Tekmar 260 and short cycling...

radiconnection

"Peter, have you had any problems with room temp overshoot when using the indoor sensor?"

Mike, this is a 2,000 sq. ft. ranch with Burham CI baseboard on almost every wall! The indoor sensor holds the temperature pretty solid--I have digital display stats and the room temperatures haven't changed... but on the other hand the stats are set for a constant heatcall of 70 degrees, while the Tekmar is set for an occupied temperature of 66 degrees. Once the indoor sensor was given control, the Tekmar became much smarter and started using lower target temps.

radiconnection

Tekmar 260 and short cycling...

My understanding is that the Tekmar 260 should control short cycling, but I'm seeing relatively short burner run times (under 3 minutes) when I allow the Tekmar to automatically adjust the differential. I've allowed the system to stabalize and determine the correct operating curve, but the burner is still firing for short 3 minute on cycles. The Biasi B4 is probably oversized for the home. I can correct the problem by manually setting the differential to a fixed value; but I am curious if this problem should exist or not?

Joe Brix

my 260

usually picks a 10 degree differential (5 degrees over and under target) With a small volume boiler, I normally set mine to 15+ degree diff. Or the boiler will short cycle when only 1 zone is calling for heat.

radiconnection

Joe, this is three zones with a constant heatcall from the stats (set for 70 degrees.) I have the optional indoor sensor and let it control the Tekmar, which is set for a desired 66 degree occupied temperature. Holds the room temps rock solid since it almost running constant circulation. But, the boiler does short cycle. System is P/S with a TV on the boiler return, and the Tekmar has full ODR authority on the heating loop. I'll try manually setting the differential, but it goes against my instincts.

Peter

Gary_27

Tekmar

I have the 256, I didn't like the short run times and small differential either. I set mine to a 20 deg. differential. I figure I'm saving $$$ with the lower boiler temperatures and am more comfortable with longer circulator run times. The nice thing about ODR is that you can tailor it to suit you.

Unknown

I recently installed a Tekmar 260 and found that the system worked much better with a fixed differential (6 degrees F) rather than the auto setting. In this case though it was with a very high mass system (3 MBTU commercial boiler described in another post). Here the problem was too long cycles, which caused overshoot in boiler temp.

I think proper auto differential operation depends on the thermal mass of the system. From what I understand, the auto operation measures the rate of water temp rise while firing, and sets the differential accordingly. The rate of rise is dependent on the system thermal mass however and there is no adjustment for this factor, at least on the 260. I believe the 262 does allow you to set the programming differently for high, medium and low mass systems.

Peter, have you had any problems with room temp overshoot when using the indoor sensor? My concern is that with a very large and high mass system (maybe 1000 Gal of circulating water) the resultant time delay will cause the feedback loop to go unstable, producing overshoot or even oscillation of temperature. A further complicating factor is that the system is now running only partial outdoor reset, with timed intermittent circulation based on outdoor temperature. I have not tried setting up indoor sensing yet, basically because the system seems to be tracking outdoor temp very well so far without it.

Unknown

Does the indoor sensor have a limited range of adjustment on the target water temp? If for example you changed the 260 room temp setting from 66F to 70F, would the Tekmar bring the target water temp up 10 or 20 degrees immediately, or would it gradually increase the target in small increments over time?

radiconnection

I think using a switch to go between the occupied/unoccupied settings would be the best way to change room temps, since the curves are already calculated. Just a guess on my part, since I am not a Tekmar expert. I haven't tried varying the desired occupied temperature in a while, but I remember doing some program changes in the past that caused overshooting until the Tekmar learned the new curve.