TROUBLESHOOTING HEAT PUMP SYSTEMS…INDOOR / OUTDOOR TEMPS VS SUPERHEAT W/ FIXED ORIFICE SYSTEMS
..If you look at a superheat charging chart for a fixed orifice system, you quickly see the required superheat varies with outdoor and indoor conditions. As the outdoor temperatures vary, so does the required superheat…pretty much the same relationship for indoor temperatures. Why? The net force pushing liquid through the metering device is the difference in the head and suction pressures, more or less. And I would guess the designers figure in some maximum outdoor temperature in conjunction with some minimum indoor temp and come up with a minimum superheat value for “worst case” scenarios.
The point being, if the outdoor temperature is 75F you don’t won’t want a “beer can cold” suction line…because by the time the afternoon temperature hits mid-90’s, the increased head pressure will have increased the “net force” pushing the liquid through the orifice, and the system will be overcharged, resulting in a lower than desired superheat.
Likewise, if the indoor temps are “high”, superheats will be high. Most charging charts use indoor wetbulb as the control variable, since wetbulb temps include the humidity factor. As indoor wetbulb goes down, the superheat will decrease, everything else being equal. The following clip demonstrates variations in superheat with outdoor conditions.
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July 1, 2008 at 4:41 pm
I just changed out a bad compresser , the condenser was already changed out 3 years ago. they also changed the lineset from 7/8 and 3/8 to 1 1/4 and 1/2″ ,the run is about 92′ and it is a 5 ton system. the returns are all in the walls [2"x4"]and is definately undersized on that end.the problem I’m having aside from return air being undersized is my SH @ SC which is probably from the return air in part. I flushed the lineset and purged with nitrogen while I resoldered the new compresser and also changed the liquid line dryer. I also checked the orfice for any restrictions but didnt find any,on the other hand ,after I recovered the r-22 from the system and took out the orfice I got a short high pressure burst of r-22 followed by another burst about 2 seconds apart. My thought was something clogged in the evaporator so I ran some n02 at high pressure to try to blow anything out. I put the system under vacuum and got it down to 259microns by my micrometer. I charged the system with a scale and put in approximately 13 pounds. I started the system ,had 71 psi on low and 210 on high side,the high side was bouncing a little bit at first and then stabilized,my SH was 2 degrees and SC was 10 degrees, I had a compression ratio of 2.6. the system was running for 45 minutes and the low side just started dropping to 30 psi and the high side went up to 250 psi. I checked the coil inside and it was icing on the first two rows of coils, then after about 5 minutes the icing stopped so I went outside to check the pressures again and had 55 psi on the low and 250 psi on the high side,I added a little r-22 and my low side was 68 psi and high side was 255 psi with a SH of 16 degrees and a SC of 29 degrees with a compression ratio of 3.2. I’m dont think the return air is the whole problem but then again I’m still new to troubleshooting with SH and SC, basically I was taught the wrong way by my former employer and I’m trying to do things the right way but I’m preety much starting over from scratch again. Any info. on this would be greatly appreciated. Thank you, Paul Duphily
July 1, 2008 at 4:55 pm
One other thing I forgot to mention, I was using an infrared thermometer to check everything. I had 70 degrees at the return grills,the t-stat was reading 78 degrees and the air at the outdoor unit was reading 89 degrees.
July 2, 2008 at 4:08 pm
I dont understand why I can have a normal superheat and a subcooling reading that is way off, and vice versa? also I was wondering If the amperage readings for fan motors and compressers are supposed to be exactly what the rating plates say the are because I have never had one be exact, they are usually off a few amps unless there was a fairly obvious issue.
July 3, 2008 at 3:01 pm
All the stuff that went on in the first comment prior to the final numbers (68,255,16 and 29) is a mystery…if the final numbers held, it doesn’t really matter…68psi and 16F SH ain’t bad, if outdoor temps are 85-90…the long line sets will affect the SH/SC values…I’d say the system is in decent shape…from where I sit…
July 3, 2008 at 3:10 pm
..name plate ratings on fan motors are usually max load capacity…better the motor pull less than more. The value on compressors is usually Rated Load Amps, which really means nothing to us, and the compressor will always pull less than the RLA value…
July 3, 2008 at 3:15 pm
So far as superheat / subcooling values with orifice systems, you charge by superheat and subcooling is whatever it is, so long as head pressure is reasonable…
July 4, 2008 at 7:31 am
..I don’t have all the answers…I’ve seen high subcooling values with normal superheat numbers.I usually blame it on undersized liquid lines, undersized orifices, restrictions, poor equipment matches, long linesets, high lifts, etc…fixed orifice metering devices are in my opinion, a cost reduction method of manufacturing. They eliminate the cost of the TXV, as well as the need to design the compressor electrics for high starting torque requirements. Several years ago, I began selling a brand of 10 SEER equipment that rarely performed like it should. Split heat pump systems would invariably require overcharging in the cool cycle to produce good superheat values. Then, in the heat cycle, they would be overcharged, producing high head pressures…I finally solved the problem by mixing 10 SEER condensers with “12 SEER” air handlers, which had TXV’s feeding the indoor coil. The condensers were all equipped with scroll compressors, and in most cases, don’t require a hard start accessory…
July 4, 2008 at 9:53 am
Thank you for your input and time,this helps out alot.
September 3, 2008 at 2:43 am
Paul, how you’re doing. you over charged the unit. It seem loke you charged the unit too quickly with liquid. You must place a little juice, gas to keep from taking oils with freons throughg the unit. took a chance of lock rotor. On a home unit, you always bring suction pressures up around 45 degrees. anything over that; your superheat rangfe starts toget wider.
Sit at the location and check the low gauge, if it starts to drop crack in two ounces at a time until the gauge seem steady. Take of the gauge and go make some money would the unit pulls down for a few hours or over night when possible. When you go back the next day; add freon till you reach the the 45 degree magic number. Now, some squol units magic number is 38 degrees and you will know by looking at the high side reading. they run close to 300 to 350. Take care yourself; I hope I helped a little. Practice more super heat, it’sood for us. have a great day.
June 8, 2009 at 1:53 pm
where is the best place to check superheat on a WSHP at the Evap or near the comp.
June 8, 2009 at 6:57 pm
..the smart-a$$ed answer is probably, the easiest place to get the thermocouple connected…if we get picky, there are two superheat values: evaporator superheat and compressor superheat. Compressor mfg’s worry about compressor superheat and like to see some minimum value, say 20 degrees (F) at the compressor inlet. Evaporator superheat would be measured at the evaporator outlet, obviously. With “air-to-air” systems and air handlers in attics with long line-sets, there could be an obvious difference between the two values. In the real world with air-to-air split systems, techs will usually measure it at the service valves (with a heat pump system in “cool”). Some mfg’s will specify where to measure the tubing temperature for the design superheat value, which is usually close to the service valves.
If your water source heat pump is the “package” design (the only kind I’ve seen) the distance between the evaporator outlet and compressor inlet isn’t much, so I wouldn’t think the superheat value would vary a lot between the two locations. But to be on the safe side, closer to the compressor, the better…
June 9, 2009 at 11:38 am
Thank You for the info, with 275 heat pumps the water coil temp changes alot and hard to get a stable super heat should I look at subcooling instead
October 23, 2008 at 11:13 pm
Wayne,
Thanks for sharing your knowledge. I have a few questions if you don’t mind, is there any way to change the balance point or the defrost time on a Lennox heat pump? It seems that if I set the balance point lower and had it defrost more often, then I would be able to get it to work below the 40 degrees F that it seems to be set to now.
Thank you in advance for your answer and have a great day.
October 24, 2008 at 8:13 am
I’m not at all familiar with Lennox equipment, but defrost controls come in two flavors, time/temperature and demand. Time/temp controls have selectable defrost initiate times, usually in the 30,60,90 minute time ranges. The disadvantage of time/temp controls is the fact they will initiate defrost cycles unnecessarily, which is of course a waste of energy…which is also probably the reason someone invented the demand defrost method. Demand controls compare outdoor air temperature to outdoor coil temperature and use some minimum temperature differential as the controlling factor for initiating a defrost cycle, pretty much eliminating unnecessary defrost cycles. With the increased focus on system efficiencies, demand controls have become the popular choice of many equipment manufacturers.
If I had to guess, I’d say your Lennox has a demand type defrost control, which has no options for changing the defrost cycle frequency. You can differentiate between the two systems in a couple of ways. Time/temp controls have a single temperature sensor that attaches to the outdoor tubing somewhere, and the control board has the 30-60-90 values printed on it. The demand controls have two sensors, one is attached to the tubing, the other exposed to the condenser inlet air flow…and there’s another post somewhere on this blog relative to defrost controls you might look at.
October 24, 2008 at 8:54 pm
Wayne,
Thanks for your great explanation. I will look into it this weekend.
June 9, 2009 at 6:25 pm
Tom, generally speaking fixed orifice systems are charged by superheat, TXV systems by subcooling. I’m assuming your system is fixed orifice. Attempting to charge by subcooling would probably be a mistake, as it is probably fluctuating also. If the water temps are fluctuating, head pressure will vary, causing suction and superheat to also vary. The same thing happens with air-to-air systems when the OD temp goes from 75 to 95…
Fluctuating superheats, or superheats following indoor and outdoor temp changes, is the downside of fixed orifice metering devices.
June 11, 2009 at 8:32 am
Thanks again,did alot of air to air units. Wshp is new to me I understand the theory of them they are fixed orifice system 12,000 btu up to 5 tons any more info would be great.
June 11, 2009 at 3:31 pm
I have one customer who had two WSHP’s, now only one. I approached them same as air-to-air. Most of the problems he had were water related. He finally learned to check the water source before calling me. His were simple open-loop (I guess you call it that) pumping creek water. Closed loop stuff is something I’ve haven’t gotten into…nobody around here will pay the cost for a state of the art geothermal system, and don’t really need it that much…