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Old 12-16-2013, 01:41 PM   #1
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Hello, I have a 2006 rheem 4ton r410a heatpump piggyback with a gas furnace. I was wondering what high side valve and low side valve pressures and temps I should see on my gauges with the outdoor ambient temp being 40 degrees F with the system in heat mode? The indoor dry temp is about 70F and the wet bulb is about 56F at the return. At one of the blower vents, the dry bulb temp is about 92F. Is there a superheat and subcool chart for this?

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Old 12-16-2013, 04:55 PM   #2
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Super heat and Sub cool charts aren't made for heating mode.

22 degree temp rise sounds low. But its best to take temp difference readings at the plenums.

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Old 12-16-2013, 06:42 PM   #3
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How do you find out if the r410a is low or not? I had to add some twice over the summer with the last time being a couple of months ago. So even if it is low how much should be added? Is there not an industry rule of thumb or something that can be used for winter service valve psi and temps?
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Old 12-16-2013, 07:00 PM   #4
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Recover and then weight back in factory charge amount plus line set allowance.
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Old 12-16-2013, 09:24 PM   #5
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Thanks beenthere. While I am sure that will do it. That doesnt really answer my question. You could use that same train of thought in the summer too. But there is plenty of documentation for what the summer psi and temps should be. Even if you do not have subcool or superheat data, for r410a in an 85 dry bulb degree ambient, you can shoot for 120 psi on the low and 250 psi on the high with a 20 dry bulb degree difference between the return and supply vents for summer and you would not be too far off. You would at least know whether or not your measurements were in the ballpark. But I have not seen any general rules of thumb like this for the winter. And that is what I am trying to find out without having to evacuate.
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Old 12-17-2013, 04:28 AM   #6
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Thanks beenthere. While I am sure that will do it.

That is the manufacturer's method/recommendation of all brands.

That doesnt really answer my question.

There is no general rule to tell you.

You could use that same train of thought in the summer too.

Sometimes that has to be done to units in cooling mode in the summer, after someone has tried charging it by a rule of thumb.

But there is plenty of documentation for what the summer psi and temps should be. Even if you do not have subcool or superheat data, for r410a in an 85 dry bulb degree ambient, you can shoot for 120 psi on the low and 250 psi on the high with a 20 dry bulb degree difference between the return and supply vents for summer and you would not be too far off.

If you wee running an R410A unit on an 85 degree day and only had 250PSIG on the high side. Your system wouldn't be working right.

A 20 degree dry bulb delta on a 3 ton unit would mean either the latent load is low, or the system is not moving 1200 CFM.

You would at least know whether or not your measurements were in the ballpark. But I have not seen any general rules of thumb like this for the winter. And that is what I am trying to find out without having to evacuate.
There are a couple brands that have pressure readings posted in the electrical panel for heat mode(and one manufacturers chart is a lot different then the others). They of course assume that you actually checked air flow, and verified that you have 400 CFM per ton.

If your unit is not one of the units that has that chart, then their is no rule of thumb to tell you.
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Old 12-17-2013, 07:51 AM   #7
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Thanks for the reply Beenthere.

"If you wee running an R410A unit on an 85 degree day and only had 250PSIG on the high side. Your system wouldn't be working right.

A 20 degree dry bulb delta on a 3 ton unit would mean either the latent load is low, or the system is not moving 1200 CFM."


Sorry I had a typo. But just by your reply, this shows that you are working off experience that has been gained over years of working in the HVAC field. And therefore you are applying "rules of thumb" and "school of hard knocks" lessons even to your posts. Especially with regard to the 400CFM rule. That is really a generic flow rate and it is actually a range (like 300 to 500CFM) and what the actual flow rate is depends on the unit and design situation.

I do not have any charts in my manual regarding winter temps and PSI. The only thing the manual says for summer is that the subcool needs to be between 8-12 degrees F. Which it was within this range two months ago and the temp differential was 23 deg F at that time. FYI, two months ago, the outside temp in my area was 80-85 degrees. We do not get much of a Fall season around here.

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Old 12-17-2013, 05:55 PM   #8
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most of the charts are based off of 400 CFM per ton. While you are correct it is also a nominal CFM, it is what most manufacturers used to design their systems and get their efficiency rating. Heat pumps re actually more efficient at 450 CFM per ton.

300 CFM is only used in areas that have high humidity, and 500 in areas that are very dry.

A 23 degree delta would mean either low air flow, or low latent load. Remember, the colder the coil is, the less efficient the unit is.
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Old 12-18-2013, 08:48 PM   #9
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Quote:
Originally Posted by beenthere
most of the charts are based off of 400 CFM per ton. While you are correct it is also a nominal CFM, it is what most manufacturers used to design their systems and get their efficiency rating. Heat pumps re actually more efficient at 450 CFM per ton.

300 CFM is only used in areas that have high humidity, and 500 in areas that are very dry.

A 23 degree delta would mean either low air flow, or low latent load. Remember, the colder the coil is, the less efficient the unit is.
Also by the density of the air. I am at 5000ft (that is the town and I often go into the mountains for jobs easily adding another 1000- 2500 ft) and we use 450CFM per ton because of the thinner air. A cubic foot is a cubic foot no matter where you are but a Cubic foot of air is lighter and a lot less dense here than it is at sea level.
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Old 12-27-2013, 04:27 PM   #10
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We are getting off track. I just want to know what typical pressures I should read on my gauges in the winter based on the system parameters I described in the first couple of posts. Or just tell me what high/low psi you would expect to see if you were doing a service call in the winter on a 4 ton r410a heat pump unit with an outdoor temp of 40F and an indoor return dry bulb of 71F and a return wet bulb of about 56F.
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Old 12-27-2013, 07:22 PM   #11
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Low side, anywhere from 56 to 73
High side, anywhere from 236 to 295
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Old 12-27-2013, 08:00 PM   #12
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Thanks!
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Old 12-27-2013, 08:02 PM   #13
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Thanks!
Don't forget. Your readings can be within what I posted. And still not be charged right, or something be wrong.
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Old 12-27-2013, 08:19 PM   #14
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Ok. I just need to know the psi ballpark so when I check it I can have some feeling for what it should be. Since this is a piggyback unit, the temp difference between the return and a supply vent due to the gas kicking in varies. I have noticed the supply temp reaching 108F with the outside ambient near 40F but other times it is closer to 94F with the same ambient.
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Old 12-27-2013, 08:39 PM   #15
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As we tried to tell you earlier. There is no ball park to go by.

Best way is to recover. And then weigh charge in to factory spec including line set length correction.

The only time a heat pump and gas furnace should run together is when the heat pump is in defrost.

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