heating design

[dpmiller]

So general plan is:

250l Harlequin Heatstream TS, twin coil

Approx 9kW ASHP

Boiler stove, a Woodfire Passiv or similar, on a charging/load valve. Quench coil also.

 

Depending on which calculator I use (Jeremy's, loopcad, other) heating load is somewhere between 3 and 6kW

 

What would stop me from following the plan in the diagram below? Forgetting about the electrical (control side of things for now) the scheme should allow either DHW or heating to have priority and also be able to offer slab cooling, whilst using the TS to act as a buffer. Crappy paint drawing sorry.

 

 

[ProDave]

So how would you heat the DHW in that scheme?

 

Thermal stores don't suit heat pumps very well, you need the water in a TS hotter than your DHW delivery temperature which is not what ASHP's are good at.  I can see why you  propose one as they work well with a stove.

 

You need better integration between stove and ASHP. Lets hope one of the plumbers comes along soon with a proposal.

[dpmiller]

I can't see a problem (other than overall DHW capacity) with just running the TS up to 55c or so on the ASHP, the COP is always going to make it a better bet than just using the immersion.

There's only three of us in the house and only SWMBO has a morning shower.

 

I'm figuring on heating the TS and UF loops on early morning E7 and then the tank will get topped up off PV during the day. Heat in the evening can come out of the TS until the temp drops far enough to bring the heat pump on again but for all the amount of heat the house will sup I don't expect to kill the TS in a couple of hours.

 

Towel rads are just a convenience thing, they're on a separate manifold with TMV so they can be set on a timeswitch and again just sup a wee bit of heat out of the main loop, whether from the TS or the running ASHP

 

Summer DHW will be handled by the PV diverter, allowing the ASHP to do slab cooling

 

in terms of integrating the stove, that's all on the electrical side IMO. The Heatstream  has an upper and lower coil and each coil has it's own stat fitted so not hard to control based on tank level. Plus the supply from the stove will have a pipe stat to enable the pump on the load unit.

If the stove happens to be lit and adds heat to the TS then good. If not,  it's not taking any heat out of the TS...

[dnb]

As ProDave says, a thermal store doesn't go all that well with an ASHP. On the DHW side you pull energy out of the thermal store by passing cold water through some kind of heat exchanger in it. A very crude 1st order approximation here: All the hot water you want in the house is probably 40deg C or hotter, so the minimum temperature your thermal store ends up at is around 35 to 40 degrees C on a daily basis in summer. So the ASHP can only take this up by 15 degrees or so to 55 deg C - it's a small percentage (approx 33%) of the total useful capacity of the store, so may not be worth the effort if it needs to be made up with yet another energy source if the stove is not running to avoid a dribbly tepid shower.

 

Of course, it's a bit different in winter when you want heating - the store can be drawn to a lower temperature to provide useful heat output for the UFH. But at this point, the COP of the ASHP is reduced. I think the design is a good start, but depending on your requirements it could be refined - how do you want it to work seasonally? Perhaps some PV would prove more efficient overall in summer than the ASHP?

[dpmiller]

Thanks @dnb  we already have around 3.2kW on the roof, on a Solar iBoost+ s yes, the summer is sorted.