I have a 3-ton water-water hydron module R410A vertical closed loop system, that is about 8 years old. I'm in central New Hampshire, so it's almost exclusively used for heating, but I did use it for cooling for a week or so this summer, at the end of August. Starting with that cooling operation, and continuing in heating operation this past week, it is occasionally locking out with high pressure faults.
At first I thought I had air in the loop causing the problem while cooling, but I have a flow meter in the loop and I have 9.5 gpm, clean flow without bubbles, and now that it is happening in heating mode too, that theory doesn't seem right. I made sure to get the air out of the output side too just to be sure, and the temperatures on all four water lines seem normal. At this point in the season I am only heating to 90 F and I have 50 F entering from the ground loop.
Here's the interesting wrinkle: it only trips right at the moment the aquastat stops calling and the compressor cycles off. At first I thought it was tripping just before the setpoint was satisfied, i.e., when the tank got to 89.9F, but it is really somehow being triggered when the aquastat stops calling. This happens only maybe 25% of the time, but I have caught in happening enough times that I know this is a consistent pattern.
Possible theories are:
1) There's an electrical spike when the compressor turns off, and somehow that spike causes a glitch in the control board.
2) Mechanical vibration, or maybe an actual momentary pressure oscillation inside the refrigerant system causes the high pressure switch to trip momentarily during the turn-off.
1) seems plausible, because we had an electrician in for some wiring (in a newly finished basement area) and he installed a new subpanel right next to the heat pump, and moved the heat pump supply to that subpanel. That shouldn't cause a problem, but the problem appeared the first time it ran after that change. It might also be the some components on the board that are supposed to protect against transients have aged and are no longer doing the job.
2) could be a problem that has newly appeared because the pressure switch has aged and is overly sensitive to vibration somehow, or because the pressure is very close to the trip point, or maybe a momentary trip on turn-off is common, and the control board is supposed to ignore that, but it doesn't because of some fault in the control board.
My questions:
Has anyone seen behavior like this before?
Any likely problems I'm missing?
Possible next steps?
My ideas for next steps:
Measuring pressure in operation (I don't have a gauge, so calling a tech or buying a gauge)
Putting an oscilloscope on to look at electrical transients.
Replacing the pressure switch and/or control board.
Putting transient supressors on the power lines.
I assume that replacing the pressure switch requires pumping out and recharging the refrigerant... Is that right?
Thanks for any suggestions and input!
A few more details for reference: 50 gallon buffer tank. Approximate loop temp 50 entering, 43 leaving, 9.7 gpm. Output entering 85, leaving 89, flow not measured. Time to raise the tank temperature by 15 F is about right for rated output. |