heating advice

[KN]

You don't have to have 3 phase, we initially looked into it but the cost to install was massive so we just put in single phase and two smaller heat pumps also you then get double the grant money ( £1,500 per unit from npower) unless you have to have 3 phase with a bore hole? Ours is a slinky.

 

[AMc]

Interesting, the 16KW air source needs single phase, but my current prefered supplier has told me the next power up needs 3 phase. The GSHP quote is so vague it doesn't actually mention the borehole explicitly and it only came up when I asked why it was so expensive, likewise the 3 phase. I suspect that as soon as it was obvious we didn't have a new build with slinky space the supplier considered it unlikely we'd do it, perhaps that's overly harsh.

The insulation and underfloor heating is a bit problematic - we have a vinery to convert and even with the best modern glazing it can't come close to an insulated wall. The renovation will all be at or better than current regs with UF, but the front isn't insulated and I don't have the funds to do the whole house nor would it be economic in the medium term.

Thanks for enlightening me on some more routes though!

 

[Flyfisher]

I would imagine that the 3-phase requirement is only related to the size of the electric motor, rather than the heat source (although I suppose a borehole might also need a bigger pump to circulate the refrigerant).

Interesting point about installing two smaller heat pumps - are they completely separate systems or can they share a single ground loop?

 

[MdB]

I would imagine that the 3-phase requirement is only related to the size of the electric motor, rather than the heat source (although I suppose a borehole might also need a bigger pump to circulate the refrigerant).

Single phase are for low power pumps and 3-phase is for high power pumps but there is a large overlap in the middle where you can use either. Most people don't have 3-phase but it has the advantage of lower start up currents which apparantly can be an issue.

Interesting point about installing two smaller heat pumps - are they completely separate systems or can they share a single ground loop?

I don't see any reason why you couldn't use two pumps with a single ground loop but this is unlikely in practise as normally you have lots of small ground loops rather than one big one. A 20kW pump from Kensa requires 4 x 50m ground loops for example. You could therefore have a couple of loops going to one pump and a couple of loops going to another.

 

[Flyfisher]

I recently bought an air compressor with a 2.2kW motor, so pretty insignificant in heat-pump terms, but it required the installation of a new power point in my workshop for it to run properly. It wouldn't get up to speed at all on the end of a 50m extension cable. I also had to have different MCB installed for the new circuit because the start up surge would sometimes cause the standard one to trip (I believe the variability arises from the differing points in the AC cycle when the motor switches on. So, if 2.2kW motors can require more than simple 'plug and play', I can easily imagine that a 16kW or more motor has some very specific requirements.

Thinking about it, the standard domestic supply fuse is usually 100A which is 24kW, so perhaps this is the limiting factor for the cut-over point to a 3-phase supply? OK, there's a power factor involved for motors so the precise point might vary, but there's no doubt that these big heat pumps place big demands on a domestic electrical supply. Still, I'm sure the installers understand all the implications and requirements.

 

[KN]

Our 3 x 85 meter trenches join together before entering the house.Then one heat pump does the underfloor and a hotwater tank and the bigger unit does the upstairs rad and another hotwater tank (lots of hot water for the B&B) so they do share the group loop. Think this is slightly unusual as one engineer hadn't seen it installed like this before. Works well and you can set separate temperatures or even turn one off if not required.

 

[KN]

Thinking about it we have 3 x100A Supply installed