High SH?

[NorthWet]

Hi guys, still trying to make sure my new heat pump system is dialed in correctly. Had a tech come out yesterday and he came up with a subcooling of 12 and a superheat of 22. Using the formula I found on udarrels site for target superheat the target superheat should be 17.5. THe unit heats and cools fine with a delta t of 23 degrees with the HP running. My question for the experts here is should I be concerned about the SH being elevated? I have the sheet the tech gave me so if you need more info I can provide that. Thanks for your help.

[BigJon3475]

It could be slightly different requirement coming from the mfg. The extra SH could be gained through a piece of missing insulation or the pipe running through the hot attic. It could be he didn't wait long enough for the system to run or there was slightly more latent heat than is on the charging chart. 5 extra degrees isn't a catastrophe.

[BaldLoonie]

22 doesn't seem out of line for today's residential equipment.

[NorthWet]

It could be slightly different requirement coming from the mfg. The extra SH could be gained through a piece of missing insulation or the pipe running through the hot attic. It could be he didn't wait long enough for the system to run or there was slightly more latent heat than is on the charging chart. 5 extra degrees isn't a catastrophe.

Thanks guys. I and the tech were not sure what the targets for SH or SC were from the manufacturer (Nordyne). There is no missing insulation and the line set runs under the house to the outdoor unit. He let it run for 10 or 15 minutes. Sounds like it is not that big of a deal, just wanted to make sure it wasn't a symptom of being overcharged or something. Any other thoughts?

[udarrell]

Thanks guys. I and the tech were not sure what the targets for SH or SC were from the manufacturer (Nordyne).

There is no missing insulation and the line set runs under the house to the outdoor unit.

He let it run for 10 or 15 minutes. Sounds like it is not that big of a deal, just wanted to make sure it wasn't a symptom of being overcharged or something. Any other thoughts?
We add refrigerant to bring too high a superheat down.

The subcooling at 12-F looks pretty good, he could have added a little more to see if the superheat would get a little closer to the target, if subcooling didn't go over 15-F; some say could let it go as high a 20-F.

Though 4.5-F too high a SH may not be too bad, as sometimes there are a lot of variables in the way that data is taken.

What SEER is it?
I always like to get the condenser discharge air-temp-split above the outdoor temp. You need the indoor CFM airflow, & especially the temp & percent Relative Humidity to get a Ballpark BTUH number according to mfg'ers data. - Darrell

[NorthWet]

We add refrigerant to bring too high a superheat down.

The subcooling at 12-F looks pretty good, he could have added a little more to see if the superheat would get a little closer to the target, if subcooling didn't go over 15-F; some say could let it go as high a 20-F.

Though 4.5-F too high a SH may not be too bad, as sometimes there are a lot of variables in the way that data is taken.

What SEER is it?
I always like to get the condenser discharge air-temp-split above the outdoor temp. You need the indoor CFM airflow, & especially the temp & percent Relative Humidity to get a Ballpark BTUH number according to mfg'ers data. - Darrell

Thanks for the reply Darrell. Doesn't sound like I have anything to worry about. It is a 3.5 ton 13 SEER outdoor unit with a variable speed AH. AHRI cert said the system was 14.5 seer. The tech yesterday said I had a TXV at the AH and a fixed orifice on the HP.

[udarrell]

Thanks for the reply Darrell. Doesn't sound like I have anything to worry about. It is a 3.5 ton 13 SEER outdoor unit with a variable speed AH. AHRI cert said the system was 14.5 seer. The tech yesterday said I had a TXV at the AH and a fixed orifice on the HP.

I falsely assumed with that high superheat, & you using a fixed orifice SH chart, that it was a fixed orifice metering device. :oops:

If the TXV is at the indoor air handler, then that is the cooling mode metering device.

The target superheat would be at wherever the setpoint is for that TXV, which is usually somewhat lower on heat pump TXV systems because they have accumulators to catch any suction line liquid flood-back toward the compressor.

The superheat setpoint might be between 7 & 9-F., which would mean that the superheat appears to be way too high.

The next warm day, check the condenser discharge air-temp & log every temp possible. My HVAC blog (you can find it) has a list of data you can take, -for us & your local tech to analyze. :det:

There may be a problem that will require a knowledgable tech to trouble-shoot. :CU: - Darrell

[NorthWet]

I falsely assumed with that high superheat, & you using a fixed orifice SH chart, that it was a fixed orifice metering device. :oops:

If the TXV is at the indoor air handler, then that is the cooling mode metering device.

The target superheat would be at wherever the setpoint is for that TXV, which is usually somewhat lower on heat pump TXV systems because they have accumulators to catch any suction line liquid flood-back toward the compressor.

The superheat setpoint might be between 7 & 9-F., which would mean that the superheat appears to be way too high.

The next warm day, check the condenser discharge air-temp & log every temp possible. My HVAC blog (you can find it) has a list of data you can take, -for us & your local tech to analyze. :det:

There may be a problem that will require a knowledgable tech to trouble-shoot. :CU: - Darrell

OK, just to clarify. The TXV at the AH is the metering device to pay attention to. Not the fixed orifice at the HP? I'm wondering if it's even worth trying to get this dialed in perfectly as it does heat and cool fine. Thanks again guys for all the help.

[NorthWet]

OK got to snooping around on some old threads here regarding superheat and saw lineset length mentioned several times as having an effect on SH. My lineset is pretty long, 70-75'. The tech measured the SH at the outdoor condenser so maybe that has something to do with it. Any thoughts? Also here is the info he provided me if anyone is interested in seeing the numbers.

70 - IDDB
61 - IDWB
68 - ODDB

117/403 (ree'r or ref'r heat mode, can't quite read his writing)
130/235 cool (Not sure what these are, pressures maybe?)

Subcooling = 12
Superheat = 22

There's also a 70.2 / 20.6 / 21.7 and I don't know what these are either. This is a R-410a system if it matters. Thanks for your patientce with a novice HO. Just trying to learn a little and understand my system better.

[udarrell]

OK got to snooping around on some old threads here regarding superheat and saw lineset length mentioned several times as having an effect on SH. My lineset is pretty long, 70-75'. The tech measured the SH at the outdoor condenser so maybe that has something to do with it. Any thoughts? Also here is the info he provided me if anyone is interested in seeing the numbers.

70 - IDDB
61 - IDWB
68 - ODDB

117/403 (ree'r or ref'r heat mode, can't quite read his writing)
130/235 cool (Not sure what these are, pressures maybe?)

Subcooling = 12
Superheat = 22

There's also a 70.2 / 20.6 / 21.7 and I don't know what these are either. This is a R-410a system if it matters. Thanks for your patience with a novice HO. Just trying to learn a little and understand my system better.
Superheat is normally taken on the suction-line near the plenum close to the evaporator where the TXV power element is sensing the temperature.

With that long line-set it is always interesting to take SH at both locations to see the temp difference.

Additionally, there are many factors that affect the nominal BTUH performance of a system.

Length of the line-set is one & depending on whether it is a horizontal or vertical run.

There can be situations where you need more subcooling than normally called for: a 30' lift on the R-22 liquid-line would need to be around 11-F of subcooling to prevent the formation of flash gas in the riser. In some of these scenarios 15-subcooling could be called; we're not working in lab piping conditions!

If the liquid-line runs very far on a tar roof-top, in a hot attic or garage, the liquid-line should be insulated to prevent liquid flashing in the line. With some conditions a little added subcooling might also be helpful. How many do the above, to properly accommodate those, (non-construed by mfg'ers), out of the ordinary conditions...?

We also have to do piping for adequate oil return, & to also avoid oil & liquid slugging of the compressor due, in some cases, to wrong headed piping.

Also as to line sizing, to avoid loss of system capacity the pressure drop for the total suction-line length should be a maximum of 2-F or 2-psig.

There are many other reasons why we have to see what the total situation is to do accurate troubleshooting analysis. :det: - Darrell

[kls-ccc]

As I understand you have a 23* delta T, that seems to me to be a large split for normal air flow {generaly araound 20*} yet your SH suggests normal to high air flow, the SH could be from the long lines. The extra number that were in the last post may be return temp and supply enthalpy. With a 68* idwb and 20.6 enthalpy supply, and assuming the airflow on the AH is set right @ 1400cfm, I figure about 41,200 BTUh, about right for a 3.5 ton. But that is assuming a lot of things. Hope this helps.

[Super Tech JJ]

Is this kinda DIY?