New unvented cylinder - recs?

on 23/03/2022, Ben Blaukopf supposed :

Your installer needs to be qualified to install unvented, G3 I think and it will then legally need to be inspected by a G3 regularly.

I'm not a cylinder expert but I'm not sure there's a lot in it - as long as it's stainless steel and decently insulated. Going for a heat pump coil would seem to make sense even on a condensing boiler since you can run it at lower flow temps. You can get tanks with coils for solar hot water, but I'm not sure the hassle for that is worth it compared with PV.

A heatpump output is likely to be in the 5-16kW range (ie smaller than a boiler), and to get best efficiency the output won't be as hot - it's better to run the tank at 45C if you can (with weekly boost to 60C for legionella sterilisation). If your hot water demand is sized for 60-65C water being mixed down with cold at taps, showers etc, that may require a bigger tank since less cold will be mixed in to reach your 38C (or whatever) target temp. Also the heat transmittance at 45C input is less than at 60C, so your

As well as the heat pump coil, one thing you might check is ports for immersion(s) and temperature probe ports, because your system will want to sense the tank temperature (if it's on a mechanical on/off stat they're doing it wrong).

Heat pump systems can use buffers/low loss headers - you can get them integrated in the tank unit but a separate unit would be fine as long as you have space (my Grant system has an integrated 50 litre buffer in the tank - I'm not convinced it's a good idea but that's what their standard install has)

Heat pump vendors have their own models of cylinders, but AFAICS this is just because they can pre-plumb some of the valves/controls to the front of them - they just buy in the cylinders from somebody else and fit them up in the factory. It's more work to retrofit to an existing cylinder but I don't see any showstoppers (assuming the plumber knows what they're doing). It's better to install the heat pump valves etc how the manufacturer expects it to be installed, rather than try and adapt it to an existing installation - that's a recipe for inefficiency.

There's Mixergy - basically it's a regular tank with a baffle at the bottom inlet to aid in stratification (ie keep the water at the top hotter and prevent dilution with cold coming in the bottom), plus some IoT stuff (top up your heat when electricity is cheap, etc). The IoT stuff may not play nicely with heat pump controls, but the baffle might be useful in reducing mixing.

Theo

ISTR all cylinders are now included under G3 because of the rules for "heat and power" saving.

I fitted a 210L Unistor. IIRC the coil on mine can cope with 22kW which is close to the full output of the boiler, and will keep it in condensing mode for the bulk of the reheat.

No harm in having a 40kW coil, but overkill and possibly more expensive than required.

To my knowledge you have to be competent, and need either a G3 installer to provide a certificate, or go the Building Control route where they will require a report.

If you are convinced otherwise, please feel free to quote the statute, or Statutory Instrument claiming it *has* to be installed by a G3 installer.