Tarm lo-limit fan switch

The "Operating Thermostat which is in the front face of the boiler has to be lifted for an adjustment which is below or above the recomendated setting. The Low limit fan switch- as far as I know- doesn't need to be lifted.

The fan thermostat is under the sheet metal, the operating thermostat is exposed, on the front of the boiler. The operating thermostat can be lifted up, the fan switch can't.

Mine just turns no lift needed.

Are we all talking about the same things? The Operating Thermostat described in the Tarm manual is on the front panel and is the draft fan control, triggered by the water temperature in the boiler. It is operated by a bulb set into a well into the top of the boiler. It has a "stop" so that it cannot, without lifting it up, set a temperature below 180F (85C), and it's maximum setting is 190F (90C). I have mine set at the maximum, and it will shut down the draft fan at about 190-194F based on the thermometer on the front panel. Incidentally, output hot water actually is a little higher than this, as measured by a sensor on the supply pipe as it exits the boiler. I'm at a loss as to why a person would want to set this lower than 180F, especially with storage, and it seems to me that 190F would be the preferred setting.

The Low-Limit thermostat is under the top cover and is located on the control panel and at the end of the terminal strip. Its purpose is to shut off the draft fan when the fire has burned out. This thermostat has no "stop" and is rotated to the desired setting. The manual states that this should be set at 190F (90C) if storage is being used. I set mine at 100C. It is operated by a bulb sensor inserted into the tube going into the smoke box. After the boiler is fired into normal operating range and the smoke box temp drops below this setting, it shuts off the draft fan. In effect, this sensor determines "fire is out" and it is time to shut off the draft fan. Once the Low-Limit thermostat shuts the draft fan "off," the boiler draft fan cannot again be turned "on" without pressing the re-set switch on the front panel.

Please note that all of these settings and instructions are in the Tarm manual and the supplemental pages furnished with the manual (at least they were furnished with my manual).

The reason I set the Low-Limit to 100C rather than 90C is that I found the draft fan would continue to operate for a considerable length of time when the boiler (and storage) was at 190F and the fire essentially was out. At 100C it shuts the draft fan down much sooner, although sometimes there may be very few coals left in the fire box which have not burned completely to ash.

I agree that some of this may be confusing to an inexperienced operator, as I was. For example, after adding storage I reset the Low-Limit thermostat but did not realize the bulb controlling this thermostat had to be moved from under the insulation on top of the boiler and inserted into the tube in the smoke box. A post to this forum and help from others cleared this up for me. Once a person has cleared the confusion, however, it becomes quite clear, and an experienced installer should have no difficulty with any of this.

Sweetheat, did you also move the bulb from the top of the boiler to the smoke box well when you set the low-limit to 90C? jebatty did a great job of describing what needs to be done and when, but please give us a call if you have any additional questions. Chris

hi sweetheat,

the smoke chamber is the open area above the top of the heat exchange tubes and behind the bypass damper. In other words, it is the cavity you look into when you take the clean-out cover off to brush the tubes.

If you are standing in front of the boiler looking at the clean-out cover, you will see four studs sticking up with wing nuts that secure the clean-out cover in place. At the base of the right-hand front stud is a small hole about the diameter of a pencil. This is the dry well that you should move the bulb to. If your installer can't get out there for a while, it may be worth looking at yourself. It's pretty straight forward and you can always give us a call while you are doing it and we will walk you through it. Chris.

sweetheat, If you move yourself, which is easy, do be careful of the copper wire the the bulb is on. This can't be kinked. There is fluid inside and if it gets kinked it won't work at all. Just pull out in one big loop and reinsert. The wire ( tube ) can be gently bent just not kinked.