How to plumb Ground source heat pump, Wood burning stove & oil combo

scotsman

I am a little confused how to specify an oil, wood burning stove and ground source heat pump system to power underfloor heating, 2 upstairs radiators and hot water on a stone cottage renovation. I can't find any good info about it. Does anyone have any advice here, or a good resource for me to read?

Thanks!

DarylP

... slightly complicated setup perhaps?
Are you looking for the GSHP to provide space heating & DHW, the WBS for space heating (& DHW), and the oil-fired boiler for backup?

If so, a OFTEC installer who also has HETAS & MCS certs....?

Good luck.....:smile:

scotsman

Yes: the oil is for backup. Am I over complicating things?

Nick Parsons

Paul in Montreal, are you there?

SteamyTea

The GSHP, along with the ridiculous anti-legionella electric heater, should supply you will near enough all the energy you need.
Occasionally, when it is very cold, or you use a lot of hot water, you may need to boost it a bit. This is what the electrical side does.
So you may be better off ditching the oil burner totally (think you have to to get the RHI) and if you really must have a WBS then just get a small one that is decorative.

Alternatively, get a thermal store that can handle the 3 inputs, some sort of controller that is programmed to give you all the heat you need and at the lowest cost or CO2 emissions (which will be the GSHP on all the time).

markocosic

GSHP and thermal store as a *packaged unit* from a single source, who takes full responsibility for designing the two to work together.

Don't complicate it with an oil boiler. Do add an electric backup if the GSHP doesn't have one already. Configure as an either-or, not both from one controller.

Wood burning stove for decor/space heat only. Better yet a flueless alcohol stove so that it's minimal faff to light, doesn't kick out too much heat, doesn't need complicated flues etc. Still real fire for caveman satisfaction?

Paul in Montreal

Over here, such a setup would best be done with an ASHP combined with any non-electric source of heat (wood, oil, gas or whatever). We have special tariffs whereby electricity is 1/3 the normal price down to an external temperature of -12C, then it goes up to about 4x normal rate. At that point, you switch to the alternate heat source. Since you're all-liquid, it's easy to do with a sufficient thermal store - you just need one with the appropriate number of heat exchangers. For hot water, you can escape the Legionella part by using a plate heat exchanger to heat your cold water on-demand from the thermal store (though flow rates and capacity are issues that need careful design). Or you could "top-up" this with an on-demand electric heater and supply warm water from your thermal store instead (if the unit will support a hot-input).

Whatever you do, you should always have some kind of backup for the GSHP, in case the compressor fails and you're without any source of heat until it can be fixed.

My system is a bit different as our GSHP is water-to-air - and has auxiliary heating supplied by a 10kW in-duct heater. So if the compressor fails, then we have sufficient heat available to stop the house from freezing. Such a system could also use a fossil-fueled backup, but it would be more complicated and not worth the expense. Electrical resistance heaters are simple and cheap and you have to consider the cost versus the expected amount of use.

Paul in Montreal.

Nick Parsons

Ah, sorry Paul. I thought your GSHP ran a wet system.

Paul in Montreal

Posted By: Nick ParsonsAh, sorry Paul. I thought your GSHP ran a wet system.

No, I'm afraid not as we need air conditioning / humidity removal in the summer. Forced-air distribution is the only way to do both heating and cooling (well, the easiest way).

Paul in Montreal.

GreenPaddy

Hi scotsman, the tricky bit of this combination is that the HP is designed to deliver heat at a lower power (rate of heat input) than an oil or log boiler, so you're trying to get a system optimised for two different styles of heat input.

I do sometimes design this way, if I can't convince the client that the with adequate insulation, a GSHP is an unnecessary capital spend and complicated piece of kit. Keeping the controls simple is key, by fully understanding the logic for each of a number of flow scenarios. I've heard a few GSHP installers say it can't be done - it can.

I'm based in Pethshire, and would be happy to design this for you, but it's not something where a few posts or bits of guidance will get you a correctly performing system.

scotsman

OK great discussion and thank you for the tips. Sounds like an option is to do a GSHP, backup electric heating, and wood burning stove for effect.

I realize the GSHP is expensive but there would be a good subsidy with the Renewable Heat Incentive scheme (18.8p/kWh). I also have the space for it.

Anyone have a recommendation on a Thermal Store\GSHP packaged unit?

scotsman

GreenPaddy - I sent you a whispered comment.