Things progress on my cost benefit analysis of putting a heatpump in:
since I know I am not the only swine around here being thwacked by the
high price of CH oil, I thought I'd share some of the salient points of
a conversation with the 'heat pump' salesman.
First of all, the basic situation here is a rural location with plenty
of land available of sodden clay.
My total requirements are around 10-15KW output, and the house is
largely, though not completely UFH.
I only have a single phase, but its a new installation with good
upstream capacity - me own substation in the garden corner. :-)
TOTAL installation costs look to be around £10,000 but with a
conservative 3:1 gain over electrical input, that places my average cost
per KWh heat delivered at 1/3rd of the cost of electricity - say around
13p a unit averaged between day and night. It is expected that the unit
would run 24x7 anyway, so lets say around 4.3p/unit cost. That compares
with pul currently at 50-60p/liter, or around 5-6p/unit (Kwh)
My consumption s around 6000 liters a year, so thats a saving of 0.7-1.7
x 6000, or between £420 and £1000 a year MINIMUM. f course the salesman
reckoned on more..but a ten year payback at current rates was felt to be
a decision point, and as I cant see anything getting better in oil
versus electricity prices (base cost to grid of coal/gas/nuclear
electricity is around 2-4p/unit: gas going up quite sharply), its
probably a decision that will look better rather than worse over time.
So I am planning on a next summer install, tentatively.
The heat pump itself is about £4000, with another £1000 for actual
installation and commissioning by the company, plus around £1000 budget
for ancillary bits and pieces - electrical, maybe pressure regulators
vessels, etc etc.
That is the bits inside the house. Outside I need to lay around 300
meters in the biggest loop possible of 40mm pipe, at 1m depth or more.
Apparently soggy wet clay is tops for this, and efficiency improves the
better the garden heat exchanger is. In clay he recommended use of a
trencher rather than digger, as getting the subsoil tamped back around
the pipe is the best way to get heat transfer, and merely backfilling
clods is not good..the trencher tends to produce more 'crumby' type
grade of spoil, and this is better. 300m is the longest run of single
pipe that is practicable: larger installations use a manifold and two
loops of pipe.
In other soils its is apparently recommended to backfill with a load of
sand as this when wet gives good heat conduction to the pipe.
thse bits and pieces are probably getting on for £1000 of pipe and
insulation a,d an estimated £1000 to hire a trencher, and generally
bugger around with the garden: we would actually lay some armour cable
and mains water pipe for hoses at the same time, so there is a bit of a
plus there as well.
Other issues outside are the need to insulate the pipes where they are
close together as they approach the pump to avoid heat transfer between
them. The pipes are filled with a glycol type antifreeze ('brine' is
apparently an incorrect term used for historical reasons) and on
account of the nature of this fluid, no mechanical joints are permitted
in the pipes underground. Insulation on the pipes is via a impermeable
high density expensive butyl foam sleeve, which he said 'is manna to
rats and mice: never use it above ground' ..so where the pipes enter the
house I am thinking of a small 'construction' filled with expanding foam
to avoid heat loss on the return loop.
The actual unit is massive. Although its on a 600mm square foot print,
its 2 meters high with potentially another 200mm above to carry pipework
in: there are also potentially extra expansion vessels needed. Fitting
this in will be my biggest problem I suspect.
The basic workings are that the ground circuit heats a refrigerant -
standard gas type AFAICT on one heat exchanger, and then that gets
'pumped' to supply the hot water heat exchanger: that takes the primary
circuit to 45-50C, and a direct immersion heater is inbuilt if you need
to go above that for e,.g. health and safety hot water tank reasons. Or
possibly to run my upstairs fan convectors that won't run off less than
65C (thermostats in them), but could be removed I suppose...
The Man was very hazy about controls: he didn't seem to think that
timing as possible, and the units generally run 24x7 (which betrays
their Scandinavian origins) based on return water temps. I hope to get
more information on this later.
I'd be looking to tweak the installation to essentially use it ff peak
as much as possible, and also to only use the hot water immersion heater
to 'top up' the temperature to our nice sizzling 60-70C we like late in
the night at the end of 'off pek' times, with probably a boost override
on a timer for when SWMBO has left the hot tap running in the sink and I
want a bath..
Likewise the internal immersion would probably want to run to boost the
upstairs radiator/convector systems - generally on a timer for toasty
morning baths, and to put some heat into te bedrooms before going to bed.
Soundwise he reckons the units is no noisier than a domestic
refrigerator - certainly better than the normal balanced flue roar from
the oil boiler, and the stink of burnt kerosene would be nice to
eradicate as well.
The only other control is apparently an air temp sensor that goes up
under the eaves on a north west facing wall. Thats the easy side of the
house as far as I am concerned.
The other £2000 is really to cater for e fact that to fit the monster
in, I will have to totally rip the utility room apart, and lay circuits
and pipes to connect it all up and make good afterwards.
SWMBO is delighted as she doesn't use what is in there anyway, and wants
other stuff fitted,...
Apparently heat spillage from the unit is lower than a conventional
boiler due to overall lower operating temps..this is not an issue in our
case as that room is stifling in winter, as all the UFH circuits run
under its floor to connect back to the manifold.
Electrical requirements he was hazy on, saying that an additional 'soft
start' unit would be required on single phase: it looks like the normal
operating current with - say a 3KW heater and a 12Kw pump operating at
4:1 thermal gain would be around 6-8Kw total. So some kind of 30A type
breaker and 30A cable should suffice. I am not clear as to whether the
thing modulates, and if so what efficiency gains or not are available.
I am more or less committed mentally to starting planning this for a
next summer install: that's the time I don't use CH at all, beyond the
oil fired Aga (we will keep that: its peanuts compared with the boiler
consumption, and is very efficient as a space heater: I wont discuss its
cooking qualities here in tis thread.), and when ground conditions will
If anyone wants to abstract any of this for the basis of a wiki, feel
free. My gut feeling is that heatpumps if you have the room for a decent
ground or pond source, are very cost competitive right now against oil,
or tanked butane, are actually probably no worse than gas, on a new
install: however the lower operating temperatures at which the
efficiencies are obtained do mean some kind of pretty large radiative
surface is needed. UFH is good, and has thermal mass in screed to take
you through the day allowing most of the energy to be off peak..but a
more typical small house installation is likely to be less cost
effective as up to three times larger radiators would be needed..or a
backup heat source to take the temps up to around 65-70C.
One thing he did say, was that with ground source, trying to use pipe
spirals to minimise the ground loop size was almost a complete waste of
time. The key here is I think that there is an incident solar energy per
unit ground, that is what feeds the base of the pump heat cycle, and
there is no substitute for ground AREA. Even vertical bores are not as
good, as eventually the localized ground are becomes cool, and
efficiency drops when you need it most..at winters end.
I didn't discuss air source pumps. or indeed the possibility of
refrigerating my floors or running cold air convectors upstairs in
summer, but I think this is possible.
That's all so far. I hope it helps others who are considering this route
to get some feel for the cost/benefit and installation issues.
For the greens, I will simply say that if I end up abstracting 50% or
more of the heat I need FROM the atmosphere/ground, and with nuclear
electricity providing the motive power, it makes far far more sense to
me to cool my garden to heat the house..than any amount of silly solar
panels and windmills ever would.