help with Strange combination of symptoms on a heat pump after re-charge

Old 07-04-14, 09:52 AM
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help with Strange combination of symptoms on a heat pump after re-charge

I have a Rudd/Rheem residential heat pump, 5 ton and about 10 years old, which started exhibiting strange symptoms after a repair tech added freon. Before I continue, I have to explain that the system has a modification which was never a problem when originally installed, but which may offer a clue. I have a heat exchanger unit mounted on the outside wall of my house, which is used to recover heat from the output of the compressor, to help heat the water in my home hot water tank. Its really just a pair parallel copper pipes in a coil, with some temp sensors and a pump. I don't know how much money it saves me, but it came with the house and was never a problem.

Well about 2 years ago, I started noticing it was taking longer than it should for the system to bring the house temp down. this is Florida, so THAT is a major problem when the electric bill comes! So a technician came and said the system seemed fine, but that I must have a slow leak somewhere because my refrigerant (I'll just call it freon) was a little low. So he hooked up his gauges and some freon. When he was done, the system took a lot more freon than he originally thought and cooling did indeed improve a great deal, but now I had two new issues. First, whenever the unit would first come on, that heat exchanger I mentioned would vibrate so loudly that you could hear it through the walls of the block construction house! And second, where the high pressure switch used to only trip once in a blue moon, it would now trip much more often, but only when the unit first came on.

Neither I nor the technician knew why this was happening, so in the end I asked him to take SOME of the freon back out of the system. He did, and we seemed to be able to find a "happy medium", where the cooling efficiency was still very much improved, but without that terrible initial vibration in the heat exchanger, and without the high pressure switch trip-outs.

So fast forward two years. I already knew there was likely a slow leak, and the system was beginning to seem like it was struggling again. So I called my HVAC friend and we basically are repeated the same sequence, with the same symptoms. He did tell to me that his gauges don't indicate excessive pressure, even on startup, and that over time those high pressure cutout switches (HPS) tend to weaken and become over sensitive. So I have asked me to get a new one, and in the mean time I have this HPS bypassed. But, the horrendous vibration of the heat exchanger is still very troubling, and I don't think it should be ignored.

So here's what I think. (Be kind, as I'm not a pro at fixing these things). The output of the compressor basically goes to a little 3 inch or so tank (probably a small bladder to absorb initial turn on stress) and from there goes to two things before it hits the main condenser coils: The High Pressure cut off switch, and that hot water heat exchanger. So common sense tells me two things. First, since the unit has been charged, there is now greater pressure when start-up occurs and second, the initial pressure must be extremely uneven (I guess I mean pulsing) to cause such an initial vibration in that heat exchanger.

I could ask to purge the system and take that heat exchanger out of the system completely, as I doubt it saves me much money. But, I think it is providing an important clue here! And besides, if it was not for that heat exchanger absorbing all this initial vibration, then maybe it would be my whole condenser coil undergoing all this initial vibration! I think the latter is much more expensive, so maybe it would be better to figure out why ist vibrating in the first place! What could cause this simple coil of copper pipe to shake so badly on startup? One thing that comes to mind is that if it were empty before the start, and then suddenly there was liquid being pushed through it at high pressure, then the coils would naturally vibrate as it filled, as the fluid pushed round and round, until it was completely filled and there no longer was any imbalance.

But that is the extent of my mental attempt to understand what might be happening. I still have no clue as to WHY, and my technician friend is baffled too (so i don't feel too bad). Should there be gas (non liquid) freon in the line between the compressor and the condenser coils (and in my case this heat exchanger too)? Or should it all be fluid? Or should NONE of it be liquid? I'm asking these questions because the initial vibration is telling me that initially there is a blend of two things in the line during start up, and since this was not a problem when the system was first installed, something MUST be different there now, and it MUST be related to this recent freon charge. Could the technicians gauges be fooling him into thinking the system needs much more freon than it should? Could it be extremely over or under-charged? Could there be air or another foreign gas in the system? After all, if there is a slow leak, then it stands to reason that when freon leaks out, air would leak in.

OK, I've rambled enough. If you're kind enough to read all this and have any thoughts, I much appreciate all comments (even theories).
Old 07-04-14, 11:15 AM
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I would lose the heat exchanger. It sounds like it is partially restricted.
Old 07-04-14, 12:58 PM
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You will have hot gas out of the compressor. You don't want any liquid in the comp( they don't like liquid).

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