Noticed the Vailaint Unistor range of cylinders for heatpumps, aren’t 28mm connections, like most other ‘heat pump cylinders’… is this an issue?
My newbie thinking is… since the cylinder heating circuit is seperated at the diverter valve which can run full 28mm index circuit to radiators, so the DHW circuit narrowing down at the cylinder to 22mm or lower isn’t bad, since it’s a sufficiently small enough run to not cause flow issues, as long as the coil in the cylinder is large enough to exchange the heat.
if that’s the case…does that mean I can use any unvented indirect cylinder with sufficient coil surface area to dissipate the output of my heat pump, even if it’s a 22mm connection???
Maybe, depends on the kW rating of heat pump and it’s flow rate. Think most Vaillant cylinders also have quite a small coil area, so wouldn’t be my first choice anyway.
However, I stopped heating cylinder via heat pump a while ago, mainly due to long piping runs, some experiments I actually found real energy usage wasn’t really much difference between immersion and heat pump. If I had known a couple of years ago I would have saved a £800 by just getting a direct cylinder, which pays for a lot of energy.
The pressure drop through the coil will be much greater than the pressure drop through a 22mm fitting, it won’t make any difference. Pressure drop is cumulative, so it’s still worthwhile having the primary pipe run 28mm even if you have to drop to 22mm.
Correct, it’s the surface area of the coil which is larger in HP cylinder compared to regular cylinders. Non HP cylinders will have a much smaller coil surface area since they are designed to run at a larger DT
How long is your pipe run! I can’t see any scenario where using a heat pump would give you a COP of less than 1. Most well configured heat pumps average a COP of around 3.5 for DHW which will use 3.5x less electricity than an immersion. If you’re getting a poor DHW COP it’s more likely due to non optimised settings rather than a long pipe run. @Zarch has done an excellent article on how to optimise DHW COP: Best Heat Pump Hot Water Settings (The Golden Rules)
Pipe run is pretty long, about 10m to diverter and then another 8m to cylinder. The CoP recorded doing hot water production is good around 3 to 4, so that isn’t the real issue. The issue is when you compare total energy usage. The immersion does use a little more energy, but only a little. Our usage profile may have some effects, water usage means we really only need to do a big heat every other day, but reheat hysterisis drives more heats than that, sometimes twice a day. Moving hysterisis wider means the whole cylinder can decrease in temp too much. But we only pay 10p per kWh, so not really much difference from gas costs in real terms.
Also our solar PV, which due to export limitations we get clipping. To prevent clipping a home assistant automation runs to fire the immersion when I am at export limit. This also saves on running costs.
So over the whole year I may pay a little more, but not enough, to warrant using the heat pump and prefer to let that run interrupted for as long as it wants to.
The water content of 16m of 28mm is about 8.6 litres. Assuming this has to be heated up from 10C to 50C (pessimistic) this will require 0.4kWh of heat energy which at a COP of 4 will equate to only 0.1kWh of extra electric energy required. The loss is tiny compared to the 4x efficiency gain from the heat pump.
Of course, if you’re getting PV export limit clipping, then it makes total sense to use immersion PV divert.