March 2025

Heating escaltion stop relay

I have a Thermolec T-18 electric heater in my furnace and it's part of a multi-stage heating system that allows you to heat with different energy sources.

This Thermolec electric heater is quite small and the idea is that you can stack it on top of an oil or propane furnace together with a heatpump module.

The controls for the oil or propane furnace connect to the Thermolec heater. The Thermolec has a pair of contacts to request heat and another pair to say whether to start the electric heater or instead sent the signal on to the oil or propane furnace.

The problem is that the Thermolec heater has an "auto-switch" function which causes it to escalate to oil/propane after a configurable time even if the control logic requested electric heating and not oil/propane. The escallation time is configurable via some dip switches and can e.g be set to 2h. The maximum is 4h in their firmware version 4 system and there is no mode to say "never auto-switch". In other words if the control system requested continous heat for 4hours then the heating will switch to oil/propane even if the control system did not want that.

Thermolec is a company that sells to US and Canadian customers. I had originally a firmware version 3.0 system which allowed a maximum time of 2 hours before it did "auto-switch". I called the company and proposed that they update the firmware to add an optional "never switch". They did not want to do that and introduced instead the 4 hour auto-switch time. 4hours worked for me on most days. During the night I lower the temperature by 2'C so in the morning the heating needs to bring it back up. When it's -10'C outside then it may take 4 1/2 hours to bring the temperature up.

It does not feel cold while the heating is running. There is anyhow at least a 1'C difference betten upper and lower floor. So the idea that one must reach an exact temperature after 4h is total nonsense especially since it is usually just 0.1'C off target when it switches to oil/propane.

I ended up buying Thermolec's version 4 firmware but I was never really happy with it. My solution was to program a "hole" into the schedule. I would program in the heating control system a time period where the requested temperature is lower. This would stop the Thermolec heater for some time and the 4h period would start over. The smallest period that I can program in the thermostat is 30min long. So for 30min there is no heating at all and then it would actually feel a little cold during that time. I have however used this work-around with a 30min hole in the schedule for several years. This winter I finally decided to build a better fix.

Fixing the Thermolec design flaw with a relay that pauses every hour

Heating systems in north america use 24V AC as a control signalling voltage. A 24VAC relay connects the C and W pins of the Thermolec heater to request heat. The relay sits in my case in a box that contains all the heating control logic. I simply replaced this relay with a relay that pauses every hour for 3minutes. The thermostat drives this relay and request heat contiously but the relay opens for 3 minutes every hour. This way the Thermolec heater never escalates to oil/propane (no auto-switch).

I am using an atmega328p microcontroller to do the timing and I have added a few LEDs to allow me to understand what's going on. That's essential for any more complicated heating system. You want to understand why something is happening. A binary counter consisting of 3 red leds shows how oftern the relay has been pausing for 3min. There is also a switch which allows the user to turn the pause feature off.

I am probably not the only one who encountered this problem with the Thermolec heater design. Feel free to copy my solution. All the documents and software are included below. I can send you a atmega328p microcontroller with pre-loaded software if you are not sure how to program a microcontroller.

The Thermolec heater has the following pin connections for the control logic. It took me a while to figure this out.

pinout of the Thermolec control interface:
--- F --
--- F --
--- C --
--- W --
--- S1--
--- S2--
--- B --
--- B --

F - F = output pins for fan conntrol. Connection closes
when fan should run. That is: the Thermolec connectes internally
the pins with each other to signal that it wants the fan to run.

C - W = input pins. Connect C to W to start heater

S1 - S2 = input pins for source selection, if C and W are connected 
and there is no connection between S1 and S2 then the oil burner (pins B−B) (or propane furnace)
will start otherwise the Thermolec will start the electric heating elements.

B - B = output pins for oil burner control,
connection closes to start the oil or propane furnace.

The relay with the pause feature connectes to the C and W pins and closes them when heat is requested but opens every hour for 3min.

24V AC relay with a pause feature to prevent the auto-switch in the thermolec heater

circuit diagram of the 24V AC relay with a pause feature (open for 3min every hour), click image for pdf file

24V AC relay with a pause feature, pinout

The circuit diagram shows a 7805 voltage regulator but I used a buck regulator sold by a chinese seller called alice1101983 on ebay. It's a small board with two input pins for DC from 8V to 100V and two output pins for 5V DC. It does not have a data sheet and I am not sure if it can really handle up to 100V DC. This buck regulator works well and it does not produce any heat. A 7805 voltage regulator would be near it's maximum allowed input voltage when you rectify 24V AC and it would get warm.

Meaning of LEDs:

Green LED: full on, the circuit is powered. Blinking, we are in a pause period. The yellow led will show if relay is on or off since this depends on the position of the switch S1.
Yellow LED: on, relay output closed. off, relay output open.

The 3 red LEDs: these LEDs are a 3 bit binary counter (counting from 0 to 7) the amount of 3min pauses that are done. The LEDs are updated at the start of the pause period.

Red LED 2	Red LED 1	Red LED 0	cnt	meaning
0	0	0	0	countinous heating for less than 1h
0	0	1	1	one pause done, continous request for heating 1h+ and still heating
0	1	0	2	two pauses done, continous request for heating 2h+ and still heating
0	1	1	3	three pauses done, continous request for heating 3h+ and still heating
1	0	0	4	four pauses done, continous request for heating 4h+ and still heating
1	0	1	5	five pauses done, continous request for heating 5h+ and still heating
1	1	0	6	six pauses done, continous request for heating 6h+ and still heating
1	1	1	7	seven pauses done, continous request for heating 7h+ and still heating

The counter wraps to 0 after 7.
The red LED counter is reset as soon as power is taken away from this circuit (green led off).

Download

heating-escalation-stop-0.2.tar.gz firmware for the atmega328p chip, source code and circuit diagram are also included