Using a heatpump on old stainless 150 L Cyl with small coil

Getting Started
1

I am fitting a Diy heat-pump to my house and hope to reuse an old stainless 150 L Cylinder which is fitted with a small coil which would cause short cycling.

What if I fed a heat exchanger primary from my 3 way valve and fed the heat exchanger secondary hot output into top of Cylinder where DHW comes out, with the water returning from the drain plug to the heat exchanger secondary return. If HX is fitted low it may work by convection or I may need an extra pump.If it works the cly should heat from top down resulting in quick heat up time..I welcome advice before I waste my time.

2

Small coil will lead to rubbish temp hot water. Heat transfer area is not that good for a heat pump. I would and do (even with a big coil) use the immersion. Leaves heat pump to do heating and cooling in cheap periods and you can also do DHW at the same time. Decent tariff it will cost about the same as gas.

3

Thanks for reply,what I am suggesting does not use any coil,the domestic hot water is heated directly from a heat exchanger which is heated by heat pump. The heat transfer area is the heat exchanger which properly sized should transfer heat as quickly as the heat pump can supply it. Hope I have explained the concept properly.

4

So basically an external heat exchanger, that concept has been used many times before. Likely you will need a pump though. The heat pump will start the flow temp based on the return temp it sees. So slowly ramps up to get a decent CoP.

You can buy ready to go kits if you shop about.

5

Thanks that sounds promising, I am curious about the kits, what would be included.

6

Usually a pump and suitably sized plate heat exchanger, insulation jacket and some fittings

7

That is exactly what I was thinking thanks, I will check if heat exchanger is big enough for heat pumps low temp.