Johnson and Starkey warm air system thermostat

[silverfoxcc]

Have the above and want to change the froom thermostat, from the old rotary'guess the temp' one to a digital display ong

None of the ones that i have looked at say suitable gor warm air systems

Any sparks out there know of compatible didital thermostats ?

TIA

[Mr Pointy]

Do you have a picture of the wiring to the room stat? They are generally pretty simple devices with 2-3 wires to them.

[S6PNJ]

I had a property with Warm Air and didn't think twice about swapping thermostats - they are typically 2 wires which call for heat when connected or break the call for heat when they disconnect. Sometimes a third wire which then feeds 240v so it provides the power to 'supply' the heater. Can you post a pic or state the model number of your old stat? I replaced mine with something like this:

https://www.draytoncontrols.co.uk/product/digistat...
Many, many other suitable thermostats are available

[silverfoxcc]

S6

Thanks

The old stat was a J+S rotary one . No temp marks just a number range 1-7 and just had two terminals


The two wires from the warm air unit run at 7.24v or thereabouts

The Timeguard one was a TRT033C and has three terminals

I have connected the two wires in all possible 6 combos and on each one the fan runs continously

And in each position it doe not 'fire up' the gas to provive heat when i adjust the reqd heat above the ambient temp

A mail has gone off the timeguard to ask their opinion.

[S6PNJ]

silverfoxcc said:

S6

Thanks

The old stat was a J+S rotary one . No temp marks just a number range 1-7 and just had two terminals


The two wires from the warm air unit run at 7.24v or thereabouts

The Timeguard one was a TRT033C and has three terminals

I have connected the two wires in all possible 6 combos and on each one the fan runs continously

And in each position it doe not 'fire up' the gas to provive heat when i adjust the reqd heat above the ambient temp

A mail has gone off the timeguard to ask their opinion.

Is the Timeguard the one you are looking to use, or is it already in use in your system? Assuming it is the one you want to use, it should be a case of swapping the 2 wires from your old rotary stat (a simple open/closed mechanical stat?) into the right terminals of the Timeguard. You'd wire the 2 wires you've identified to contacts 1 and 2 or 1 and 3 depending on the no call/ call orientation (when it is closed/open). Ignore the RF element, it's just a random diagram.

[aeropilot]

silverfoxcc said:

S6

Thanks

The old stat was a J+S rotary one . No temp marks just a number range 1-7 and just had two terminals

I still have the old J&S rotary dial stat on the wall in my living room from when I had the hideous old warm air system.....and funnily enough, I now have the same digital stat as posted by S6 above in another location to control my nice cosy and much cheaper to run, combi boiler radiator central heating

[Mr Pointy]

Are these the instructions you have?
https://www.timeguard.com/media/1884/TRT033C_Instr...

They are badly written as the wiring diagram seems to bear no relationship to the picture of the back plate. As far as I can see your existing two wires need to be connected between the backplate terminals marked Live & Switched Live. Ignore the earth connection.

7.2v sounds odd. Can you check there isn't a blown fuse in the heater control system?

[TheInternet]

S6PNJ said:

Assuming it is the one you want to use, it should be a case of swapping the 2 wires from your old rotary stat (a simple open/closed mechanical stat?) ...

It sounds like it's one of their thermista-stats which is not simply open/closed.

[Mr Pointy]

OP, do you know if you have the Basic Control system or one of the more advanced versions, eg System ET? If it's the latter then it does look like the thermostat is not a simple contact closure. You could try calling J&S to see if there's an updated Thermistastat but responses over of Diynot aren't very hopeful:
https://www.diynot.com/diy/search/98408695/?q=John...

https://www.johnsonandstarley.co.uk/contact-us

[gooman]

Is the original stat is a "Modairflow" stat that looks something like this: https://phc.parts/product/johnson-starley-102797/T...

If so, it's NOT a simple on/off thermostat.

It's a thermistor. Here's the diff ...

A traditional thermostat tells the boiler either "everything's warm enough" or "it's too cold". It's on/off.

The J&S Modairflow stat tells the boiler *how* much colder the room is than what's desired. The boiler then modulates - rather than providing full power until it's warm enough again, it backs off and provides less heat. This is needed on a warm air system - warm air heating warms up the air really quickly (much quicker than a traditional rad system) but also cools down more quickly.

If you replace it with a traditional thermostat, you're likely to regret it. On cold evenings you'll feel it getting colder between cycles.

We moved into our place with a J&S warm air system only a month ago. I've replaced the ancient rotary timeswitch with something better, but I've not yet looked into modifying the stat (though that's part of my long term plan).

[mrgonzalez161]

@gooman, thanks for explaining the difference between a thermostat and a thermistor. I had no idea and I think this is the crux of my problem.

In April 2019 I had a Nest smart thermostat installed to replace the thermosta-stat of a Johnson & Starley Hi-Spec J25RS System E-T. For all intents and purposes I believe the E-T works the same way as a Modairflow. I wanted a smart thermostat to keep costs down and it came with free installation from Nordec (https://www.nordec.co.uk/nest-heating-control/) who stated it is compatible with my warm air system. Nordec operate in the South London area.

With the colder weather now I have indeed regretted replacing the traditional thermostat. Last February we were very cosy but as this year as you predicted the cycles do appear to be cooler with the boiler appearing to flit between on and off rather than staying continuously on until the desired temperature is reached. I can only assume that it is down to a thermostat (regardless of how 'smart' it is) being used rather than an intended thermistor.

It sounds like I will have to have the old thermosta-stat re-installed (I thankfully kept it) and the Nest equipment removed. You mentioned that you upgraded the old timeswitch which I'd be interested to hear about. You also mentioned modifying the stat, have you managed to look into it? Any comments or help would be much appreciated.

[gooman]

I fitted a Sonoff Basic wifi relay to replace the original timer in the boiler (I'd need to take the cover off to take pics, so will try and do that sometime soon). I have a Home Automation system which controls the Sonoff. That lets me do all sorts of clever stuff like presence control (so the boiler turns off if we both go out during "on" times, turns on if either of us come home, and only turns on in the morning if we're both home).

I haven't yet managed to work out how to replace the J&S Thermistat. I *think* that what I'd need to do is replace the thermistor in the circuit with a digital potentiometer, controlled by a WiFi controller like a Wemos D1 mini. But there's no simple instructions for doing anything like that, so it would be back to basics and a lot of work ... unfortunately I have too many other big jobs to sort out!