Air handler or thermostat responsible for 2 degree temp swing?
mpd
| Posted in General Questions on
I have a Mitsubishi SVZ-KP36NA ducted air handler controlled by an MHK2 thermostat. Rather than working like a simple On/Off thermostat I know that the MHK2 is communicating constantly with the air handler. The problem that I have is that temps that I set on the MHK2 are always controlled within a +/-2 degree range. What I want to know is: which unit is responsible for this?
I have ceiling cassettes upstairs connected to my heat pump that can keep the temp at the set point constantly within +/- 0.5 degrees and am wondering why that’s not case with my air handler and which unit is at fault.
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Replies
I can't say about this particular unit. But I've been doing a lot of work lately with air handlers with thermostatic control of the fan, and it's common for them to have a temperature sensor that has a resolution of 1C, which is equal to 1.8F.
I'll add here that many cheap thermometers aren't calibrated, and have sensors with poor resolution. If you are using a cheap mass market digital thermometer, then there is a good chance your "test equipment" is also at fault.
As DC mentioned, common temperature sensors -- and I mean the actual semiconductor devices used to sense temperature -- typically have a 1 degree C or worse tolerance, so +/1 1 degree C, which is an "error band" of about 3.6 degrees F -- pretty close to what you're seeing. The very best sensors are around 0.2-0.5 degree C, which still gives you an error band of 1-2 degrees F or so.
Basically don't expect to keep the temperature dead on target here, and don't trust cheapie thermometers to be accurate. For RELATIVE measurements, an old school analog thermometer (the glass tube kind) can be useful. For GOOD digital ones, expect to pay some decent money to get something you can trust.
Bill
I''ve got temperature sensors in the rooms but in this case I'm using what the MHK2's temperature sensor is showing on its display - in Heating mode it very definitely overshoots by 2 degrees. I might point out that you can find this complaint cropping up over the years on various forums.
And of course I also have upstairs Mitsubishi MLZ-KY06NA Ceiling Cassettes which keep the temps within 0.5 degrees constantly. I don't know why Mitsubishi would use a good thermistor in the ceiling cassettes and a lousy one in the MHK2.
I've yet to hear from Mitsubishi (I've got a request in via an HVAC contractor) as to exactly which unit is actually controlling the temperature (i.e. actually what's happening in the communications protocol between the MHK2 and the air handler). I have some idea about what's being transferred from documents on the CoolAutomation CoolPlug and also from the github project derived from the "Hacking a Heat Pump" blog but not enough to answer my question.
They're probably not using thermisters, they're probably using a silicon direct to digital sensor, which are laser trimmed, but not always dead on. They are usually pretty reasonable for relative measurements though, which means making a measurement against itself at some future point in time.
It's possible that the thermostat is "seeing" something other than ambient air, and is misreading the temperature as a result. Check that the thermostat isn't in the path of air from the minisplit, and check that nothing behind it in the wall may be warming or cooling the thermostat unintentially (like refrigerant lines in the wall behind the thermostat). You may have a situation that is causing the thermostat to have a time lag when reading the temperture of the room, which will manifest as the temperature "overshooting" the target since the thermostat doesn't "see" the actual temperature in the room, so it has a delay before it shuts off the unit.
I've seen people complain before about rooms running cold, just to find out the sun comes in a window and shines right on the thermostat, as one example. I'd look for issues like that.
Bill
I would agree that the accuracy of measuring devices is limited, their repeatability is usually quite good within the narrow band they are working in.
IOW the device might be reading 68 degrees when it is 70 degrees, but it will always read 68 degrees at 70 degrees
I have found that minisplits for instance seem to maintain temperature better when they are working to do their job, IOW not oversized
Big fan, small room, less accurate
Correct, accuracy is how well the measurement agrees with a fixed reference, usually a calibration standard, and is good for measurements relative to other pieces of measuring equipment (assuming they're all calibrated too). Precision is how well the measurement agrees with itself, which means how stable the measuring device is over time in this case, and that's good for tracting relative chances over time with the same piece of equipment.
I expect the issue, while it could be a flaky sensor in the thermostat, may be due to the thermostat not "seeing" the true ambient air temperature. This can be due to air currents, stuff in the wall behind the thermostat, stuff under or above the thermostat, etc. I've also seen thermostats mounted on solid materials (i.e. concrete walls) sometimes lag the true temperature of the room, so that they read "late", which can make for over (and usually also under) shooting temperatures. Sometimes in those cases, mounting the thermostat slightly off the wall with an air space behind it can help.
Bill
There was another post with a Mitsubishi that could not hit its set point in the end the poster installed Mitsubishi generic thermostat interface and a conventional thermostat and was able to maintain the room at the set point.
I doubt the problem is the sensor itself. My guess is Mitsubishi is trying to set itself apart from the competition with control software and seems to be failing.
Walta