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Author Topic: ASHP System Design  (Read 5547 times)
andyrob
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« on: October 20, 2012, 08:23:25 PM »

I'm soon going to install a dual ASHP system into a small barn conversion and wondered if anyone would like to answer a couple of questions and comment on my ideas?

In my house, the Ice Energy / IVT GSHP  installation has an oversized pump (it's waiting for an extension to the UFH to be completed). To avoid excessive cycling of the pump there's a buffer tank which has two inlets and two outlets. The heat pump output flows to the tank bottom, up through the volume of water in the tank, out the top and back to the heat pump. When the UFH pump is running, the same water that's circulating in the buffer tank from the GSHP is pumped around the UFH circuits as well.

So, the GSHP does not pump to the UFH directly, but neither is it indirect, there must be a name for this? I reckon there are one or two very logical reasons not to have the pump-buffer-UFH in series, as it were,  for example when zone controls might operate and reduce flows in the UFH circuit which would cause cycling.

For my DIY ASHP installation in the barn, I am using two Calorex 4.5kw units, one for DHW and one for UFH. If you're wondering why two, I bought the ASHPs and associated tanks out of a social housing re-design.

For the UFH, I want to have the ASHP connect in series with a bog-standard, one inlet/one outlet, 90L tank. I'll not use actuators on the floor circuits so as to retain a fixed volume of circulating water. To help the ASHP raise and maintain floor temperatures in cold weather, the two existing heating elements in the tank could be operated (automatically or manually) - say during off-peak times only.  I think the Calorex controls itself by monitoring return temperatures and maybe external weather compensation.

For the water heating, I'll have the second ASHP heating a 150L tank via one of the two indirect coils (the lower one?). A solar thermal collector, or wood-burning stove back boiler, can feed the other, higher coil. There is an electric element in this tank as well, and it could be used at times when the ASHP might struggle on its own, eg when it's cold, dull/dark, the WBS is out and there's not enough DHW.

Any issues with these ideas?

Thanks in advance for any advice, comments or suggestions.

Andy
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A.L.
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« Reply #1 on: October 21, 2012, 10:50:59 AM »

hello,

Quote
The heat pump output flows to the tank bottom, up through the volume of water in the tank, out the top and back to the heat pump.

- why not the conventional arrangement of hot in at the top, return at the bottom?


Quote
For the water heating, I'll have the second ASHP heating a 150L tank via one of the two indirect coils (the lower one?).


- if you intend to use solar it should be the upper one to maximise volume available to solar, but would this be enough volume?

- if this a conventional boiler coil it may be too small to distribute the heat causing cycling, normally for heat pumps coil should have equivalent of 3m2 surface area, although your heat pump is quite small, I would speak to Calorex for their recommendation.

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dhaslam
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« Reply #2 on: October 21, 2012, 12:23:14 PM »

It would be better to  feed one larger buffer tank with  both heatpumps and  have conventional  controls on the underfloor heating ciecuits.     Air source  heat pumps should normally be  used with off peak electricity  but may need to be used during the day in very cold weather.   A large tank gives flexibility and  good controls on output can direct heat where and when it is needed.   

It would be better to heat DHW at the normal  heat pump  running temperature and top up with  the immersion or solar or both.   This can be achieved by reserving the top of the buffer tank  for DHW  or by using a separate cylinder. 
The COP is quoted by Calorex as  about 2:1 at 0/55C  and about 3:1 at 20/55C   but these figures may be  difficult to  reach in practice.  By running both heat pumps at a  lower temperature  the efficiency would be much better,  particularly in colder weather.   

In setting up the system  you should allow  leave provision for a stove to be connected if that is a future possibility.         

   
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rt29781
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« Reply #3 on: October 21, 2012, 02:36:35 PM »

When we originally installed the heat pump we plumbed it to heat the 500lt tank that we have rather than put the heat directly in the floor.  After taking advice from a refrigeration forum (http://www.refrigeration-engineer.com/forums/) we now have it plumbed directly to the underfloor loop with a 50mm recirculation loop.   So the heat pump is set to 40C and circulates around the 50mm PVC pipe with the underfloor circuit flowing across this circuit and that seems to work well  So we probably have 3m of 50mm plastic pipe with a 4 way junction in the circuit (like a tee piece but with one extra connection).  The underfloor loop connects to the 4 way junction and takes off the 40C water into the floor.  The 50mm pipe is looked upon as a balancing pipe making sure the flow to the heat pump is high at all times.  So water constantly circulates in the 50mm pipe and then the underfloor circuit takes off water at the hot end into the underfloor system.  Our system has an added complication that we have solar water at 40C also coming into the underfloor feed.  The return from the underfloor goes circuit goes into the solar heat exchanger (but could return into the 50mm pipe near to the water feed into the heat pump.   This seems to work well and avoids the need for a tank.  The heat pump is a 12kW Trianco (more like 8kW in practice) air to water.  Out tank is now only a scavenger for the solar heating.  If we did this again I would have no tank just a heat exchanger for the solar circuit and the balancing pipe for the heat pump circuit.

http://www.refrigeration-engineer.com/forums/showthread.php?22114-Addition-of-a-heat-pump-to-solar-heating-system
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Nowt currently, Aberdeen.....well actually very well insulated extension with passive solar that seems to heat the house....
tsh
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« Reply #4 on: October 22, 2012, 08:12:09 PM »

Too many variables. To add another one, there may be some value in using low heat water to feed the lower coil of DHW in the cool days where the underfloor load is lower (so you get some reduction in cycling, and also improved COP). This is in effect a pre-heat for the incoming cold feed before you boost it with the top coil (but the area of this coil is likely to be small, and I think if you take this route you'd want a valve to switch the flow and use both coils when you're trying to heat the DHW).

Probably, you're unlikely to get an optimum solution...
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sam123
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« Reply #5 on: October 23, 2012, 09:27:34 AM »



In my house, the Ice Energy / IVT GSHP  installation has an oversized pump (it's waiting for an extension to the UFH to be completed). To avoid excessive cycling of the pump there's a buffer tank which has two inlets and two outlets. The heat pump output flows to the tank bottom, up through the volume of water in the tank, out the top and back to the heat pump. When the UFH pump is running, the same water that's circulating in the buffer tank from the GSHP is pumped around the UFH circuits as well.

So, the GSHP does not pump to the UFH directly, but neither is it indirect, there must be a name for this? I reckon there are one or two very logical reasons not to have the pump-buffer-UFH in series, as it were,  for example when zone controls might operate and reduce flows in the UFH circuit which would cause cycling.


Swedish style GSHP donīt usually need puffer tank for UFH, if UFH is assembled to concrete. You should open all your thermostats and if you want some floors more cold than others, you just have to turn flows in your UFH manifolds. If your GSHP is too powerful now, you can add some hysteresis to control on/off cycles. On the other hand, there is no negative effects with buffer tank. Just little a bit more expensive to build.

I donīt know any "name" for that. We call it just UFH with buffer tank  Smiley And it should be connected to serial. GSHP->buffertank->UFH->back to GSHP. All made with circulation pump inside your GSHP and temperatures controlled by heat curve of your IVT. Hope this helps?

Cheers, sami

 
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andyrob
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« Reply #6 on: October 23, 2012, 10:53:22 AM »

Sami

There's no problem at all with my GSHP setup - the buffer tank is only there because the pump is sized for the UFH area to be doubled in size later, and also for when I occasionally shut off an unused zone or two.

The design of the buffer tank circuit is what makes me ponder. It has no no direct connection to the UFH loops, the IVT heats and circulates 100 litres of water in the buffer tank with its own internal pump, and the other pump on the UFH manifold draws out and returns that same water through a second pair of draw off points on the buffer tank.

I'm just wondering whether this is the only/best/correct way for a buffer tank to be plumbed, or whether a simple in-out, in series,  circuit - as you described at the end of your post - is all that is necessary for my barn's ASHP which I want to run only during E10 cheap rate periods as I do with the GSHP in the house.

Incidentally, thanks to all for the ASHP circuit design suggestions so far. I'm going to digest them all later today and come back to you. But, A.L., is there any reason for the convention of hot in at the top of a buffer tank and out at the bottom. I just assumed (as heat rises) that the opposite would be the case.
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titan
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« Reply #7 on: October 23, 2012, 10:58:14 AM »



In my house, the Ice Energy / IVT GSHP  installation has an oversized pump (it's waiting for an extension to the UFH to be completed). To avoid excessive cycling of the pump there's a buffer tank

The standard "other" option is just to have 30% of the UFH loops uncontrolled. For me this is a central hall area open to the roof which is good for getting heat upstairs where there is no other heating. We haven't run the system in full ( winter) heating mode yet and I may activate the weather compensation depending on how it all works out. The problem with heat pumps optimisation is I doubt any two houses are the same
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Bodidly
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« Reply #8 on: October 23, 2012, 11:57:35 AM »

Sorry OP bit off track but titan I would not hesitate using the weather compensation, our system is totally controlled buy this and it's very good.

As for buffer tanks sorry can't help as our system does not use one and all zones are continually open.

System is IVT greenline C6
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sam123
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« Reply #9 on: October 23, 2012, 02:03:04 PM »

I'm just wondering whether this is the only/best/correct way for a buffer tank to be plumbed,

Hi

GSHP, Best way is to connect puffer in serial and use only internal pump of your GSHP.

ASHP is a little bit tricky. I would keep in mind that:
- You donīt get benefit using ASHP at cheap hours, because itīs probably colder at nights and COP drops if you overheat more than needed.
- DHW can be pre-heated to +35 to +40 with ASHP, after that you get poor COP. So little electric boiler should be used after pre-heating.

I would try to make system where both ASHPs are somehow connected to UFH and DHW is only pre-heated with ASHP. My friend just assembled Nibe F-1145 GSHP and because the old electric boiler was in good condition, he pre-heats DHW with plate heat-exchanger (0.9m2 ebay.de Ģ120) from his GSHP. Heat exchanger is connected just after GSHP, so benefit depends if GSHP is running or not. Incoming tap water is something like +5 degree here in Finland.

Something like this: http://www.ebay.de/itm/B3-23-40-Edelstahl-Plattenwarmetauscher-0-92m-BHKW-NEU-/140864855153?pt=L%C3%BCftungs_Klimatechnik&hash=item20cc331c71
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andyrob
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« Reply #10 on: October 23, 2012, 03:04:55 PM »

Sami

One of the best kept secrets here in the UK is the Economy 10 cheap rate tariff. Five hours at night but three hours in the afternoon  and two in the early evening.

I hadn't thought about the colder air temperatures at night, that will be significant. I'll have to set the ASHP to come on in the daytime when the sun is shining (on the PV). My wife will be happy not to have to keep getting out the Hoover and putting on the washing machine as soon as we see a bit of sunshine.

To keep the COP as high as possible, your idea of using the ASHP primarily (even mainly) as a pre-heater for DHW is a good one. I can connect the ASHP into the lower heating coil in the water tank and use the electric elements if necessary above.

Having said that, A.L. was right I'm sure, in pointing out that the inefficiency of a heating coil fed from (preferably) low temperature ASHP may well not heat the DHW quickly enough, so perhaps I should use both coils in in the tank, in series, on the ASHP circuit. And top the temperature up with off-peak electric immersion heating as necessary. I was intending using a WBS on the second coil sometime soon, but I'm ignoring that added complication just for now.

Andy
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A.L.
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« Reply #11 on: October 23, 2012, 03:28:36 PM »

hello again,

But, A.L., is there any reason for the convention of hot in at the top of a buffer tank and out at the bottom. I just assumed (as heat rises) that the opposite would be the case.

- in general counter current heat exchangers are more efficient than concurrent ones. With 'in at the bottom' the flow in the coil and the flow in the buffer tank would both be upwards. This would reduce heat transfer and warmer water would return to the heat pump.

- with flow downwards in the coil the 'heat rises' (and cold sinks) effect will aid the process
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