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Tekmar 360 and 4 way mixing issues
Pete, it most likely your mixing valve is oversized.
Tekmar 360 and 4 way mixing issues
I have radiant heating powered by Viesman Vitocell 50 controllled by Tekmar 363 and the system has 4-way mixing valve Heatlink SM4. The problem is that the reset control does not seem to be able to keep more-less constant temperature in the system and the temperature fluctuations seem to be huge. For example, the control calculates the mix target 33 degrees (all measurements in Celsius) and the mix supply oscillates between as low as 22 and as high as 42 degrees. I am not a specialist in this area but this does not seems to be efficient way to run the system. When watching the actions Tekmar takes, it appears to be reacting too late to the mix supply temperature changes - it keeps opening the mixing valve too long when the mix supply is raising and it keeps closing it too long when the mix supply is dropping, thus the overshoots on both sides. I thought maybe the mixing valve speed was entered in error but it is correctly set at 120s. I had this problem since the system was installed but the installer does not want to deal with this. I tried to get the software updates from Tekmar directly but they don't seem to be very customer-oriented company. Can you let me know why do you think I have this issue and what steps should I take to improve the situation.
Slow Down the Mixing Valve Speed
I agree that the most likely reason your temperature is overshooting is because your mixing valve is either too large or because your mixing valve is set to open and close too quickly.
Slow down the mixing valve actuating motor speed and you should be OK.
Change it from your present 120 second setting to the setting that will open and close the valve as slowly as possible and see how your temperature control works with a much slower valve speed.
On my Tekmar 354 the slowest valve speed is a setting of 30 which works great. Not sure what the slowest valve speed setting is on a Tekmar 363 - but you should visually see that the valve is moving in much smaller increments.
If the mixing valve speed is set faster than the system temperature response time - it will cycle.
There is also a SOFT START feature that allows the mixing temperature to ramp up more slowly. I am not familiar with that function, but it sounds like it might also be helpful in your situation.
Let us know.
Pete - let us know
Pete - let us know what happened when you slowed down the mixing valve actuating motor speed.
If the mixing valve speed is set faster than the system temperature can respond - it will cycle. If set too slow the control will be sluggish. Find the best valve speed setting for your system.
I would be frustrated by the sound of a feedback mechanism that can't find it's groove. I noticed quite a bit of hunting on a Viessmann mix valve that I installed, ultimately the delivery is quite even and consistent so I never investigated the fluctuations from target. if mean supply temperature = target, perhaps this oscillation, while annoying, is still an effective means of regulating output.
While your mix valve may be creating a sine-wave target temp, the constant circulation that it enables is, ideally helping spread the burner cycles and improving steady state efficiency.
If a new Software version exists that has made improvements that might effect your situation, it's not as thought you can just boot it up on your existing control. It's understandable that they would not want to get too deep into this sort of stuff with a homeowner, these sort of questions should be asked by your contractor on your behalf. In my experience Tekmar has very good support.
Guys, I think we got it licked!
When I read Doug's suggestion, I went to look at the Tekmar manual and the settings for the mixing valve motor are defined as time that it takes from full close to full open. So to set it to 30 would be telling the control that the motor is much faster that it actually is. But having nothing to loose, I changed the setting to 30 seconds and in disbelieve watched the mixing valve starting to behave correctly: the control now opens and closes the valve in small increments and it is now able to control the supply temp almost precisely +/-1C ! Thanks a lot, you guys are the best! After I posted my question here, I also found local Tekmar rep who determined that the mixing valve is over-sized (I have 1 1/4 piping and should have 1 inch) but short of re-piping it with smaller mixing valve, he did not have a solution. So again, thanks a bunch!
Process Control 101
Pete - Glad that slowing down the speed of your mixing valve has solved the problem of temperature overshoot and cycling.
Thanks for asking this important question. Your heating system will operate more efficiently when it controls properly.
Basically, what you are doing here is "tuning" the speed of the process control system (mixing valve) to match the relatively slow speed of temperature change of the heating system.
When the speed of the process control system (mixing valve) is set too fast - it over-controls (makes larger changes than necessary) and causes temperature overshoot and cycling.
When the speed of the mixing valve is slowed down - it makes smaller adjustments that nudge the temperature back into the control range.
Too much is said about mixing valves being too big. The real problem is usually that the mixing valve is set to operate too fast.
The mixing valve has a speed range of 30 to 230 which is used to match the speed of the control system to the response speed of the heating system.
Process Control 101 says - If the process system cycles you should cut the speed of the controller in half. If it still cycles, you should cut the speed of the controller in half again. This will eliminate the cycling and get you in the correct ballpark for good control.
Temperature control is a case where faster control is not better control.
Thanks for asking the question.
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