Critique my over-complicated heating design

[Wil]

Hi all, 

 

I’ve been living with the cold farmhouse over the winter since we moved in last year. We have an old oil burner which heats some rads in an open vented configuration. Also a small gravity fed DHW tank which appears to only heat from an immersion. It’s time to drag our house kicking and screaming into the present and prior to some extension works, I’d like to get the basics of the heating system in place. I’m lucky enough to have plenty of space in a plant room (old garage) next to the house and the time to start again. I’ve used Jeremy’s spreadsheet to come up with a 28kW heat loss for the house. I’ve done the obvious like stopping unused chimneys and filling the loft with 350mm insulation (50mm existing micafill, 300mm earth wool). But it’s still a leaky old place with 2ft thick solid stone walls. 

 

See attached diagram for my latest hare-brained scheme. Here’s where I need the collective smarts of those present to critique it and point out my glaring errors.

 

I have access to a relatively cheap 22kW GSHP and can probably do a lot of the install work myself so considering that route. I’d keep the oil burner as a backup/ top up for a few seasons to ensure the HP and other systems were doing enough to keep us warm. I’m not going for the RHI as I know I can’t have a backup burner there. I plan to fit 3.68kW solar PV per phase onto my 3 phases to give an 11kWp solar system although almost all my load is on L3. L1 and 2 will almost always be surplus when the sun is shining (usually 50W each phase base load) which can dump into water/ heating/ PHEV/ future batteries.

 

DHW is simple enough in that a new UVC with a large coil to take the output from the GSHP (or oil in super cold times). It would then have two backup/ dump immersion heaters to take solar output when available. There’s a passive coil through the TS to put a little heat into the water before it reaches the UVC.

 

Heating is more complex and hence the interest in a Thermal Store to correlate some of my heat sources. As part of the extension works I have an opportunity to site a wood burner with back burner close to the plant room (while also useful in the house).

 

1. In times of high demand with only the GSHP/ Oil available (we’re not in and haven’t lit the fire or the sun hasn’t been shining and the TS isn’t hot) then the GSHP would bypass the TS and feed the rads and UFH direct.

2. As soon as there was heat from the fire or the TS was up to temp from excess solar, the GSHP would switch to a coil in the TS and the TS would feed the heating system (the 3P valves changeover on the TS/ GSHP loop). This would allow the GSHP to run against a buffer tank as well (although apparently it’s an inverter driven HP so should be able to just ramp down).

 

Basically- what have I missed and why won’t this work? I could just drop the whole TS and wood burner idea and have the oil/ GSHP direct feed the rads and floors, but I think I’ll have excess solar in the shoulders and summer to be able to dump into the tank.

Are the 3P valves a nightmare waiting to happen?

What do I do about expansion vessels in this scenario?

 

Thanks in advance for your help.

[ProDave]

I would keep the thermal store, needed when using the wood burner, but I would not heat it with the GSHP, I would only heat the UVC from the GSHP.

 

Use all 2 port valves (I hate 3 port valves, and choose carefully, some can be normally open, some can be normally closed.

[Wil]

Thanks Dave, interesting on the 2P- 3P thing- this would increase the number of valves but I guess provide some level of resilience? and no let through as the valve changes position?

 

So basically the GSHP does the heating direct, or if the TS is hot, it does the house and the GSHP switches off? I hadn't really considered that but might be a good shout. I'd have to be careful to size the TS correctly to get decent heat out. I suppose if it was a little smaller it could pass the heat from the fire on quicker. Would you put the oil into the TS or just run it direct like the GSHP? 

 

If the TS isn't fed by the GSHP or Oil, does it just become a heat source like them until it's temperature drops to a level where the TS or GSHP can do better?

 

I have it in my head that simple relay control could do most of this but getting a simple and reliable control system may be more complex than I imagine!