Heat pump not right

18 years ago

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I have a 5 ton Bryant heat pump that just hasn't been it's old self lately. The compressor and TXV were replaced last fall. Seemed to do OK during winter, but this spring coil began freezing up in AC mode. Tech comes out and sees low suct pressure and immediately adds freon. Schrader valve is leaking too so he replaces core. Coil still freezes up, mainly at night. Comes back out and replaces TXV. Suction pressure is still low and coil still freezes. He is stumped now. I ask about airflow and sure enough turns out the insulation in the air handler is coming loose and needs to be secured. Air flows are now back in the 400 CFM/ton range. He says "let's let it run a couple of days and see how it works. You might have a cap tube plugged."

Being an engineer and tired of techs guessing, I've taken a few measurements. Based on these, what should I expect him to do or look for when he comes back out.

Outdoor amb 87F, WB 77F, Suct 65psi/48.5F (11.5 superheat), Disch 241psi/88F (27 subcooling), Indoor return air 72F, WB 60F, Supply 58.6F, WB 51F, liquid line temp @TXV inlet 89.6F, TXV outlet 89.3F, Evap disch line temp 48F. Liquid line sight glass clear and dry.

Equip documentation says target subcooling should be around 12F at these conditions. Refrigerant overcharge or something else?

Thanks in advance for your advice.

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Comment (1)

udarrell_2007
18 years ago
Well, I had three psychometric data sheets at 350-cfm/ton/ 400-cm/ton & 450-cfm/ton that showed the unit well under 5- nominal tons.

"The nominal tonnage will always vary considerably as the Total Heat Enthalpy conditions through the evaporator vary."

I lost all my work when I closed a window instead of a TAB! I won't redo them, bad hands & fingers.

I got 19.4-F Subcooling - 241-psig SCT 109 minus 89.6= 19.4SC. That is still too much but is probably due to the lighter than rated Btu/hr heatload.

Lighter heatloads don't boil off R-22 as fast therefore the TXV slows to maintain Superheat setpoint & more of the R-22 will be in he condenser resulting in a higher SC.

At 350-cfm/ton nominal tonnage was 3.5-ton 42,228-Btu/hr;/ @400-cm/ton 4-ton or 48,260-Btu/hr & 450-cfm/ton 55,882-Btu/hr.(5-ton 60,000)

Depending on the nominal Rating of your unit, here is what a nominal 5-ton system would deliver at these conditions:
2,000-cfm 400-cfm/ton.
Air entering coil 80db 67wb
Air leaving coil 61db 57.6wb
Total cooling 60,111 Btu/hr or 5.01 ton of heat transfer
Sensible 42,631
Latent 18,086
(SHR) Sensible Heat Ratio 0.70
Indoor SA/RA delta-T 19-F
Condensation Rate 17.07-lb/hr
- udarrell
======================
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Here is a link that might be useful: Latent Heat Removal
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