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Heat Pump problem? (7 Posts)
Heat Pump problem?Hi-I haven't worked on too many heat pumps so bear with me, but just from my A/C experience I know this one has a problem. It is a Carrier heat pump R-22 unit with flow check pistons and solenoid operated reversing valve, defrost board and a suction line accumulator(I think they all have these) Got a call that it was icing up on a 20 F day, but didn't get there till monday when it was around 36 F outside. Seemed to be operating normally with a bit of frost on the coils, I checked the defrost and reversing valve operation, and all seemed normal....fan stopped, it thawed out, and then when it began operating again there was heat inside. I put my thermometers and gauges on and took the following readings: Compressor discharge temp 136F Liquid line temp 76F Indoor temp 65F outdoor temp 36F. On the high (discharge) side, I was reading 168 PSI which gives me 88-90 condensing temp so I believe this gives me 12F of subcooling....a bit off the mark according to the chart on the unit. The readings that threw me were these: From the liquid line service valve coming back to the outdoor coil, before the flow check piston, I was getting 55 PSI / 30F and it was frosting up right at the service valve, then up towards the piston and on. Shouldn't the pressure be the same as the discharge here since it is before the metering device? Does this indicate that the piston on the indoor coil is stuck/ blocked? It seems that the pressure is not high enough to get the refrigerant through the outside metering device. I wasn't able to measure the superheat because it looked as though someone had removed the third schrader valve and crimped/brazed it shut, so I'm kinda in the dark here, but it seems as though there is either a restriction at the service valve, or at the indoor piston. Put it on emergency heat and am planning on heading back when it stops raining/snowing. Any thoughts/suggestions are appreciated.
TemperaturesI am a bit confused about your description. The discharge side looks good, but it appears the suction is way down there. To troubleshoot, look at suction temperature leaving the reversing valve. It should be somewhere close to the outdoor ambient, minus 10 or 15 degrees. Moisture will cause a blockage at the outdoor coil metering device that comes and goes. Another indicator is the air temperature drop across the outdoor coil-look for about 10 degrees; lower if this is a high efficiency unit. The unit charge should provide about 20 degrees of liquid subcooling in the heating mode. These are all "rule of thumb" numbers, but should get you in the ball park. Last, but not least, what is the indoor coil air temperature rise?
Icing vs Frosting ... Liquid service valveI'm not sure about your description either.
Frosting up (other than a paper thin layer) means the defrost controls are not functioning properly. Icing is caused by something else:
1. Insufficient refrigerant in the outdoor coil or insufficient load on coil.
A. Low on freon.
B. Restriction in line or metering device.
C. Bad outdoor fan motor / capacitor / relay.
D. Accumulator orifice restricted.
E. Wrong size metering device.
2. Improper drainage at base of unit.
A. Unit not properly elevated off pad.
B. Snow, leaves, debris under unit.
3. Head (liquid) pressure too low causing low suction pressure.
A. Air flow too high across indoor coil. Set blower speed to achieve 400cfm per ton.
B. Restriction at indoor metering device, coil, line.
4. Not defrosting long enough during cycle or frequently enough.
A. Check defrost thermostat /sensor.
B. Set defrost control to cycle more often. 30 or 60 minutes instead of 90.
5. A recent ice storm. No heat pump can overcome this.
On a Carrier the outdoor piston is in the service valve, therefore, the pressure reading in the heat mode is basically meaningless because it's after the pressure drop from the piston.
With the indoor airflow at 400cfm per ton and the I.D. coil and filter clean, your discharge line at the compressor should be 165 - 180 deg. At the discharge service valve it will be 10 - 15 deg lower. But you must first be at 400cfm per ton to take this measurement. A higher discharge temp indicates undercharge; a lower discharge temp indicates an overcharge.
The suction superheat should be 1 - 5 deg in the heating mode, measured at the coil outlet.
Check your cfm by turning on the heat elements only (Em.Heat) and measuring the wattage (volts x amps). Measure the temp rise at the same time with one accurate thermometer in the supply duct and out of the sight line of the heat elements. Then use this formula to determine cfm:
Btu = Watts x 3.413. Cfm = Btu's / (Temp Rise x 1.08)
Example: 15kw = 51,195 btu's. If you had a Delta T (temp rise) of 30 deg. 30 x 1.08 = 32.4. Then 51,195 / 32.4 = 1,580 cfm.
This would be good for a 4ton unit, but too much for a 3ton unit. Incorrect airflow is one of the biggest but most over-looked problems in heat pumps. Check your wattage with an amp meter, don't assume it from the name plate.
Please post your findings.Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
Thanksfor the replies. The client decided he didn't want to fix the heat pump as it only heats an entrance area of a commercial building, so he is leaving it on emergency heat for now. I'm sure he'll call when he gets his electric bill. In the meantime, I'm trying to learn all I can about heat pumps, I've got my Modern Refrigeration and A/C book out, and I understand the mechanical end of things, but need some help on the application. Are there any good books....maybe titled "heat pumps for the A/C technician?" ; )
Anyway, sorry for my confusing description. To clarify: Indoor temp was 65F, outdoor temp 36F. The compressor discharge line temp after the service valve was 136F, and the liquid line temp (before the piston/service valve) was 76F. Pressure readings were 55PSI (30F) on the low side (Carrier, so after the piston) and 168PSI (90F) on the
high side. First thing I checked was airflow, cleaned the filters and checked the indoor coil, all clean. Took static pressure readings, airflow is fine....around .4" and 1172 CFM on a 3 ton system. I didn't do a temperature rise calculation though. Also, the defrost mode is working fine, all frost is cleared from the coils when it runs. It is set at
30 minute defrost intervals. From the looks of the unit, I thought the piston would have been right before the distributor. This explains why the frost began right at the service valve, and the pressure was so low. Is there a way to check the pressure right before the outside piston? What is the function of the 3rd schrader service valve? It looked like it was connected to the suction line near the accumulator.
I know the refrigerant has to be below the outside temperature to absorb the heat from the air, so if the air is 36F, that means the refrigerant passing though the piston will have to be saturated below freezing, and then gain some heat/ superheat as it moves through the outdoor coil before heading to the compressor. What would be ballpark readings for these temps? If the refrigerant has to be that cold entering the piston/outdoor coil, how could it not freeze up right away? I'm used to A/C where any frost or ice is a bad thing. Since the liquid line has to be well below ambient, is it normal, depending on the outside temp, for the liquid line to be frosted before it heads through the coil where it will absorb heat? Like you pointed out, the temperature leaving the outside coil after the heat absorption seems more important. Since I had 168PSI high side pressure, and a 76F liquid line temp, would that put my subcooling around 14F or am I calculating this the wrong way?
This post was edited by an admin on December 15, 2010 6:25 PM.
Suction conditionsYou had said that suction was 55 psi. That would be a saturation temperature of 30 degrees; so the outdoor coil could be no colder than that temperature. That's in the ball park for the process at hand. The 3rd port is common suction downstream of the reversing valve. The outdoor refrigeration process is the same as indoors, and manufacturer's have managed to mate the indoor and outdoor coils closely enough so that the charge works both ways without having to store some of it in an accumulator. The amount of charge is critical, particularly because of the use of a piston restrictor rather than an expansion valve. The compressor discharge temperature tells you that the outdoor coil is not flooded and returning liquid refrigerant to the compressor. Last, but not least is the loss of capacity that the 30 degree coil causes-nature of the beast. From your description it's working properly. Frost is a normal condition any time the coil temperature is below 32 degrees and will vary based on the amount of moisture in the outdoor air.This post was edited by an admin on December 21, 2010 9:34 AM.
thanksfor the info Tom, its helping to fill in the gaps in my knowledge. Another question, and probably one I should have asked first, is what are the design temperatures for the 2 coils in a heat pump, lets assume standard efficiency. I know in A/C the evaporator is around 30-35F below the return air, and the condenser is 25-30F above the outdoor air passing through it. It would seem to me if the outdoor coil were to be that far below the outdoor ambient, it would absorb quite a bit of heat from say 25F air, but I would think it would frost up rather quickly and be spending a lot of time in defrost, but I'm not sure.
StandardThe heat pump I am most familiar with is my own, a Trane 4 ton 13 SEER unit. Outdoor coil delta-t runs about 8 degrees at 50 degree ambient. Yes, there is a lot of frost build-up, particularly when the outdoor air is around 38. At 17 degrees, it's capacity is down to 25,000 btuh, which is half that at 72 degrees and the COP is down to 2.0. At outdoor temperatures near freezing, it's practically worthless, except it is still producing heat with less than half the input of resistance heat. Wirh the new 2-stage heat pumps being offered, there is an opportunity to really gain some efficiency, because the unit can be sized to the heat load rather than the cooling load and still have reasonable dehumidification at light loads. Unfortunately, this won't happen because in the housing market the choice is always the builder's model-lowest first cost. When I did a house addition in 1990, the bank refused to pay for any more than the cheapest unit. The situation is still the same today....end of rant.