Apologies for the long post, very keen to get some pointers on how to fix this issue.

I have an existing heating system which consists of a 16kw Woodburning Stove which provides heating and hot water via a Gledhill Torrent ECO 250 Solar thermal store. We previously also had an oil boiler connected to the thermal store to provide backup when the stove was not on. We recently removed the oil boiler and had an installer comissioned to install a ground source heat pump, I explained at the start that I wanted to keep the stove and use the heat pump to as backup similar to the oil boiler previously. The installer said this was fine and proceed to install the heat pump.

However when it came to connecting into our existing system, the problems started. Only at this point did they realise that our system was open vented (which should have come as no suprise with a wood burning stove connected to it). Our heat pump is Valiant flexoTherm 11kW and it can apparantly only be connected to a pressurised system, I think it has a pressure switch which cuts out at 1 bar. Our installer proposed connecting the heatpump to the unused solar coil in the thermal store which would allow the water in there to be pressurised. I did ask if the coil was big enough and they said it was.

The installer then struggled to work out how to wire the controls for this system to switch between the wood burner and heat pump, so I got my own electrication to wire it. The controls work fine, when there is demand for heat and the thermal store is not hot enough from the wood burner a circulation pump activates to circulate water from the heat pump buffer tank through the solar coil to heat the thermal store. The flow and return get warm as expected but the water in thermal store barely rises in temperature even over a few hours.

I now realise that the solar coil (surface area 0.34m2) is nowhere near big enough for this purpose and a different solution is required. I'd like to know what the best solution is before I go back to my installer. I think there are a couple of options but am keen to know if there are others I'm not aware of:
1. somehow adjust or trick the heat pump to operating at a lower pressure than 1bar and connect the buffer tank directly to the thermal store via the disused boiler flow and return.
2. use a plate heat exchanger to connect the buffer tank either to the thermal store boiler flow and return or directly into the pipwork running from the thermal store to the radiators and UFH manifold.

Any input or experience welcomed, particularly about sizing a plate heat exchanger for this purpose.

James

I agree with PiH's points (although I'm no expert on heat pumps so that doesn't count for much).

Apart from operational choices, I'd be looking at the commercial/political aspects as well. That is, it seems your installer doesn't really understand what he's doing. So how was he hired? Are there other parties involved? What guarantees are there? Is there some sort of regulator or other trade scheme that he's part of? Is Vaillant involved? What other options apart from getting him back are there?

Also, wood burners and oil boilers produce heat at high temperatures, whereas heat pumps generally produce it at lower temperatures. Is your heat distribution system (radiators etc) designed to be used at lower temperatures? Or are you planning to drive the heat pump at high temperature, with consequent loss of efficiency?

Are you sure it's a solar coil as it sounds very small at .34m2, data plate on ours says its 1.6m2.

Another option maybe to replace your buffer with a small indirect cylinder with a massive coil in it. Heat the cylinder with the HP and pump the contents of the existing TS through the coil. Newark copper Cylinders designed and built our cylinder and I'm sure they'd be able to advise on heat transfer viability through a custom cylinder/coil.

If you're going from a high temp boiler/WBS set up to a relatively low temp HP, has someone verified the sizing of your rads/UFH will work with the HP......I'd not be relying on your installer!!

I think the OP said they wanted to continue to use the wood-burner as the main source of heat, rather than the heat pump?

But it would be useful if they gave some feedback on what's been posted so far.

Thanks for the input so far. Plan was / is for GSHP to keep underfloor heating going whilst the woodburner is out when we are at work then we'll put woodburner on in the evenings. We've run the house like this for a few years with the oil boiler doing the same job. There is also an element of if something happens to me and my wife can't chop and cart logs, the house will still be warm.

Solar coil is 0.34m2 according to the data plate on the thermal store.

Installer is MCS Certified and HETAS registered so should be able to do this and said they could at the start.

In terms of the emitters, the ground floor is all UFH the majority at 150mm spacing, one room that was laid earlier is at 200mm spacing but there isn't anything we can do about that now and the wood burner is in that room anyway. Upstairs is radiators, some are big enough and some aren't. The calculations have been done and as we renovate rooms we will upgrade the radiators.

The way the system has been plumbed, the heat pump is blind to the heating controls in the house. It can only see one zone which is the thermal store and it is simply heating it's buffer tank when the temperature outside is low. The heating controls in the house control a circulation pump to feed water from the buffer tank to the solar coil when required.

WiA's suggestion is interesting but also approaching Valiant is a good idea.

I haven't mentioned hot water here becuase we have essentially have two separate systems, the heat pump DHW cylinder is the primary option here and the thermal store can provide hot water at mains pressure via a PHE. We have a thermostat on the thermal store and two motorised valves to determine which provides hot water.

I'm essentially trying to work out what the correct solution is before I go back to the installer.

James

Perhaps a missing point from the decision making is - how is the primary fuel paid for. If the wood is free (perhaps apart from the work of cutting up) and the electricity is charged then one solution becomes desirable but if the wood is bought in and the electricity comes from solar panels then perhaps different solution would be needed.

If a GSHP connection to the TS can be done by other than the solar coil then it should be fairly simple to give the wood burner priority in heating the TS by means of thermostats properly placed.

Of course the environmentally sensitive thing to do would be to ditch the polluting wood burner and use the GSHP from either your own solar or from the evermore sustainably green grid. (says he with a 40 Kw gasifying wood burning boiler)

Update:

Both Valiant and Gledhill gave similar advice that either a plate heat exchanger should be used or the thermal store be swapped for one designed for a heat pump with a much larger coil in it. Though neither were prepared to specify exactly how it should be installed.

Having read the advice given about about heat pumps and thermal stores, and because its likely to be simpler and cheaper to do I'm going to get a pate heat exchanger installed and the method WillnAbderdeen suggested seems the best.

As space is a bit tight in the thermal store cuboard, i'm thinking to have the the flow from the heat pump buffer tank permanantly going through the pressurised side off the PHE to the radiators and UFH and use motorised valves to open a bypass of the buffer tank and switch on the pump on the unpressurised thermal store side when the thermal store is hot enough from the stove. Does this make sense and would it work? It would minimise the amount of extra pipework required.