HR92 needed for all radiators?

[Sueziemc]

Hi

I've spent a lot of time researching which smart heating control system to buy and I'm just about settled on the Evohome but I'm just a bit unsure of how many HR92s I need.

I currently have CM927 controlling my whole house. I have 8 rads in total but want 4 of these in one zone and then the other 4 I want to control individually.

I was ready to buy the "Honeywell evohome Wi-Fi Connected Value Pack B", expecting to use the controller itself for the single zone with 4 rads in it and then out the 4xHR92s on the 4 individual rads for fine control. However, having spent some time reading this forum it seems that this might not work?

Will the ATC928G3000 act as the thermostat controlling all rads that don't have HR92s? Or do I need to invest in 8xHR92?

Thanks

Sue

[Richard]

Hi Sue,

I can only tell you how the evohome system was designed to work and the brief answer to your question is that yes, you do need HR92's on all radiators for evohome to work as it was designed to...

The main reason for this is unlike a single zone thermostat heating system, (in multi zone mode) evohome controls your radiators with HR92's (and therefore the zones they are in) and this is done primarily by controlling the flow of water to each radiator. So in a system that is only partially zoned, an 'uncontrolled' radiator would come on each and every single time a HR92 controlled radiator has heating demand. This would result in the uncontrolled zones potentially overshooting their set temperature as well as the energy that you would waste in zones that maybe didn't need heating, when say a bedroom zone did.

I hope that helps explain, but if you have any questions at all please ask.

Thanks,

Richard

[Sueziemc]

Thanks Richard, that makes sense and I'm glad I asked as it obviously makes a huge difference to set up costs!

Back to the drawing board and further research for me!

Thanks ahain

[Richard]

Thanks Richard, that makes sense and I'm glad I asked as it obviously makes a huge difference to set up costs!

Back to the drawing board and further research for me!

Thanks ahain

No problem at all, if you have any further questions feel free to contact me - https://theevohomeshop.co.uk/content/175-contact-us

[sharpedge]

I think that fitting all radiators with HR92s may not be the only solution. If you have ordinary thermostatic valves and some areas where you don't need to control the temperatures in the way an HR92 does ie you need these areas just to have some background heat when other rooms are being heated. If this is the case then leave the ordinary thermostatic valves on these radiators and fit HR92s on the ones where you do need the control of the temperature. These secondary radiators would only come on if the thermostatic valves opened AND the boiler was supplying heat due to an HR92 opening.

[Richard]

I think that fitting all radiators with HR92s may not be the only solution. If you have ordinary thermostatic valves and some areas where you don't need to control the temperatures in the way an HR92 does ie you need these areas just to have some background heat when other rooms are being heated. If this is the case then leave the ordinary thermostatic valves on these radiators and fit HR92s on the ones where you do need the control of the temperature. These secondary radiators would only come on if the thermostatic valves opened AND the boiler was supplying heat due to an HR92 opening.

As I have commented above, the evohome system was not designed to operate like this and you will be fully reliant on the zones with HR92's calling for heat to heat other zones. The HR92 controlled zones will be using an aggregate demand percentage to heat the zones in question, which may mean both the controlled and uncontrolled zones don't hit their target temps correctly. There are also software limitations when doing this, which will prevent certain setups being possible (for instance S Plan Plus cannot be catered for if you only partially zone a system).

The evohome Installation Guide page 42 & 43 shows the possible evohome configurations and as you can see S Plan Plus is not in there (hence Figure 4 is recommended with HR92's on all radiators).

This forum is here to give the correct information to customers, unlike some of the 'hearsay' ones out there... I appreciate things can be 'bodged' certain ways (I have tried and tested all of them myself), but we always recommend best installation practice for very good reason.

Unfortunately, there have been too many installations of evohome in the past where customers and installers have not installed evohome products correctly (or thought they were doing the right thing based on limited knowledge) which then makes diagnosis of 'system issues' very difficult (hence we always ask for pictures from our customers of the installation before we commence with any such technical support).

[Tigermad]

If you don’t have an Abv fitted you will need a bypass radiator. I know lots of people with evohome that don’t have a hr92 on their towel rail either. Just a normal trv. Are you saying this is wrong?

[Richard]

If you don’t have an Abv fitted you will need a bypass radiator. I know lots of people with evohome that don’t have a hr92 on their towel rail either. Just a normal trv. Are you saying this is wrong?

The evohome system does not account for 'loads' it does not know about in its fuzzy logic. I have already tried my best to explain why above, so no need really for me to go over this again.

[chippy419]

I have always fitted a auto bypass on a boiler protects the pump

[Richard]

I have always fitted a auto bypass on a boiler protects the pump

Ummmm, interesting point of view... ABV's are normally fitted to ensure minimum flow rates are achieved through the boilers heat exchanger to prevent damage to the heat exchanger when system flow rates are reduced.

But how do you set an ABV on a fully modulating pump?

I personally look at the minimum flow rate of the boiler and ensure it has somewhere to go rather than an ABV when a boiler has a modulating pump these days (unrestricted airing cupboard radiator or towel radiator).