Rattling Piston ??

[carlb7]

My heat pump was overhauled today by the original installer. They replaced the evaporator coil with the correct "matched" coil to outside unit. A new TXV came w/coil and it is the required .157 in size. System was pumped down to 500 microns and recharged with virgin refrigerant. Some of you may remember that the temperature would drop 7 to 8 degrees at upflow after startup and then recover. Well this continues. The tech said he could hear the rattling of the piston in the TXV. This noise is not foreign to me, it did the same thing with the old coil. Questions: Is this a normal sound with this TXV? Is this a normal temperature drop with the Ruud heat pumps?

[karsthuntr]

The piston is to distribute the ref. through the cap tubes evenly. Yes they do rattle on some units. We just remove the piston in those cases (with factory blessing) and the problem goes away.

[WhoIsThat?]

The tech said he could hear the rattling of the piston in the TXV.

In a car engine it would mean the piston skirt is undersized due to wear.

[carlb7]

That brings me back to the question about the temperature drop. Has anyone had experience working with a Ruud achiever series? The temperature drops only at start up, the tech seems to think that there is vapor in the evaporator coil that is eventually replaced with refrigerant coming from the accumulator which is a cooler temperature that is the cause of the drop. It can make sense to me if there other Ruuds act the same way. If this is the case is there any action that can prevent this from happening?

[Kevin O'Neill]

A piston and a TXV are both refrigerant metering devices. Normally you do not have both on the same coil, just one or the other. It sounds like you have a check valve rattling. Strap a magnet to the check valve and it will normally stop the steel ball in the chech valve from rattling.

The distributors that I am familiar with do not have a loose piston th them. There is sometimes a cone that is brazed in place, or the pressure drop in the distributor tubes controls the distribution of the refrigerant. It is possible there is a loose piston in the distributor, but I am not familiar with that style.

[carlb7]

The coil required a txv conversion kit to make it the properly matched coil. When the installer opened up the package there was what I thought called a "piston" that was sized at .157 that was part of the txv. This is what they believe is causing the rattling. Don't know if the magnet would help me with this. The temperature drop they are not sure of except the explanation given in my prior post. They are going to ask Ruud if they could explain it.

[Kevin O'Neill]

I have found that it takes as long as 15 minutes for the temperature drop across the indoor coil to stabilize.

[beenthere]

A piston and a TXV are both refrigerant metering devices. Normally you do not have both on the same coil, just one or the other. It sounds like you have a check valve rattling. Strap a magnet to the check valve and it will normally stop the steel ball in the chech valve from rattling.

Rheem uses a piston with their TXV to keep the refrigerant flow even to all distributor tubes.
Its not like a piston for metering refrigerant.

[carlb7]

Heat pump starts up, vapor from compressor and line-set are fed to the evaporator giving a certain temperature. Refrigerant from accumulator is making its way to evaporator with a "cooler" temperature, temperature begins to drop at air handler. This coincides with the liquid line feeling cooler coming from the evaporator. Refrigerant recycles and the "warming" process. begins. Could that make sense?? If so is there any way to "fix" it ??

[Kevin O'Neill]

Refrigerant from the accumulator goes to the compressor, then the condensor, then the liquid line, then the evaporator.

I am not sure I understand your problem.

Are you heating or cooling?

Is the supply air too cold and for how long.

Untill the system stabilizes, temperatures will probably not stabilize.

[Kevin O'Neill]

This is a chart on the temperature drop across evaporator coils with various metering devices using a dual channell data logger. The TXV is both with a standard blower and a Variable Speed Blower. See Attached.

[carlb7]

Refrigerant from the accumulator goes to the compressor, then the condensor, then the liquid line, then the evaporator.

I am not sure I understand your problem.

Are you heating or cooling?

Is the supply air too cold and for how long.

It is in heat mode. Let me give you a example of what takes place:

Stat calls
heat pump starts up
Upflow air from handler goes to 96 degrees.
Heat pump is on for about a minute
Temperature begins to drop from 96 degrees all the way to 88 degrees. (Liquid line goes from warm to cool)
It takes about 3 minutes for the temperature to recover to the 96 degrees.

I understand it takes time for things to balance out but I'm just trying to get an understanding of why it starts out at 96 degrees, stays there for a minute or so, and then drops 7 degrees. This will happen at every start up.
,

[yelram]

Heat pump starts up, vapor from compressor and line-set are fed to the evaporator giving a certain temperature. Refrigerant from accumulator is making its way to evaporator with a "cooler" temperature, temperature begins to drop at air handler. This coincides with the liquid line feeling cooler coming from the evaporator. Refrigerant recycles and the "warming" process. begins. Could that make sense?? If so is there any way to "fix" it ??

Blower delay, a heat trip, or a crankcase heater. The liquid is going to condense where its coldest, which is outside, and when you change the pressure within the system, that liquid is going to have to boil off before you can force any super heat into it, in the process, it is going to absorb alot of heat initially (specifically between the accumulator and suction side of the compressor), that wont be realized in the condenser coil until the ratio between liquid and gas is correct. Alot of times, if we change out a compressor and its cold, it takes forever for the suction pressure to start getting into the correct range, I usually put it into defrost, to rob some heat from inside to help boil off the refrigerant.

^just read your post.

The thing is, at a point of temperature stasis, it will produce some heat, but as soon as that boiling off refrigerant cools down the suction side of the compressor, it lowers the suction pressure significantly enough to lower the temperature of the condenser coil, both because of what I already mentioned, and that it initially can push against the liquid side up until the point where all the refrigerant starts into circulation.

Think of it this way, a heatpump moves heat, it doesnt make it, so it has to get it from somewhere, the speed of the initial rise is the anomaly, and its corresponding cool down is the response of stealing heat from a rapidly cooling piece of copper. It gets that first heat fairly easily, but as the copper cools down, it has no where to extract the heat, until the liquid in the evaporator begins to boil off, and make its way back around to the suction side.

I imagine a TXV type metering on the accumulator would probably nix that to some degree. It would keep the suction pressure artificially increased, until the refrigerant coming back to the compressor had a certain amount of heat saturation. But that would be just one more part to break, and it seems to be a pretty trivial problem