BCO is strict about TMV requirement for baths and showers have them already. I'm wondering if I can set my cylinder temperature low enough to not need a TMV. That would make whole system run at the restricted temperature (45C?).

 

I can't think of a reason we'd need warmer than that at any taps.

 

A cooler tank is good for ASHP efficiency but not so good for hot water volume. That should be fine though.

 

Tank would probably have to do some legionella magic periodically though so that may be tricky / make it impossible. I suppose the legionella cycle could be done using an immersion heater wired in parallel with a NO 2-port valve that closes whilst the water is too hot. There's a time delay issue there, but is anything like this ever done? 

Set the cylinder to be nominally <48C and be done with IMO.

 

Sack off the legionella cycle entirely. Not required. 

 

 

The only real risk to normal people is really the shower (cold pressure drops; hot water red hot; jump out of your skin and whack your head on the shower valve as you slip before getting scalded whilst out for the count)

 

If you were old and demented you might leap into a bath you've just filled at 60C; or if thick you might fill a bath with scalding water at then throw a baby into it; but 48C isn't going to save you from either of those anyway.

 

 

I think most all scalding is now in "care" facilities with shonky recirculating hot water systems dialled up to 11 to try kill legionella; or scumlord properties with dodgy immersions in direct electric tanks sat at 80C with non thermostatic mixers for showers etc. 

In my last build I set the DHW to 48’, no TMV,s, larger DHW tank and ASHP COP very good.

If your really concerned just stick a TMV on the cylinder outlet, same as all thermal stores have.

One that responds quickly and doesn't need a large delta between the hot inlet and the mixed temperature in order to operate.

One downside of a low hot water temperature is that you asking for a higher flow rate through your pipes as it will be further from a 50:50 mix of hot and cold.

 

I was thinking 12 L/min for a shower, but it looks like it might be lower:

   "In the USA the maximum flow permitted from a shower head is 9.5 litres a minute and the UK government is looking to bring in an even lower rate of perhaps 8 or 9 litres a minute." [link]

 

15mm Hep2O which I am using has a maximum flow rate of 6 L/min if you are trying to stay under the 1 m/s flow velocity recommendation for low noise applications. Netherlands guidelines of maximum 1.5 m/s for hot water (2.0 m/s for cold). If all the shower water was going through the one pipe, the flow velocity would be higher than 1.5 m/s so could get noisy.