Replacing TRVs with electrically-operated valves

[Peter Martinez]

New to the forum:
I have an oil-fired central heating system with one thermostat in the house which controls the boiler and pump. I have tried fitting TRVs to some of the radiators but they "fight" the main thermostat - the TRVs could all be off with the boiler/pump still running. I wish to replace the TRVs with electrically-operated valves, each with a separate thermostat in the same room. Ideally auxiliary switches on the valves will be wired so that the pump/boiler runs if ANY room is cold but shuts down if all rooms are warm.

I say "electrically operated" because I am not sure what is the right name for these things. "Thermostatic radiator valve", or "motorised radiator valve", but they don't seem to have motors in them, just a wax cylinder like in a TRV, but with a heater as well.

The Honeywell MT-4 device seems suitable. I think I can just replace the TRVs with these, but I am not sure which type I need. The cylinder in the present TRVs expands when hot, pressing a button on the radiator fitting to turn it OFF. I want the replacement to turn the radiator ON when I apply 230v to it's heater.

Question: Does this mean I need a NC (normally closed) type of valve (such as the Honeywell MT4-230S-NC)?

Another question: I need the auxiliary switch to be ON when the radiator is ON, i.e. when the valve is OPEN. My electrical training tells me that's a "normally open" switch so you can see why I am confused. Does the auxiliary switch come ON three minutes after 230v is applied to the device?

Any other advice would be most welcome.

Peter

 

[stem]

The MT-4 device is designed to work as part of a Honeywell Evohome System which will provide individual radiator control and a boiler interlock you are looking for.

However, the problem of 'fighting' that you describe with your existing set up will be because of bad design. My guess would be that either the golden rule of not fitting a TRV to the radiator in the room where the thermostat is located, has been broken, or the room thermostat has been fitted in the wrong location for your property.

 

[Peter Martinez]

The MT-4 device is designed to work as part of a Honeywell Evohome System which will provide individual radiator control and a boiler interlock you are looking for.

However, the problem of 'fighting' that you describe with your existing set up will be because of bad design. My guess would be that either the golden rule of not fitting a TRV to the radiator in the room where the thermostat is located, has been broken, or the room thermostat has been fitted in the wrong location for your property.

Stem: Many thanks for your reply. I am sure you are right. That's why I am here asking questions. I believe I can replace each TRV with a thermostat and an MT-4 (or similar) and wire the aux. switches together to replace the existing single thermostat. I don't need Evohome.

Are you able to answer my two questions: (1) Do I need an MT4-230S-NC? (2) Does the auxiliary switch close when the valve opens?

Many thanks.
Peter

 

[bernardgreen]

A Normally Closed switch is closed when the device is in-active / quiescent / at rest.

With a radiatiator control valve I would expect a Normally Closed switch to be closed when the water valve was closed ( the quiesent state )

Hence if you want a contact that closes when the water valve is open then the actuator should have a Normally Open switch

 

[stem]

Ah sorry my mistake, I was getting the part numbers mixed up. I was thinking of the radiator controllers that communicate wirelessly.

I've never used an MT-4 which is a wired version that has an integral microswitch. But 'NO' is what most electric room thermostats use to refer to the state when the thermostat is satisfied, which would be correct for a boiler interlock. But there are exceptions so if no one comes along that has used one, I would check with the manufacturer to be sure.

You will need to run a cable to each radiator though, so the wireless version would be easier if you wanted to fit it to several radiators.

Edit
Having a quick look on the interweb, those for sale are generally 'NC' which would be what most systems would require, so Bernard is probably correct.

 

[Peter Martinez]

A Normally Closed switch is closed when the device is in-active / quiescent / at rest.

With a radiatiator control valve I would expect a Normally Closed switch to be closed when the water valve was closed ( the quiesent state )

Hence if you want a contact that closes when the water valve is open then the actuator should have a Normally Open switch

Bernard:

I am still confused about "normally-open" and "normally-closed". I think I need the VALVE to be normally closed and the SWITCH to be normally-open, but I want to be sure. TRVs work by CLOSING when they warm up, but I want the valve to OPEN when I apply 230v (which warms it up?) which is the opposite. I also want to be sure that the switch closes when the valve opens, but as Stem says, I might have to ask Honeywell.

Peter
[in a 250-year-old stone farmhouse in Cumbria]

 

[bernardgreen]

 

[bernardgreen]

I meant to add that it isn't clear as to what the state of the switch is, it looks like the NC or NO suffix refers only to the state of the water valve.

This suggests that with a NC actuator the water valve is closed when no power and the switch is open. I assume that with power the actuator is pushed upwards to open the water valve and close the switch. The expamding wax drives the pin upwards. (( That is the opposite of a "normal" TRV where the expanding wax drives the pin down to close the water valve. ))

 

[stem]

It's a pity it's not like an room thermostat that comes with both NO & NC contacts. Then if it works in reverse, it's just a matter of changing the wire from one terminal to the other.

 

[Peter Martinez]

It's a pity it's not like an room thermostat that comes with both NO & NC contacts. Then if it works in reverse, it's just a matter of changing the wire from one terminal to the other.

I wouldn't be surprised if there is a standard change-over microswitch inside it but only the N.O. wires fitted. The Honeywell V4043 is like that. However, that wouldn't solve the problem if the valve works the wrong way. If the MT-4 is just a standard TRV with a built-in heater, it WILL be wrong.

This morning I was in town and put these questions to two plumbing/heating emporiums. Neither of them had a clue.

Peter

 

[Peter Martinez]

I meant to add that it isn't clear as to what the state of the switch is, it looks like the NC or NO suffix refers only to the state of the water valve.

This suggests that with a NC actuator the water valve is closed when no power and the switch is open. I assume that with power the actuator is pushed upwards to open the water valve and close the switch. The expamding wax drives the pin upwards. (( That is the opposite of a "normal" TRV where the expanding wax drives the pin down to close the water valve. ))

Bernard: I think you have got it. Thanks. The NC refers to the plumbing not to the electrics. I found some more info elsewhere which says you can wire the switches on several MT4's in parallel to control the boiler. That must mean they are normally open. Honeywell's response may well confirm this, but I have a feeling I just re-invented the wheel!

Peter

 

[bernardgreen]

Install Evohome. Job done.

After a bit of setting up and programming

Jesus. How to over think things?

How to over complicate things. Batteries to replace, ( 24 months or sooner ) wireless comms and that neat little display to show how clever the TRV is....and display faults and error codes. How quiet is the motor driven actuator ?,.

Honeywell's response may well confirm this, but I have a feeling I just re-invented the wheel!

More like you are selecting a basic and functional wheel that does the job reliably instead of a glitzy and over complicated wheel.

 

[endecotp]

https://www.honeywelluk.com/Documents/Full-Specification/pdf/956.pdf
Page 2, footnote to table 1.
Also the caption to figure 6.

If you want the radiator to come on when the power is applied, you want the NC version.

It says remarkably little about the aux switch, but it does say it's for pumps & fans so I believe it should do what you want.

 

[endecotp]

Your confusion is probably because "open" and "closed"mean exact opposites for valves vs. switches.
An open valve lets water flow. A closed switch lets electricity flow.

 

[Peter Martinez]

Your confusion is probably because "open" and "closed"mean exact opposites for valves vs. switches.
An open valve lets water flow. A closed switch lets electricity flow.

Yes that was certainly part of my problem - I am an electrical engineer, not a heating engineer - but I also had a mental picture of one of these "wax actuator" devices as being a TRV with a heater strapped to it, and that would have closed the valve when it got hot. I will just wait to see if I get anything back from Honeywell then buy a couple next week and report back here. Thanks to all who responded.

Peter