New system: Thermal Store or Gravity fed and thermostatic mixers

We are having our small detached house enlarged by about 50% to a total internal volume of roughly 250 cu m. It is 1956 brick built and apart from there being no CW insulation in the original walls (yet) the planned insulation will be about the current standard.

We are now researching a water heating system within our budget. Mains flow rate is 21 l.p.m. bucket test.

There will be a downstairs shower room and an upstairs shower/bath and separate en suite, and conventional water filled radiators.

We already have an Intergas compact 24 OV only 2 years old and are happy to stay with an open vent gravity system if needed. The header tank is in the loft and its base is 2550mm above 1st floor level. There is room to raise it possibly by 900mm before it starts to get close to the rafters.

We want to avoid fluctuating temperatures in the showers. There are usually only two of us but we expect problems with washing machines and kitchen taps all fighting for water.

We don’t feel easy about Megaflows but like the idea of a thermal store with a plate exchanger. We are in a hard water area but understand that a good installation can help reduce scale formation by keeping temperatures to a minimum.

We understand that using the thermal store for central heating water as well could be a good option.

We have heard of some models that have a choice of plumbing points for the optimum cold return.

In the worst case if our plumber ends up advising against a thermal store due to either cost, complication etc, we are not sure of our options if we stay with a gravity fed simple HWC

It seems the choice is between low pressure mixers with a cold supply from the same header as the hot, or pumps, and a thermostatic mixer at the shower end.

We wouldn’t want electric, nor a combi boiler, nor a megaflow as mentioned.

Would there be enough power in the showers to prevent them running hot or cold whenever a tap is turned?

I like thermal stores, but is there a reason you don't want an unvented cylinder? Not sure why you are thinking you need to raise the header tank, are you developing the loft?

Keeping the open vented system and installing Thermostatic mixer, with a shower pump will be fine,

You have good water flow rate too.

You could have the taps using mains water, with just the showers using the pump and header tank.

Generally it's only really combi boilers that have the issue of temps fluctuating when other taps are used.

Thermal stores are terrible for boiler efficiency as they have to be kept too hot for a condensing boiler to actually condense. Make the best use of the excellent boiler you have - put an unvented cylinder in, and have a hot water priority setup. The boiler will ramp up to max power to heat your cylinder quickly, and can have an OpenTherm control for best heating system efficiency.

The OP said they are not comfortable with an unvented, so there is really no other option than a thermal store if you want mains pressure hot water, unless they install a combi.

You are correct in that most thermal stores have too small an exchanger to deliver decent hot water unless they are kept hot. There must be some that are designed to be used with heat pumps, no?