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

[Bruno Prior <bruno@xxxxxxx>]

Bill,

I tend to agree with a lot of what you are saying. I went for a heatpump
primarily because of the environmental rather than financial benefits. I
was trying (unsuccessfully) to present Kensa's arguments for why their
kit isn't expensive, but I'm not totally convinced either, although I
hadn't done the detailed maths like you. All the same, there is a big
gap between what you have calculated and what Richard put to me. I'm
going to forward your message to him, and see if he wants to explain how
the maths might work differently.

I would tweak your assumptions in a couple of ways. Firstly (and this
only makes it worse), someone with the requirements you describe would
probably be better off (if you can afford the extra space) going for
their 16kW twin compressor model. This would be much more efficient to
run, as it is effectively 2 8kW units, so you can run at only 8kW when
that is all the house needs (which would be all but the coldest days),
whereas the 14kW unit gives you a choice of 14kW or nothing. But, as I
say, that might make matters worse financially, as it would increase the
capital cost.

On the other hand, I think £2000 may be overly conservative for laying
the ground loop. I was in the lucky position of working in a business
where we have trenchers working for us a lot of the time, so I can't
talk about the market rate for a one-off installation, but £2000 seems
over the top for what (in dry weather) should be no more than a couple
of days' work. And you allowed nothing for installation of the oil-fired
boiler, to set against this fairly hefty allowance for installation of
the heatpump ancillaries.

Interesting that you found a claim on Kensa's site of a COP of 4:1 (I
hadn't looked at it recently, and it has changed dramatically since I
was researching my heating options). I think Richard would be more
confident with a figure of 3:1 (or a little over), so I'm not sure they
should be using 4:1 on the site. From what I've been told, I agree that
4:1 is "optimistic".

On the other hand, you can almost certainly do better than 5p/kWh for
the electricity. Heatpumps are much best suited to underfloor heating
(they run very inefficiently if trying to produce the sort of
temperatures you need for radiators). Underfloor heating is best
installed in a screed bed, which will act as a thermal store, emitting
heat gently over a long period once it has been pre-heated. So you
should be able to do most of your heating outside peak hours (say run it
for a few hours early each morning) - sort of like a superior
night-storage system. In which case, you should be able to get a
suitable contract with an electricity supplier to supply electricity at
cheaper rates than 5p/kWh at the times that you would use most of your
power. Not that this is going to change your conclusion - the payback is
still too long.

One other thought - do you actually use the capacity of your 17kW
boiler? Most boilers were dramatically over-specified in the past. I
think you might find the comparison was rather kinder if comparing 6 or
8kW units. I'm not quite sure what you mean by "30kW p.a.", but
if that
is intended to mean 30MWh p.a., then you are only utilising around 20%
of your boiler's capacity, which is probably realistic. That may still
mean you need 17kW for peak demand, or it may be that you could scale
down a bit. Have you done a heat load calculation for your house?

[And (while we're on the subject) have you looked at the options to
reduce the heat demand - insulation, double glazing etc? These are
almost always a more financially and environmentally efficient way of
saving heating costs than complex solutions like heatpumps. The best
result might be achieved by a combination of the two.]

In contrast to boilers, which tend to be overspecified, Richard
recommends installing a smaller heatpump than would be needed to supply
your peak heating demand. The logic is that the days on which you need
16kW (say) are few and far between. Most of the time, if you need heat,
your demand is likely to be much less than that. So it is bad, both for
the efficient running of the heatpump and for your initial capital
outlay, to install a heatpump with sufficient capacity to supply those
infrequent peak kW. If your peak demand was likely to be 16kW, he'd
probably recommend installing a 12kW unit, and if your real peak demand
was less than the capacity of your existing boiler (very likely) he'd
probably be recommending a smaller unit still. On the really cold days,
you top up your heat demand with something like a flow boiler (fancy
name, from what I can tell, for an electric heater built into the
heatpump system). Of course, the flow boiler is an extra and pushes the
capital cost up again, but not by as much as you saved by installing a
smaller unit.

There is another factor in this, which hasn't been mentioned, and which
again shades things against heatpumps on a financial basis. Your boiler
is presumably also providing hot water. The heatpump can supply hot
water too, but you will need a buffer tank (more expense and space) or
preferably a pressurised hot water system (even more expense and space).
And the heatpump cannot achieve 60 deg.C., which means it cannot raise
the tank periodically to the temperatures required to prevent
legionella. The water it produces is more than hot enough to supply
steaming baths and showers, but it needs topping up with another heat
source occasionally, for legal and health reasons. So you also have the
capital cost of the additional heat source.

That is one of the main reasons I have an LPG boiler - it is not my
primary heat source, as you guessed, but (a) the supplier of occasional
very hot water, and (b) a backup in case there is a problem with the
heatpump. Richard swears that (b) is unnecessary, but I wasn't inclined
to gamble, as there is no service contract available. Richard's answer
to (a) is to install a solar hot water system for hot water 80% of the
year, with the flow boiler providing the extra heat when the solar
system can't deliver. It's a nice idea in theory, but not practical for
many people (including me - my house is listed, so I can't put solar
panels on the roof), and involves yet more expense.

As you can see, the detail (that I am aware of) tends to reinforce your
calculations. On the other hand, you do get benefits with the heatpump
that weren't considered.

For me, the biggest benefit is that it can be run in reverse (this is a
Kensa speciality, although another additional extra), so it can provide
cheap chilled water in summer, to pass through a basic fancoil forced
ventilation unit to cool the building. This is another reason for the
LPG boiler - it allows me to continue to supply hot water to the
pressurised hot water system whilst running the heatpump in reverse
producing chilled water. Of course, this is yet more expense, which
probably would be little better (on a financial basis) than installing
conventional air-conditioning systems, but it does make for a very tidy
and efficient (and therefore environmentally-friendly) installation.

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

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

And perhaps the next most important factor is fuel-price volatility.
Fuel prices may currently be at a point that favours use of oil. But
history suggests that they won't stay there. My heatpump insulates me
far better than would an oil-fired boiler against whatever fluctuations
in price may occur in the future. For example, you might want to look at
emissions trading, and the impact that people in the energy market are
predicting for it. Some are arguing that it could affect fuel prices by
as much as 40%, athough I personally doubt any government would tolerate
that. But they are more likely to implement measures to restrain the
price of electricity than of fuel oil. I would think that it would be
prudent to do your financial modelling on the assumption that prices
will move against fuel-oil.

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

Cheers,

Bruno


W T Taylor wrote:
> BP> Yes. I've got one in my barn conversion. Got a few teething
troubles
> BP> with the controls to sort out (mainly to do with how it
combines with
> BP> the LPG boiler), but basically it works fine. Just a question
of getting
> BP> the right bit of kit having priority at the right time (it's
quite a
> BP> complicated system).
>
> BP> For a UK manufacturer, have a look at Kensa Engineering's
website -
> BP> http://www.kensaengineering.com.
>
> BP> The statement in another message that the payback on all these
> BP> technologies is too long is probably wrong for heatpumps now.
You can
> BP> get £1200 grant from the government towards the cost. Take that
off the
> BP> typical £3000 - £4000 for buying and installing an
average-sized
> BP> domestic heatpump and ground loops, and it's not much more
expensive
> BP> than buying a normal boiler. In theory, it shouldn't take long
to make
> BP> back the money from the energy savings. At least, that's what
Richard
> BP> Freeborn at Kensa argues. In fact, he had some justification
for why his
> BP> kit was no more expensive than a boiler, though I struggle to
see how
> BP> that is not an exaggeration.
>
> Thanks for the link, interesting to get upto date info on heat pumps.
>
> Unfortunately that site rather confirms my assertion that you will
> never recoup your capital outlay on these systems.
>
> Currently my house is heated with an oil fired boiler with an output
> of 17kW. Its getting on, so I'm thinking of replacing it. Looking at
> the SEDBUK site the most efficient boilers are from Grant. A suitable
> model is the Vortex 15kW, costing about £1300. Looking at the Kensa
> list, the most suitable heat pump gives a nominal output of 14KW;
> unfortunately this pump costs £5230 + VAT, a total of £6145. Take off
> your £1200 grant leaving a cost of £4945. But its going to cost a lot
> more to have someone install a heat pump, 'cos someones got to dig a
> big hole in the ground to bury the heat collecting pipe. I would guess
> that this would cost in the region of £2000. This gets you to a total
> capital outlay of £6945. Take off the boiler price leaving you with an
> excess capital outlay of £5445.
>
> I spend about £400 p.a. on oil. Even if you pay nothing for your fuel
> it will take nearly 14 years to recover your capital costs. If you
> take the efficiency that Kensa give (a ludicrously optimistic figure)
> you will get 4kw out of the heat pump for 1kw input. If you pay 5p per
> unit that gives you a cost of 1.25p per unit of heat delivered.
> Currently oil is about 16.5p per litre, which works out at about 1.9p
> per unit of heat delivered, assuming 90% efficiency (the best boilers
> are about 97% efficient). On the basis of 30Kw p.a. heat used this
> gives an advantage in running costs of about £190 p.a. to the heat
> pump. On this basis it will take 26 years to recover the capital
> costs. If you use a more realistic, but still optimistic, figure for
> efficincy of 3kw out per 1 kw in you get to a saving of £80 p.a. which
> will take you 78 years to recover.
>
> In fact these are simplistic figures which exclude things like loss of
> interest on capital, so they are biased in favour of the heat pump.
>
> I see that you are using LPG for your main heating system. As this is
> the most expensive fuel available (apart from peak rate electricity) a
> comparison is likely to favour a heat pump more; but then you can't
> take the boiler price off your capital costs so the repayment period
> is still likely to be large.
>
> Heat pumps may become economically viable if fuel prices rise
> significantly; I believe in the 70s they made sense; but at the
> moment, with rock bottom fuel prices I am afraid that they don't.
>
> There may be good reasons for installing a heat pump; economics is not
> one of them.
>
> Bill