RE: Ground Source Heat Pumps and HA (OT)

["Neil Ball" <neilball@xxxxxxxxxxxxxxx>]

I've got a ground source heat pump which has been running fine over
the last
12 months since it was commissioned. As long as your heating system can
cope
with lower flow temperatures then a heat pump can be worthwhile considering
following the major price increases in oil and gas recently. However you
need to look at the calcs carefully to see if it will be appropriate for
your needs. The seasonal co-efficient of performance (COP) is key, this is
the measure of efficiency - how many units of heat energy do you extract
for
a given unit of energy input. The heat source also plays a part in this,
the
closer the heat source (e.g. the ground) and heat sink (the water in the
heating system) are then the better the COP as well. If you have a well
insulated house heated by underfloor heating (so only need a flow
temperature of up to 40 degrees) using ground loops in excess of a metre
deep then seasonal COPs of 4 or higher are achievable. If you have an older
house heated by radiators that need flow temperatures of 60 degrees plus
then you begin to reach a COP of 1 i.e. you may as well have used an
immersion heater instead. Some decide to fit a heat pump and boiler
combination so that in spring/autumn the heat pump can meet the load but in
winter the boiler takes over when the heat pump cannot cope. This is often
controlled by the built-in controls of the heat pump.



You also need to look at the electrical loads required for the heat pump
and
be sure your current supply can cope - the compressors draw a large current
on startup, my 2.2kW compressor can pull 30A for example and that is with
soft-start control. There are some inverter-driven speed-controlled
compressor models starting to reach the market now and these have much
lower
starting currents and can match the compressor output directly to the heat
demand at any given time but these are more expensive and not very common
just now.



If you decide a heat pump is worth using then you also need to factor in
how
it interfaces with your heating system - larger units use a thermal store
as
a buffer vessel as you need quite high minimum flow rates to keep the heat
pump happy. In smaller units you need to keep a number of loops (UFH) or
radiators uncontrolled to maintain the minimum flow rates required. Most
modern units employ weather compensation to try and minimise the flow
temperature from the heat pump wherever possible thereby keeping the COP as
high as possible. You will also find that many do not provide any form of
external interface for data exchange or even simple heat demand control.



Finally you will find there are many companies selling heat pumps who have
no engineering background at all - they are really professional salesmen
&
women who have moved on to the next big thing. These companies often
dismiss
the problems of low efficiencies with radiator based heating systems,
ignore
any controls issues (I've heard salesmen telling potential customers that
as
the unit is so efficient it simply runs 24/7 and if you are too hot simply
open your windows!) and use the high pressure sales tactics that were
common
with double glazing etc. If you had mains gas then it will still be more
efficient and green to use a high efficiency condensing boiler to generate
the heat at source rather than using a heat pump which is fed from the grid



As mentioned, the self-build group would be worth a post or two to get some
real feedback from heat pump users and if possible some recommendations for
manufacturers/suppliers worth speaking to.



Neil B.



_____

From: ukha_d@xxxxxxx [mailto:ukha_d@xxxxxxx] On Behalf Of
biggles299
Sent: 31 July 2008 11:16
To: ukha_d@xxxxxxx
Subject: [ukha_d] Ground Source Heat Pumps and HA (OT)



After many years of faithful service our oil fired central heating boiler
is
on its last legs. I
have been seriously considering using a ground source heat pump as an
alternative.
Is there anybody out there in the group who has any first hand experience
of
this technology
or better still is running an installation already.
Thanks
Charles





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