Re: Turn your home into a mini power station...

[W T Taylor <bill-taylor@xxxxxxx>]

Bruno,

BP> I would tweak your assumptions in a couple of ways. Firstly (and
this
BP> only makes it worse), someone with the requirements you describe
would
BP> probably be better off (if you can afford the extra space) going for
BP> their 16kW twin compressor model.

That may be so, but all I was trying to do was to establish relative
economics of heat pumps vs conventional heating. I used my house
because I know its heating requirements and chose the nearest
applicable pump. Obviously if I was seriously looking at heat pumps I
would be doing a lot more research.

BP> On the other hand, I think £2000 may be overly conservative for
laying
BP> the ground loop.

Its a fair cop! I was trying to guess at the costs for installing the
ground loop, and I assumed about 3 days for 2 men and a digger, but
its probably an over-estimate.

BP> And you allowed nothing for installation of the oil-fired
BP> boiler, to set against this fairly hefty allowance for installation
of
BP> the heatpump ancillaries.

The £2000 was just meant to allow for the ground loop. I didn't
include boiler installation costs as they should be similar for both
systems.

BP> Interesting that you found a claim on Kensa's site of a COP of 4:1
(I
BP> hadn't looked at it recently, and it has changed dramatically since
I
BP> was researching my heating options).

http://www.kensaengineering.com/pdf/singleprices.pdf
gives a COP of 4.

BP> On the other hand, you can almost certainly do better than 5p/kWh
for
BP> the electricity. Underfloor heating is best
BP> installed in a screed bed, which will act as a thermal store,
emitting
BP> heat gently over a long period once it has been pre-heated.  - sort
of
BP> like a superior night-storage system.

Yes, but then you start getting into issues of responsiveness and
controllability. It's important to me that the system provides the
temperatures that I want when I want them. It may be possible to design
a system like this that works satisfactorily but it isn't a trivial
task and I would say impossible in a retrofit.

BP> One other thought - do you actually use the capacity of your 17kW
BP> boiler?

Digging out my heat loss calculations from 1990, when I designed the
system, heating load ranges from 8kw to 15kw depending on the
insulation level, with a temperature difference of 22 deg C. Add on the
hot water load and you get a requirement of 13kw to 19kw. So in terms
of the original design the boiler was the right size. As it happens I
installed as much insulation as was feasible and the controller that I
used does not allow reheating of water and space heating at the same
time, so the maximum load is probably about 8kw. But reality intrudes
here; the boiler was the smallest that I could find and I doubt that
there are any smaller oil fired boilers available today. In fact using
a slightly larger boiler has the advantage of extra output being
available if necessary with very little impact on capital cost or
efficiency, as long as a decent controller is used.

BP> I think you might find the comparison was rather kinder if comparing
6 or
BP> 8kW units.

Right, lets try.
Going back to my first example, I see that Kensa claim that some
installations can get away with VAT at 5% on the extremely spurious
grounds that heat pumps utilise solar energy. If we use that figure
for VAT and reduce the installation cost to £1000, which I think is
pretty realistic, we get a total installation cost of £5291. With a
boiler price of £1500 (a very high price) we get a capital difference
of £3791. Assuming 30Mwh of energy consumption (heating and hot
water), an oil price of 16.45p/l + VAT and electricity at 5p/kwh inc
VAT and a COP of 3 we get and annual saving of £65 giving a repayment
time of 58 years.
Using an 8kw system we have a pump price of £4247; this gives an extra
capital cost of the heat pump of £2547. But now we are only using the
heat pump for space heating so the energy use will drop. Lets say it
drops to 20MwH p.a. Using the same figures as the first example we get
an annual saving of £44 and a repayment time of 58.4 years, but you
then have to make some provision for water heating, so the capital
costs go up.

BP> I'm not quite sure what you mean by "30kW p.a.", but if
that
BP> is intended to mean 30MWh p.a.,

Yes, it should have been 30Mwh p.a.

BP> [And (while we're on the subject) have you looked at the options to
BP> reduce the heat demand - insulation, double glazing etc?

When I renovated the house I put in as much insulation as was
feasible. Two windows that needed replacing were double glazed, the
rest left as they were, but with good draught exclusion. As this is an
old house the windows are fairly small and double glazing them has an
insignificant effect on the heat loss, but a large negative effect on
the aesthetics. In fact I think that, considering the size and age of
the house, the fuel consumption is quite small.

BP> In contrast to boilers, which tend to be overspecified, Richard
BP> recommends installing a smaller heatpump than would be needed to
supply
BP> your peak heating demand. The logic is that the days on which you
need
BP> 16kW (say) are few and far between. Most of the time, if you need
heat,
BP> your demand is likely to be much less than that. So it is bad, both
for
BP> the efficient running of the heatpump and for your initial capital
BP> outlay, to install a heatpump with sufficient capacity to supply
those
BP> infrequent peak kW.

Yes, but again that is changing the basis of the comparison. Having
lived in houses with inadequate heating, I want a system that will
maintain comfortable temperatures on the coldest days, and I want it
to be maintained automatically. If you install an undersize pump you
are either going to have to suffer the cold or install a secondary
heating system, which, unless you use electric heaters, is likely to
negate the capital saving of using a smaller heat pump.

BP> Another factor that you didn't take into consideration, although
hard to
BP> quantify, is maintenance costs. Richard claims that maintenance on
his
BP> heatpumps is negligible, that they should run with almost no
maintenance
BP> for 20 years or more. Of course, he would say that, and I have no
way of
BP> knowing whether it's true.

That's a valid point. Maintenance of my boiler consists of replacing
the jet and cleaning out the heat exchanger; about a fiver for the jet
and 20 minutes time. Obviously, if you have to pay someone it will
cost you £50 - £100 a year. I agree that a heat pump should not need
any regular maintenance.

BP> But these are details. The principal reason (IMHO) for choosing a
BP> heatpump (or almost any other form of heating) over an oil-fired
boiler
BP> is because burning oil to heat your house is a poor use of a scarce
BP> resource (whether or not you believe in global warming).

Unfortunately that is another argument that doesn't hold water when
you examine it. In this country most of the electricity we use is
generated from fossil fuels, mainly coal and gas. Electricity
generation and distribution is a pretty inefficient process, according
to my book on heat pumps around about 30%. If your heat pump has a COP
of 3 you get an overall efficiency of 90%, which is pretty close to that
of a modern domestic heating boiler. By using an electrically driven
heat pump you haven't saved any energy, you've just moved the
pollution from your doorstep to someone elses.

BP> And perhaps the next most important factor is fuel-price volatility.
BP> Fuel prices may currently be at a point that favours use of oil. But
BP> history suggests that they won't stay there.

That depends on your interpretation of history. The only time that I
know of when heat pumps have approached break even is the early
seventies. Oil prices became very high for a period due to a
particular set of circumstances when the OPEC countries had a virtual
monopoly on oil supply and were sufficiently united to be able to
force a price rise. The result of that is that oil users search for
other sources of oil and other fuels. Ultimately this has lead to the
breaking of the power of OPEC and, until recently, a surplus of oil.
Yes, oil prices are volatile, but they haven't reached the levels that
they were in the 70s over the last 20 years.

BP> I would think that it would be
BP> prudent to do your financial modelling on the assumption that prices
BP> will move against fuel-oil.

It is certainly possible that oil prices will rise dramatically, but
if they do other energy prices will eventually follow. You can only
make an assessment on current knowledge. If you take the view that
fuel prices are going to increase significantly in the fairly short
term, whilst electricity prices stay at their current level, you have
a reasonable argument for investing in a heatpump.

If oil became too expensive I would stop using it; I'd probably use
wood as I have enough to be self sufficient in it. It's
environmentally neutral as well.

My post was just meant to demonstrate the point that, currently, heat pumps
are not economically viable. I used oil as the fuel, because that was
the fuel that I had figures for, not because I advocate oil as a fuel.
If I had used gas as the comparison fuel heat pumps would have looked even
worse.

BP> These are just a few extra details, which may not affect your
BP> conclusion. Richard at Kensa may well have stronger reasons why the
BP> financial argument is not so biased against heatpumps as you think.
I'll
BP> forward any information he wants to provide to the list.

I would be interested in seeing his comments, but I rather think that
most of the other subscribers to this list may well be less
interested: the subject has become rather off topic for a home
automation list!

Bill