TXV: 15 degree superheat vs mfrs spec

[gary_g]

A co-worker's 2 year-old scroll compressor failed last week (grounded out).

It was replaced under warranty. The tech also replaced the indoor TXV and liquid line filter drier.

After charging with R-22, the tech stated that subcooling was 9 degrees and the superheat was 15 degrees.

Manufacturers specs for subcooling are 5 to 9 degrees, and 7 to 9 degrees for superheat for a TXV system.

When questioned on why the superheat was left at 15 degrees, and not per mfr's specs, he stated that "sluggging of the compressor was less likely at 15 degrees".

Questions/comments:
1. Comments on 15 degree superheat vs spec of 7 to 9 degrees?
2. Will the 15 degree superheat provide maximim efficiency from the cooling cycle?
3. The tech commented that the original TXV was bad and caused the compressor to fail. The heat pump hadn't been on the a/c cycle yet this year. Does this comment make any sense?

Thanx.

[beenthere]

3. But its had lots of defrost cycles.

2. Depends on line set length and configuration.

1. Same as 2.

[udarrell]

Adjustable TXV or fixed SH setting, if adjustable I'd set it at 10-F SH?
I might wonder if the replacement was properly selected for the application. Also, if the sensor bulb was installed on a cleaned area & correct location & good contact on the line, after brazing the TXV in.

Where was the SH temp taken & if at the condenser, on a long run it would be higher there. Best to ck SH at exit of evaporator. - Darrell

[gary_g]

The heat pump is a matched 13 SEER condenser and coil, 5 tons. The air handler is an existing oil furnace.

The lineset is about 30 feet in length. Not sure if the SH was taken at the condenser or evaporator.

The TXV is adjustable.

Thanx.

[Shophound]

When questioned on why the superheat was left at 15 degrees, and not per mfr's specs, he stated that "sluggging of the compressor was less likely at 15 degrees".

Questions/comments:
1. Comments on 15 degree superheat vs spec of 7 to 9 degrees?
2. Will the 15 degree superheat provide maximim efficiency from the cooling cycle?
3. The tech commented that the original TXV was bad and caused the compressor to fail. The heat pump hadn't been on the a/c cycle yet this year. Does this comment make any sense?

Thanx.

Are you saying the compressor failed while running in the heat mode? Possibly meaning it failed late in the heating season, and the auxiliary heat strips were on all the time, so nobody realized the compressor was dead until the thermostat was flipped over to cooling mode?

If the above is true (that the compressor failed while running in the heating mode) then I would not suspect the indoor TXV, since it is either internally or externally bypassed when the system is in heating mode. The outdoor coil also has a metering device. If it restricts or fails it would starve the outdoor coil and possibly not allow the compressor motor windings to be sufficiently cooled (which could lead to the winding insulation breaking down and shorting to one another or to ground).

The piece of info missing is when the compressor failed. If during the heating season, attention should focus on heat mode operation to see if something there caused the compressor to fail.

[jrbenny]

As mentioned earlier, defrosts.

[udarrell]

Excellent points, shophound.
Additionally, the indoor internal TXV or external check valve must be open during the heating cycles.

If the indoor internal TXV check valve "sticks closed or nearly closed" that would also cause the system to pull-down, & cause a possible overheated compressor problem & possible burn-out.

If the indoor check valve is external to the TXV, then only the check valve would need changing out.

The indoor check valve has to be closed during the cooling cycles & fully open during the heating cycles.

If the indoor check valve doesn't completely close during a cooling mode cycle, liquid would by-pass the pressure reducing portion of the TXV, flooding the evaporator & filling the accumulator with liquid, possibly slugging compressor.

Check both heating & cooling modes, & make sure refrigerant flow is okay & the check valves are working properly, if system is not too old they are probably -in the TXVs. - Darrell

[catmanacman]

if the house had a high load 15 degrees of superheat could be ok at the time have him come back and check it now the the house is conditioned

[amd]

if the house had a high load 15 degrees of superheat could be ok at the time have him come back and check it now the the house is conditioned


That only applies to systems with fixed metering devices.