Why Limit Tank Temperature?

[Rory]

So, I was feeling pretty good about my wood-warmed house today, and after loading the Tarm I went snowshoeing through the woods to my parentss house and managed to cop a loaf of fresh bread my 80 year old father had just taken out of the oven. When I got back home, there was trouble in paradise. My boiler had shut down, apparently overheated, though my tank was only 165F and the circulator was shut down. After some perusing of the manuals, I determined that the Stiebel unit was responsible, and I scrolled through the parameters and found that the circulator was set to shut off when the tank reached 140. I bumped it up to 180, and the pump came on and started cooling the Tarm down. I expect when it hits 165, I'll be able to reset the fan and resume burning.

I'm no heating expert, but it seems like shutting down the circulator is only going to contribute to overheating the boiler. My tank is a rubber lined box built from the design of a friend who has been using a similar one for a few years now without any problems.

Where should I set the upper tank limit, if at all?

 

[in hot water]

What's the temperature rating on the rubber lining? That will be the limiting factor. ideally the tank could charge to 180, 185, even 190F. The higher the tank temperature the more BTUs you have available.

As I understand it there are different types of EPDM marterials with different max. temperature ratings. I'm not sure what problems you would encounter if you run a 140F EPDM product to 180F or higher on a regular basis?

hr

 

[Rory]

I think the liner is rated long term at 170F, but if I want to protect it, wouldn't I be better off to limit the boiler sooner? It seems like stopping circulation is only going to hasten the boiler overheat.

 

[jebatty]

If you turn down your high limit control (front panel), that may give you the extra boiler capacity to take the shutdown heat without tripping the overheat limit.

 

[NHFarmer]

I have an EPDM liner in my tank and I have no high limit on my circulator,I would rather overheat the tank than overheat the boiler.I regulate tank temp by adjusting my fires.As I approach 180* I just add less wood or let my fire go out.seems to work for me.I like to keep it as simple as possible.

 

[Rory]

I suspect that the reason for the function is because this controller is primarily used in solar systems, and it's not practical to shut down the sun.

Simple is good.

 

[Brialin]

I just replaced my 7 year old epdm liner. I used to heat the tank to 180-185 degrees. I was told the rubber cannot take that kind of heat. I am going to lower the water temperature to 170 and hopoefully get 10 years out of the liner.