so I'm on bulk LPG out in the luverly countryside and the bills are
starting to bite. LPG feeds into a 5 year old condensing boiler that
does central heating and hot water via 2x350liter megaflow for our 10
bedroom B&B. Works lovely, just expensive and about to get more so I
Got a heat pump man in and he has suggested we look at an ASHP. We
have a lovely outdoor pad about 10 feet from the boiler room, and 10
feet from 3 phase power. Not enough land for GSHP and anyway the
digging sounds appalling.
His suggestion is to plug the ASHP 50deg output into the return
circuit to the boiler. This way when either the hot water or the
central heating (microbore radiators) call for heat, then the ASHP
kicks on, and the boiiler will also kick on if the return is too low.
This sounds very simple, and doesn't mean replacing any radiators or
messing about with anything other than a single cut in the return
pipe, and some control electrics.
Any thoughts on whether this is a rubbish idea? I would imagine the
boiler would need it's settings adjusted so that it wouldn't turn on
so much. The ASHP loop would have a valve so if it was bust I could
just divert past it.
Cheers ! Simon
If the ASHP is heating to only 50 degrees, you want to carefully check
the return temperature into your boiler. It might be more than 50
already - mine is when the heating is on. And the hot water is also
hotter than 50, so it can't do that either.
A little search suggest you want to install underfloor heating to make
it work properly. Ouch.
Seems to me the best use would be as a pre-heater - put the two tanks in
series, and heat the first one to 50(ish) with a GSHP, then use the LPG
to take it up the last bit to be usable. (you probably have a legal
minimum as a commercial premises, and I bet it's a hand-scalding 60+)
This would cut you *hot water* gas bill to maybe 2/3 it's old value, and
not touch the room heating part. This may not be useful.
On Mon, 07 Jul 2008 13:13:48 +0100, The Natural Philosopher wrote:
Maybe you could still do that with (mainly/some) off-peak leccy. If you
plumb the heat pump into one heat bank, store off-peak heat in another
(big, very well insulated) heat bank and combine the outputs such that the
cooler but more plentiful HW from the first is topped up with a small
amount of hotter water from the 2nd to get 60C (or whatever).
Combining the two temperature sources could be done by thermostatic mixing
valves, hotter water to the 'hot' input of the TMV and cooler to the
'cold', so that as the output reaches the set temperature the TMV reduces
draw from the hotter source and mixes in water from the cooler one.
One can think of various arrangements of the two stores, using one or more
plate heat exchangers or coil-in-tank heat exchangers (the latter
could work even in hard water areas if primary water from the first
cylinder were piped through a coil in the second, hotter, one, and thence
to a PHE to heat DHW). Possibly an area for a bit of DIY (<gasp!>)
For Simon M's original situation I think the same approach would be
applicable but with his LPG boiler providing the hotter water (either
instantaneously via a PHE from the boiler primary, having it act as a
combi but just for the top-up heating of the DHW, or via a heat store).
For CH Simon might find that 50C was adequate for two seasons anyway, with
a switch to the LPG boiler only when full heat is required in very cold
weather. Even if the heat pump was entirely unable to contribute to CH in
these conditions he'd still be saving for much of the year and on DHW.
Thermal storage is the way. However, I would have the heat pump heating the
lower section and the boiler the top. The DHW would run through a bottom
mounted pre-heat pancake coil to ensure a very cool section at the bottom
and then into a DHW plate heat exchanger. This bottom section the heat pump
heats. In summer the heat pump may heat all of the store. Off peak
electricity would pre-heat the store overnight and if not up to temp the
boiler kicks in to top up.
As I have written, the key is keep the return temps from the CH and DHW low.
Sophisticated control can do this. Control can also dictate when the boiler
However, the OP has two megaflows which he would be reluctant to replace
because of cost. As I highlighted, Heat the megaflows via plate heat
exchangers and a bronze pump to ensure a cooler return temp. Then have a
3-way valve operated via a weather compensator for CH. The return temps
will be low. The heat pump then raises the return temp. If it cannot
because the return is higher than what the heat pump can raise, then it is
by-passed. If it can then it preheats and boiler does the rest.
The megaflows can be converted to heat banks though. Depending on DHW usage,
both megaflows can be converted to heat banks for the lower temp heat pump,
and the boiler, depending on size, can be converted to a combi, and then mix
the water for DHW and CH.
funnily enough we did have a borehole drilled last year for drinking/
washing water, to 71meters, so I could wean myself off Yorkshire
Water. Been on the borehole now for a year and love it. Council
agrees, purest water they've tested for years. So I could call the
drillers and ask if the borehole could co-exist with a GSHP feed
somehow. I wouldn't want to drill another hole as they cost thousands.
I hadn't thought that the hot water tank would be stored at 60, so
potentially the ASHP at 50deg would be taking heat *from* the tank! I
would expect to put the boiler output for CH down to 60 degrees or so,
right now it seems to aim for about 80-85deg going out. I would
possible try and swap the controls on the two hot water tanks so that
they call for heat at, say, 50 degrees, and only go to 65 once a week
to solve bacteria issues.
I think I need a real plumber to double-check the assumptions here.
Not a plumber, a real heating engineer.
You cant use the borehole bore,because you will be sending sub zero
brine down there..likely freeze the water on the way up.
You need to heat bathwater etc to 60C to avoid possible bacterial
You don't need to heat CH to 85C, but you DO need a much much larger
radiator area. you have 15 C drop from 25C room temp to 40C CH temp,
but a 45 degree drop from say a rad at 70C..three times the output per
unit areas effectively.
Youre making this harder than it need be. If your heat pump has blown
air output rather than HW, all the issues listed go away. And are
replaced with more different issues... but lots of people are ok with
that, and its cheaper to install and significantly more energy
You really cant use a 50C source to heat rads, unless you replace them
all with bigger ones, or only use it when the weather's mild and 50C
is enough. Or add ufh, or... etc - you can but its work and expense,
and you pay for it in lower efficiency.
Do you meter the water going into the cylinders so you can determine
daily hot water consumption? You can then calculate daily hot water
usage per guest.
Do you have a good location for solar panels?
I can't really do solar panels for our main building as it's under
100foot tree's that have a TPO on them. Do have solar panels on our
new build and they are indeed fantastic! That really is free energy..
yeah I know about the tiny CH pump at 100W or so.
3 of the 10 bedrooms in the main building are currently electric
storage heaters, so we would change these to oversized rads this
winter in any case. Several of the smaller dinky radiators are looking
a bashed anyway so replacements for them are on the cards in any case.
This is to address the concern that a HP at 50deg might not heat the
building in winter.
Did watch the LPG boiler in operation, and for heat (bearing in mind
this is summer) it seems to sending out a supply at 50-65 and the
return is coming back at 39deg usually. When the boiler kicks on for
hot water it sends out at about 80deg and I've not recorded the
return. I've contacted the hot water cylinder people to see if the
controls on the cylinders can be swapped so they store water at, say,
50deg, and then go to 65deg once a week to kill any bacteria. No word
Generally our LPG usage seems to be about 70% heat to 30% hot water.
Just read a reply in another note about HAUTEC 42's that can supply
at 65deg so I have fired off an email to the manufacturer asking for a
spec sheet and UK supplier details.
well if its on for 12 hours a day, and the average output from a 1 sq m
solar panel over the day is about 100W...it should add 50% or more to
the actual heat output. :-)
Good plan. Get insulation in if possible.
That is apparently doable. The other option is to have your heatpump
drive the DHW primary, and use an immersion to 'top it up'..possibly on
cheap rate. I thik that route looks optimal for me. If te heat pump can
take it from say 25C top 50c, taking it to 65 with an immersion isn;t
such a bad deal.
I estimate my oil usage is more like 95% house heating. and 75% of that
is in the November December january february months. But that relates to
a large house with no kids in it. ;-)
Sadly they don't seem to be IN the UK..only Ireland.
Almost any heat pump is better than oil price wise, but air source is
not so good in efficiency.
LPG is pretty pricey these days too..so wouldn't recommend that.
strangely enough, electric storage done proper (which it never is) is
not as bad a bet as it used to be.
Do you have any garden at all for a ground source? the efficiency gains
A heating man you need, not a drains man. One specialising in heat pumps.
One who understands control as well.
Look at improving insulation in a serious way.
The cost of the ASHP buys a lot of LPG.
ASHPs quote a decent COP, however that is peak COP and the real COP is
likely to be much, much, lower. Don't be fooled.
Can the flue outlet be firing at the air intake of the ASHP? This means
warm air is being sucked into the ASHP, raising efficiency. Check with ASHP
makers that this is OK.
Heat the cylinders using plate heat exchangers and a bronze pump. This
raises efficiency creating thermal layering, heating the cylinder from top
down. The cylinder return pipe is then at a very low temperature and the
ASHP can then cope in pre-heating. Coils in cylinders are very inefficient.
So, the return from the cylinders, via the plate heat exchanger, into the
ASHP, out and then into the boiler. That is, in series.
Fit a weather compensator, which means the ASHP will be the prime source of
CH for most of the year - only when very cold will the boiler kick in. The
ASHP can preheat the DHW and the boiler tops up.
Fit DHW blending valves on the cylinders.
To get the two to combine efficiently needs a decent control system, which
would then make it shine. When the ASHP can cope, the boiler stays out, only
coming in when needed. When the boiler does come in, it supplies warm air
into the ASHP to raise its efficiency.
I would be inclined to fit both ASHP and boiler in series. This way the
ASHP is always on, when there is demand. Then use a two stage step switch
controller, with the ASHP as primary heat and the boiler secondary -
master/slave. The trick is to keep the return temperatures low from CH and
DHW. This can be done by:
1. Using plate heat exchangers heating the DHW cylinders.
2. A weather compensator to do the CH.
Also, get the flue aiming at the air intake of the ASHP. If the boiler can
be a two pipe system, convert it and have the exhaust right up to the ASHP
intake and boiler air intake well away from the ASHPs intake.
Some calcs have to be done, but if heating the cylinders by plate heat
exchangers, the two cylinders coils can be used for CH take off, converting
the cylinders into thermal stores. Then the ASHP and boiler only heat the
cylinders ensuring a low return temperature - not heating the CH directly.
The weather compensator can operate on the CH circuit alone using a
modulating 3-way valve being detached from the two heat sources. Then no 2
stage step switch controller is needed. This ensures a low CH return
temperature most of the time, ensuring that the bottom of the cylinders are
cool raising the pre-heating ASHPs efficiency.
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