Heat pump question. I realize that heat pumps do not put out the highest heat level especially if you are used to fossil fuel forced air but what is a reasonable heat rise to expect? I have a digital thermometer probe in a register and one laying on the return vent. I am getting 11 degree rise normally. It is 23 deg outside but it does not seem to vary too much even if it is about 15 or 50 outside. Is this about what I should expect? I believe it is a 3 1/2 ton 12 seer unit.
The house is 1724 Sq ft with 2×6 walls with cocoon insul and supposedly r-41 of blown cell. in the ceiling which is cathedral running from 8′ outside to 12′ at apex. Put in a little too much door/windows but effect is great. Have 13 panels of glass doors ( basically Peachtree steel with wood thermal break, full view)6 moderate sized Anderson mid grade windows. Over an unheated (currently 60 deg) full basement walkout on almost three sides.
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heat pumps do not do good in very low temp with it 23 outside you might get 85 to 90 at vent. That why there have a heat backup. The most you will ever get out the vent be around 110 degrees.
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I would be very happy with 85-90, I have 68.4 at return and 80.8 at register. I am amazed that it can function in extreme cold. I check to see if aux heat strips are on and about the only time it is on is if I raise the demand temp more than 2 deg over room temp. If I have been away and have reduced it and take it up to the usual setting of 72 then the coils come on and I see 105 or so. The oil fired system in the other side of the house was so hot I put a smaller nozzle in it to cool it down.
Edited 1/7/2004 11:49:47 PM ET by RASCONC
A few years ago I put in a 15 seer Trane (XL 1400) with over sized air handler.( 2-ton) Trane supplied enough data figures to build a chart of out door temps vs delta-t across the coil (indoor). The performance is dependent on many factors, especially the frost condition of the condensor coil, and the speed of the out door fan, which is 2-speed. The air handler fan is variable speed, so I have to wait until it has reached top speed to take readings, which is 7.5 min after run start.
Here are some od temp and corresponding delta-t readings for 100% performance.20 deg.od=14.4 delta-t, 22=14.8,24=15.0,26=15.4,28=15.6,30=15.9,32=16.2,34=16.5,36=16.8,38=17,
40=17.3,42=17.6,44=21,46=24.5,48=26.6,50=27.4,52=28.2,54=29,58=30.6,
60=31.4
With a dry/clean out door coil, I often get 110 percent performance to the figures from about 25 deg od to 42 deg od. ie., at 30 deg od, I often see 17.5 delta-t. Below 20 deg od, the performance falls off to 90 percent.
I take test readings every year to see if the performance is holding, and plot the points on a graph. I will know if the system needs attention if the performance starts to drop from the "new , well tuned condition".Trane field reps, Trane dealer, and factory input help were combined to get the pressures dead on, and the system has performed beautifully for 5 years. Variable speed air handler ramps up over a pre set schedule, and is quiet, smooth and efficient. No, I have no vested interest in Trane.
It is easier to check the heat pump performance in the heat mode than the cooling mode due to the variation the indoor humidity has on the cooling effect. If running good perf. in heat, I figure the compressor, coils etc., are doing the cooling job well.
The system has backup ele coils, which come on if a 2 deg variation in actual vs thermostat setting, and whenever the system goes into defrost mode. Rarely, does the system require backup heat other than defrost. Hope this gives you some idea what to expect. PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
Edit.. my digital probes are in the return plenum just before the air handler, and in the output plenum just past the handler case, so I am getting pretty true "across the coil" readings. You could do the same, and then compare to an exit register to see what kind of duct loss you have. And, you can take a reading at the return to compare to the air handler return to see how much temp loss in the return duct system.
Leaks and weak insulation can have a huge effect in these two areas.
Paul
Edited 1/7/2004 1:21:24 AM ET by Paul Hayden
Thanks, that is pretty much what I was lookng for. Your system is definately a little more upscale than my Bryant. I am taking the return reading about 15 ft from the coil and the discharge about 25 ft downstream. At 9.9deg outside aux heat was kicking in. I have got the sensitivity on the thermostat a little too close. The room temp does not vary much over .5 deg. I will check to see if I can tell the aux to come on a little sooner.
I think my ducts are pretty tight and since all is in semi insulated space (no crawl space or attic) any loss is not a real loss.
Thanks again. I still think I will get my hvac guy to check pressures. The system was charged before power was available and never checked. We took it for granted that since it was providing at least 10 deg delta t that it was probably ok. It kind of got my attention when my elec bill was 127 for Nov and 262 for Dec.
You are wise to have the system checked out, something is not right.
My system may have more bells and whistles, but temp across the coil should not be too different, and you have a new system, with good efficiency potential.
You could compare Dec. heating degree days for your area with same for Richmond, Va. to get a very rough comparison. I have nat gas heated water coil in central forced air downstairs,(air handler is 28 years old) and heat pump up stairs. House is 3500 sq ft,a lot of glass, long showers,a lot of electrics on. Total gas+ ele for Dec was about $160. Ele rate is 8 cents per kwh. Life styles, etc., aside, I think you can reduce your power costs by a lot. A wider range before heat turns on would help reduce the number of cycles with power surge and poor efficiency while pressures stabilize. However, your refrigerant is likely not correct. Your installer should have checked it while fully operating for 15 minutes at least, even if you have to adjust thermostat to keep it running. Good luck and let me know what you find out, and suggest you take some benchmark readings for future comparisons. PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
Finally got my hvac guy to come by. Low on refrigerant! He put his dig. probes like you mentioned, mine read exactly the same on return side and about 6-7 lower at register about 25 ft down stream from his. After charge I was getting 100+ at register with oat around 32 deg. That should make things a lot better.
Our rate is around $.075/kwh. I am running two refrigerators, a Jacuzzi outside at 100 deg, the heat pump, and in Dec I had a monster amount of Christmas light on for 6 hrs a day also. The old half of the house has an oil-fired furnace set around 56 deg.
The low refrig. also explains why I was getting wet filters ( installed just below the updraft coil), it was probably freezing up and dripping down rather than running down to the pan.
Now my only issue is a rogue thermostat that sometimes when the power goes off for a short period during the cooling season will revert to about 65-68 deg set instead of coming back to set temp ( usually 76-78). Tried to duplicate it but was unsuccessful. Will play with it later. Not usually a real problem unless going out or town for a long period
Thanks for help and advice.
Bob
Rasconc, Glad you had it checked. Not to make you nervous, but 100 deg in heat pump mode is pretty high for a 32 deg od temp. Are you sure the backup strips were not on when you took that reading? You can check that by turning the thermo up to kick in the heat strips, and read output then. Are you running 400 cfm per ton?
Re: the wet filter in an updraft unit. You are correct; the low refrigerant over chills, ice blocks the air flow, that causes more buildup, and eventual meltdown soaks the filter. This would be a good time to spray coil cleaner and rinse water on the coil now that the refrig is balanced, and take some heat readings for the log book, as benchmarks. Glad I could help, and thanks for the update. PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
Sorry to be so slow getting back. I am not sure about flow, did not check what tap fan was on, rated for a max 1785 cfm so at 3.5 t should be ok.. Just checked and 40deg oat (raining slightly), 72 return (at pre-filter 16ft or so before coil), 102 discharge (just above coil, in start of duct). Hot flashing wife has had to go back to t-shirt attire. May kick on heat strip and get her to go for topless.
I guess I had been brainwashed to think that heat pump output temp was way below fossil fuel. I shudder to think how many hours and kw's have been wasted. We keep ours set about 72 winter and 78 or so summer.
Another issue is that our t-stat seems to be possessed when there is a short power outage. Sometimes while in cooling mode it will revert to about 68 if power is interrupted. Not a big deal if home. If gone for a long weekend the cats give us a bad look when we return to the 1700+ sqft refrigerator. I need to train the folk who look in on the cats to check and adjust.
I think I will be able to shame my installer into a warranty replacement. We tried to duplicate it but could not. It is supposed to return to the same mode and setting unless an extended outage, then to the same mode and 68 or so in heat and 78 in cool I believe.
THanks for update. That is a mighty fine delta-t.You should see some major savings now. Suggest you can the thermostat in back seat of dealer. It is costing your money. PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
Heat pumps don't give much rise in temp. But your reading are much better than mine were on my old york system. Maybe it needing recharging every year had something to do with that. I'm over to gas and a total new system now, much better!