Hi Toodles!

Have you got anywhere with this? since I think we might have the same question. We’re getting a dual fuel towel rail and ufh for the new shower room. There will be a timer on the electric supply to just give a boost in summer, (when lots of PV).

at present we’re still on gas for the heating but want to make sure we have to re-do as little as possible when we change to ashp. 
I hope that you can help and thanks very much!

Hello Judith, I came to the conclusion that as long as I remain ‘Compost Mentis’ I would continue to remember to close the inlet valve to the towel rail before the electrical heating element is switched on and then open it again after the electrical supply turns off at night. How this works is that during the summer, I can leave the valve open or closed as the ASHP is not running anyway. During times when the heating is likely to come on, I’ll leave the valve open so there is flow through the towel rail providing some background heat to the rail. Bearing in mind this heat is much lower than the electrical element provides, I then close the valve ahead of the electrical element being powered up. Just the one valve is used in this way, as that is sufficient to control the flow and then there is no chance of me inadvertently causing a pressure buildup and damage to the plumbing. Our ASHP does not heat the DHW as this is handled by the Sunamp Thermino unit. I have considered whether it might be worthwhile to have a motorised valve to operate with the electrical element but well, lif is too short to stuff a mushroom! Regards, Toodles.

@toodles 

thanks!

this seems to be yet another feature we need to consider ourselves since it’s too detailed for builders/plumbers/electricians. Our electrician is our best bet of guidance since he’s done a lot of his own stuff. I naively thought electric towel rails always included timers since it’s too expensive to run them continuously! 
I have a head like a sieve so I wouldn’t dare rely on remembering!

@judith The Eddi does the switching for me so at least the worstthat will happen if I forget to open / close the valve on the towel rail is that we have a cold TW during the day or the electrical element might upset the heat pump’s temperature sensor’s sensibilities! The Myenergi Eddi relay board has been programmed to do the switching for me as a secondary job to diverting solar energy to heat the Sunamp element. Regards, Toodles.

@toodles 

I asked the plumber who was installing this yesterday (we’re only going to use the electric version in the summer when the water isn’t being pumped) and the plumber’s response was along the lines of-

-there’s no need to worry about leaving air in a dual fuel radiator to take up expansion (something else I’ve seen suggested) because there’s lots of scope for expansion in the pressure expansion vessel of the whole system. The air suggestion is for electric only.

-in the summer when the pump isn’t flowing then the electric just warms its location not trying to warm the whole system 

We’ll try it when it’s finished (no electric connection yet). Any comments on the plumber’s view?

@judith I have an update of a sort to report: We use the electrical element during the winter too! I find that (particularly in mild weather) the radiator below the towel rail is little more than lukewarm and without the electrical supplement, so will the towel rail be. (Or as Mildred in ‘Men from the Ministry’ used to say ‘It may luke warm but it is cool!’) So the Eddi switches on the element in the evenings and I continue to close one towel rail valve during this time so keep the water from flowing. I then try to remember to open said valve again after we have showered when the element is cooling down. The heat pump heated water again flows through the towel rail until the next evening….

Regarding the water pressure; I had no concerns about the overall system pressure just the expansion that would take place were both towel rail valves to be shut and then the water contained in the rail being heated up and expanding - hence only shutting the one valve avoiding any pressure build up, and this in itself being sufficient to prevent the element-heated water from flowing away all the time.

Regards, Toodles.

Aaaaah!

your situation is more complicated than I realised!