Heat pump supply temps

Problems related to residential installations.

Heat pump supply temps

Postby cliffdweller » Sun Feb 26, 2012 2:15 pm

Mostly out of curiosity, I put a data logger on my HVAC to look at temperature-time curves. Result wasn't quite what I expected. On heat demand, the supply temp climbs quickly and steadily from about 72.2 (Tstat set at 70) to the mid eighties then levels off for a period of 1 to 3 minutes, then climbs again for a few degrees and levels again. This climb and hold pattern repeats every 2 to four minutes until the heat demand terminates. I haven't calibrated the sensor, but I'm using a precision temperature sensor (LM34) which should be accurate. Is this a normal temp/time history? Are these saturation related cycles? The unit seems to be working well, but I expected a rather smooth uninterrupted increase in temp over the run cycle. Any insight greatly appreciated. cliffdweller
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- Heat pump supply temps

Postby heatseeker » Mon Feb 27, 2012 9:29 am

Are you talking about a heat pump electric heat strips or gas? In either case I would assume that when generating heat from a heat pump it is a gradual warm because a heat pump needs to capture heat from the ambient temperature and then release it into the house so it would depend on ambient temps. Gas would probably be a steady temp rise and electric strips would also rise sharply over a short time. Usually gas and electric have a plenum temp around 130.
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- Heat pump supply temps

Postby cliffdweller » Mon Feb 27, 2012 10:43 am

Its a 5 ton Rheem air-exchange heat pump with electric auxilliary heat. I had all three banks of aux heat de-energized so I could look specifically at the performance of the compressor/refrigerant cycle.
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- Heat pump supply temps

Postby heatseeker » Mon Feb 27, 2012 11:30 am

What? Why are you fooling with the wiring? Your heat pump has aux. heating for a reason. If it is below 35 deg you will need aux heat or if your system is not maintained you will also lose efficiency. If you have routine maintenance performed you will not have to test your equipment. Keep it simple and stop trying to break your heater/ air conditioner. Better yet stop trying to hurt yourself.
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- Heat pump supply temps

Postby cliffdweller » Mon Feb 27, 2012 1:38 pm

Ahh Heatseeker, you have jumped to too many conclusions. I have a 20kW loadshedding generator up here on the mountain, so when we experience protracted power problems, I can trip the breakers to the electric aux heat. That allows me to continue to run the heat pump on my generator for whatever heat it can extract. Ambient up here was in the mid 40's when I collected the temp/time history on the heat pump. The outdoor unit (RPNE-060JAZ) ARI heating design specs include data for 47 F (dry bulb) and 17 F (DB), so the unit was operating well within its performance envelope and no aux heat would have been called anyway.
Your initial reply sort of confirmed my thinking about what the temp history ought to look like. I'm beginning to think my unit has a slight refrigerant overcharge - within spec, but just a bit more than needed.
Thanks for your input.
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- Heat pump supply temps

Postby heatseeker » Mon Feb 27, 2012 2:39 pm

Yes I figured there was more info; It sounds more like an undercharge as opposed to overcharge. Heat pumps are especially sensitive when it comes to air flow also in the duct work. But a air flow prob would actually produce warmer air in the interim before it cut out.
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- Heat pump supply temps

Postby heatseeker » Mon Feb 27, 2012 4:06 pm

My bad you are right overcharged too much subcooling in condenser however a sever undercharge would also cause insufficient heat, but yea overcharge * too much * not enough heat.
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