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Mr Roboto
November 15th, 2004, 08:32 PM
Can suction and discharge be checked on a heat pump in the heat mode? I was told that it couldn't and you had to place it in cooling to check it like a conventional a/c.

mdshunk
November 16th, 2004, 01:56 PM
Ummm... sometimes. Some heat pump manufacturers put three ports on their units. Two are to the lineset, and one is hooked directly to the suction line of the compressor. This is handy, but most manufacturers only have the two valves open to the lineset. In heating mode, both lines are high pressure. If you'd hook your conventional gauges on in heating mode, you'd immediately ruin the low side gauge. I have a "heat pump gauge set" that I cobbled together with two high side gauges. To get the readings you need and to charge the unit, you need to put it in a/c mode. Generally, you also have to block off most of the condenser coil with cardboard or something to simulate a 90 degree day to get decent readings and to charge the unit properly. Charging a heat pump in the winter is tricky, tricky.

Mr Roboto
November 16th, 2004, 03:04 PM
Ok, what if the unit has the third port coming right off the suction line at the compressor is it still under high pressure in the heat mode? Also, do you think the hot gas method of checking the charge is good enough? I mean like to also check it with a Gage set to compare the two.

It seems to me that the gage's would be a lot more reliable.

mdshunk
November 16th, 2004, 03:57 PM
Ok, what if the unit has the third port coming right off the suction line at the compressor is it still under high pressure in the heat mode?Nope.
Also, do you think the hot gas method of checking the charge is good enough? I mean like to also check it with a Gage set to compare the two.

It seems to me that the gage's would be a lot more reliable.I don't know what the "hot gas method" is. Using a gauge set and some temperature measuring equipment is the only way to properly charge a system.

Mr Roboto
November 16th, 2004, 04:29 PM
This is out of the manual I have for it. I have checked it this way but I not sure how accurate it is.

Hot Gas Method
The following procedure can be employed as a method to check
for system charge in the heating mode by measuring the hot
discharge gas at the compressor.
1. Allow the system to operate at least 20 minutes.
2. Attach and insulate an electronic thermometer's probe
to the vapor service valve (large line) at the base valve.
NOTE - Make sure that the probe is well insulated from
the outdoor air.
3. Allow the system to operate at least 10 minutes.
Afterwards, use an accurate electronic thermometer to
measure the temperature of the discharge gas at the
probe.
4. Using the electronic thermostat, measure the outdoor
ambient temperature.
5. For check purposes the temperature measured on the
hot gas line should be equal to the outdoor ambient
temperature plus 110F (+ or-4F). e.g: Outdoor Ambient
45F then the temperature measured by the
thermometer's probe should be 155F for a system that
is properly charged. If the temperature measured by the
thermometer's probe is higher than the outdoor ambient
plus 110F, the system charge should be adjusted by
adding refrigerant to lower the temperature. If the
temperature measured is lower than the outdoor ambient
plus 110F, the system charge should be adjusted by
recovering charge to raise the temperature
NOTE: When adjusting the charge in this manner allow the
system to operate for at least 10 minutes before taking the
next temperature reading.

mdshunk
November 16th, 2004, 04:46 PM
That's funny, but it will get you in the ballpark. Using that method, you'll have a perfectly functional system, but the charge will never be right on. The type of electronic thermometer required to do this on a cold day is more expensive than a gauge set. This company probably recommends this method, since you must be certified to install a gauge set. An uncertified person can face a fine of $27,500 per offense just for screwing on a gauge set, since it is considered accessing the sealed system.

Mr Roboto
November 16th, 2004, 04:56 PM
Ok, what should the suction pressure be just for knowledge purpose?

mdshunk
November 16th, 2004, 05:16 PM
There is no "one pressure" that will answer your question, since pressures vary with temperature. In cooling mode, suction between 60 and 80, and in heating mode suction between 35 and 50 would be some ballparks. Depends in large part on the ambient temperatures, and the engineering features of the equipment. Heat pumps are quite regularly seriously overcharged in the winter, resulting in refrigerant needing to be recovered in the cooling season for proper operation. It's not uncommon for HVAC technicians to reschedule a "check up" to revisit systems in the spring that they've had to charge over winter, just to make sure that they've not screwed up the charge.

Mr Roboto
November 16th, 2004, 06:21 PM
Ok, thanks for your input.