Electric override of TRV's

[ericw]

Several previous threads have mentioned overriding a TRV with a heater but have drifted off into using relatively expensive Rondostats etc, without any practical details of a low cost system being discussed.

In order to get an idea of the practical aspects of turning off/setting back a TRV with a small heater I have experimented by attaching a small heater to the sensor bulb and plotting the rise in temperature of the said bulb.

For maximum effect the heater has to be tightly coupled to the sensor bulb but not interfere with its normal operation.
In principle it is possible to wind a length of enamelled resistance wire around the bulb to achieve this. Unfortunately suitable wire is not easy to come by in small quantities (www.wires.co.uk is a possible source) so I used a power transistor clamped to the bulb with a cable tie as my heating element and sensed the bulb temperature with a DS18B20.

The attached graphs show the temperature rise obtained by putting various levels of power into the heater.

My application is to convert the existing bedroom TRV's into pseudo zone valves to shut off the heating upstairs during the daytime. It would appear that by using 1/2 watt or less per valve this can be effectively achieved.
As a plug-in digital timer can be had for £5 and suitable 5 or 12v switched mode power supplies are under £5 on EBay the whole system can be built for less £20.

As the temperature rise is dependant on the power level an adjustable set back could also be achieved by feeding the heater with a PWM signal.

[wookey]

Very useful bit of work eric. My understanding of this was that it used at least a watt, and all commercially-available (wax) devices used at least 3W and often 5W. This was the main reason I decided this was not a good-enough solution. But at 600mW I guess it is viable (still inefficient in comparison to an HR20, but acceptable).

Note that HR20s can be had for 25euro each, probably less if you shop around (I got 5 for 50 quid s/h).

I got a few metres of nichrome off some chap on the net who had loads, for some nominal sum, after some searching. I agree it's suprisingly hard to buy just a bit of it (and not tens of expensive metres). I can send you a bit if you want to play?

[MikeM]

Yes, a very useful bit of work and something that I had wondered about for some time. I bought some Danfoss thermo electric actuators (TWA-A NC) to play with. They are rated at 24V 2 Watts. See:

http://www.navitron.org.uk/forum/index.php/topic,12073.0.html

These are 'normally closed' so power is applied to open them. I haven't actually used them but was taken by your idea of a plugtop PSU and simple time switch for On/Off control.

At present I am too using Rondostat HR20 (Lidl £20.00 specials) and am very pleased with them !

[KenB]

I had a go at this back in 2005 and found that 1W of power was sufficient to close the TRV in approximately 20 minutes. Faster response could be achieved with more heat, but as room heat-up cool down are fairly slow processes I reckoned that a response time of 10 to 20 minutes fully open to fully shut was satisfactory.

I revisited the idea over Christmas following some ideas about distributed dc power.  A 12 to 24V ring main could supply low voltage dc to electronic devices in the room. The TRV "heater" would be just one such other addressable device.

The original thread from September 2008 is here  http://www.navitron.org.uk/forum/index.php/topic,4638.0.html

Of course you only need to power the heating element/resistor/power transistor whilst the circulation pump is running and the boiler is enabled.

The few watts that you use on these resistors is minimal compared to the wattage of the central heating pump or the boiler itself - so no need to get hung up about it.

I'm currently looking at using modified PIR devices to measure room occupancy plus give some indication of the temperature of the internal walls of the room. PIR sensors normally have two thermoelectric sensors in reverse polarity to cancel out background radiation, and make them more responsive to changes - i.e. movement of warm bodies.

A single PIR device could tell you the true temperature of the structure plus if anyone enters the room.




Ken

[SimonHa]

...As the temperature rise is dependant on the power level an adjustable set back could also be achieved by feeding the heater with a PWM signal...

Interesting... I think you'd want a switch on time of under 20 mins for unplanned bedroom occupancy changes though!

One other thought is whether you need higher power for the phase change of the wax, but once the valve has changed state, you could reduce it to the lower levels you suggest. Note: if so, this could also applied to commercial actuators too. Admittedly we are splitting hairs here