air source heap pump hooked to LPG boiler

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 suspect!
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
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simon m wrote:

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.
Andy
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Andy Champ wrote:

You can change that by lowering the pump flow rate. And teh boiler efflux temp. Then if the heatpump is doing enough on its own, the boiler won't fire.

Not necessarily. Larger lower temp radiators will do the same job really.

I am looking at a GSHP with top up immersion heater to raise the water the last 20c or so.
And NO oil boiler.

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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!>) experimentation :-)
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.
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On Sat, 12 Jul 2008 12:20:05 +0000, YAPH wrote:

D'oh! Or of course Simon the OP's Megaflo of DHW combined with lower-temperature heat-pumped DWH via either another Megaflo or thermals store/heat bank.
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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 cuts in.
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.
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On Sun, 6 Jul 2008 12:09:17 -0700 (PDT) someone who may be simon m

The alternative is to drill vertically to get enough length for the loops.
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David Hansen wrote:

VERY expensive and complex. But a good solution if you can do it.

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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.
Simon
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simon m wrote:

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 contamination.
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. .
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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 efficient.
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.
NT
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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?
Martyn
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Hi All,
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 back yet.
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.
Simon
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simon m wrote:

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 are marked.

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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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simon m wrote:

Sounds good to me. ASHP would seem able to generate a +30C rise or so with good efficiency.
The final answer is in the overall cost benefit analyisis though.

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