Another one for the plumbers

The universal answer is 42:

IANA plumber, but AIUI the thermal store always needs to be hotter than the desired water temperature, because the heat exchanger isn't perfect. That limits the headroom of how cool you can go.

I suppose there is less static water in a thermal store so less opportunity for legionella to breed. And the rest of the pipework is just the same after the tank.

That raises a question about what happens to any legionella in the pipes:

- at 60C constant, any water takeoff would kill off legionella in the pipes

- on a weekly boost to 60C, water may not be taken off into all the pipes to kill legionella lurking there (eg a leg for a rarely used shower)

- on a thermal store setup as you propose there would never be temps high enough to kill off legionella in the pipes

So I suppose it comes down to what you mean by 'exposed to atmosphere': while neither a thermal store nor an unvented cylinder are exposed, it may happen somewhere in your intake feed. There's less static water in a thermal store to encourage breeding, but the same pipework downstream where it may breed. OTOH copper pipework is somewhat antibacterial.

Depends on your risk calculations...

Theo

How would you know what the actual temperature is without running the water and measuring it, or fitting a digital thermometer to the output? My W-B Greenstar has no digital display to let me know what the temperature is of the water coming out of the boiler. The boiler thermostat knob doesn't even have a meaningful display. All it states in the manual is "The temperature range available is between approximately

Seem to me that it would cost very little for boiler manufacturers to add a digital display of temperature.

Or even if there is any legionella in domestic pipes.

Are there any statistics on legionella infections in domestic situations ?. The only part of a domestic setup that might be a problem is the flexible shower hose that is connected to a mixer tap. If this is not used regularly then the water inside this flexi tube will become stagnant.

This summer my loft (and hence cold tank) got up to 47C every day and a plastic tank is more at risk then a copper tank and pipework where copper is a natural biocide.

The store has a thermostat to control the boiler. Clearly this is only an indication of the likely outlet temperature and the actual will decay with draw off.

My boiler displays water temperature.

You also have to take into consideration that any new water entering the system is treated to prevent such things as legionella and in a domestic environment is it going to be a problem?

Seems unlikely here. Mains fed underground ->water softener->22mm copper to store-> 22 and 15mm copper draw off to kitchen and bath/shower rooms. Weak point seems to be shower head but you have that whatever the source temp.

With a thermal store the water content is never "consumed" as DHW - it is used either to heat rads or UFH, or via a heat exchanger to heat incoming mains water "on demand". So any biohazards from the water are minimal to none in normal use.

The main influence that temperature will have is on the amount of energy you can store. The higher the temperature, then the more energy stored, and th less frequently the boiler will need to recharge it, and the longer the sustained delivery of heat can be.

Also you won't be able to use the store to heat anything to a temperature higher than its content (and in fact will be limited to a couple of degrees lower probably). So if you are driving conventional rads, and need them to get to 70 in the depths of winter to get the place warm enough, you would need to raise the temp of the store to do that.

The thermal store temperature is controlled by a thermostat - so the DHW heated using the store will be close to the store temp as it comes out of the heat exchanger. You may lose a bit on the way to the taps depending on length of pipe run and how well insulated it is.

Many do have digital readouts of current flow temperature.

Seems to be a general consensus that a thermal store can be exempt from routine Legionella prevention.

Mine only does DHW.