was "If we never use it that's fine, but I certainly don't want cost/upheaval of retro fitting one".

Sorry, not relevant to this thread.

 

No but very interesting.

I am a great believer that you can enormously reduce heating cost with the right design. For instance if you lived in a large thermos flask you wouldn't lose much heat at all. But a front door on the roof wouldn't be so convenient.

 

Limited oxygen supply would be a bit of a downer too!

Passive House has mechanical ventilation (with heat exchanger between Exhaust and Inlet air) which actually provides more air changes than conventional trickle vents etc. So by any metric air quality is very good Could probably fit one of those in a thermos flask I guess

 

For anyone interested:
en.wikipedia.org/wiki/Passive_house‎

90 second soup-to-nuts video explaining it:

 

Thanks Kristen - yes thats exactly what the Northern Electricity building was based on. Though, in addition they had this huge heat sink/source in the basement.

I just ran my house heating program again, and even with triple glazing - the greatest heat loss for my house is through the windows, albeit the walls are close behind. Having said that I think the u values for modern double and triple glazing are much improved - as is wall insulation.

 

Passive house would have, in broad terms, walls, roof and floor U-values of 0.10-0.15 or better, windows 0.80W, and air-tightness of 0.6 air changes per hour (current building regs = 10 air changes per hour!) and an annual heat requirement of 15kWh/m². Current UK housing stock is around 225kWh/m² :(

 

I didn't know that u values got that low. So I have just had an informative Google. I have assumed 2.1 for my glazing - I am not sure of the actual figure. Its a minefield finding u values - they all seem to coflict and nothing is dated so you have no idea if you are looking at old or new data.

I have just seen a 2005 document stating a government maximum of 2.0. I think I must ask my supplier.

 

The Brickies keep joking that I could lose a tank in the cavity! Its something like 3x the normal width of insulation.

Indeed. One of the pitfalls is that, for windows, the u-value is often quotes as the centre of the pane of glass, whereas in fact the frame will normally be far worse. Passivehouse requires that whole-window u-values are tested and quoted, which should at least provide a more level playing field!

But there is no way to know, for sure, if they have used, say, Argon gas fill or the glass coatings they promised to ... I've seen some horror stories around that too.