Honeywell Evohome Questions What Do I Need...

[*Dan*]

I’m thinking of going for the Honeywell Evohome in my house and while I understand the logic behind it I’m struggling in what I need for it to work seamlessly in my house with my current heating and water setup.

I have an unvented water cylinder in airing cupboard and a potterton promax SL boiler in cloak room downstairs. The water cylinder has two Honeywell zone valves (one for downstairs and one for upstairs heating). There is also another valve as you would expect for the water system. I see you need the relay box which comes in the kit to connect to the zone valves - would I then need two of these to connect to each of my valves?

For the hot water Evohome kit I see you can get a sensor that actually goes in the cylinder - there seems to be a blanking plug on the side of my cylinder in the picture below. Is this to allow for something like an extra sensor to be immersed in the tank?

The zone valves at the moment are controlled by danfoss controllers (one downstairs and one upstairs). The downstairs controller also has a temperature sensor in the lounge as the controller itself is right by the boiler in the cloakroom. Just trying to get my head around how it would be set up (understand the TRV side of things on the radiators) with my system and if it is even possible as it’s not cheap but if it worked properly would be very beneficial in our house and for our lifestyles.

Here is the unvented setup in airing cupboard upstairs and in second picture you can see a brass blanking plug (I think) - is this where I could locate the sensor for the water probe?

 

[ericmark]

Interested to see answers, the question to me is, when you fit eTRV heads to radiators do you need motorised valves? As I see it the idea is to move from digital to analogue, so instead of heating turning on/off which will always cause some hysteresis, we slowly increase and decrease flow through the radiators, but this can only work if the water heater can be modulated.

As far as I can work out your water heater has a flow thermistor which I would assume works by measuring the return water temperature to modulate the water heater, there is no direct connection to the eBUS so best any external control can do is switch the water heater off/on. As a result I question if fitting a device which can only control by switching on/off is going to improve control?

So basics is to ask ourselves what we want to achieve, any eTRV head will allow times to be set, and the temperature control will be more accurate. However we need a method of working out when the water heater is no longer required, or reverse, this is where we are told EvoHome excels. The "thermostat" basically looks at each eTRV head and says if all heads show target equal or higher than actual switch off water heater, other wise switch it on.

However I seem to remember limited to 6 eTRV heads, so with large installations where you can't connect to eBUS then you could fit more than one "thermostat" each one will control a motorised valve, which in turn controls water heater, but then we have to ask what happens when the software tries to reduce hysteresis by mark/space methods (turning off and on and changing the ratio between on and off times) will this help or hinder?

Sorry I don't know the answer, and since I don't have EvoHome I have not looked at software options. You can't throttle back a motorised valve, it only has to options open or closed, and you are going to be switching three valves with at least two EvoHomes unless you remove valves and just use the eTRV heads, and so the water heater will be switched on/off many times a day, not a clue if this is a problem or not.

 

[*Dan*]

Thanks for detailed reply that has helped and as you say maybe the only way is to remove the motorised valves. If that was the case I would be questioning if it is really worth the effort/time/money versus the benefit I’m really going to get from this setup in the long run.