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Subject: heat pump system subcooling requirements for a

downflow air handler.I have Rheem...
subject: heat pump system subcooling requirements for a downflow air handler.I have Rheem 6 ton variable speed split heat pump system. My air handler is installed in downflow position. Air handler is on first floor and ducts go through the crawlspace to supply first floor vents. When I want to verify refrigerant charge by subcooling, I look at the nameplate to find the subcooling design value from the manufacturer y found out that there is an additional 20 degrees required if the airhandler coil is installed downflow.
My question is, does this apply to my system (read somewhere that downflow is mostly used for attic installations)? most importantly I'd like to understand why the difference would be so big?
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Answered in 1 hour by:
12/22/2017
Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 9,402
Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial
Verified

Hello, sub cooling condensed liquid from the condenser coils varies from 10F to 30F or more depending on the volume and temperature of the air across the coil.

The best way to charge a heat pump is with a sight glass in the liquid line, charge until there is no liquid stream and no bubbles in the sight glass and you will be very near a perfect charge.

The final pass tubing in that coil are laid out in such a way that more air flows across them in the down flow configuration than in the up flow configuration, thus greater sub cooling of the condensed refrigerant in heating mode.

.

My goal is to give you the best experience possible! I hope I have earned a 5 star rating! Please remember to rate my service by selecting the 5 stars at the top of the screen before you leave today. If you need more assistance, please use the reply box and let me know.

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Hello again, we can continue with any sort of follow up questions you wish.

Having said that, any heat pump charged in heating mode, in the winter, should have its refrigerant charge checked in cooling mode in the summer. Going strictly by super heat and sub cooling has led a lot of technicians astray, that is because there is not such thing a system built perfectly for the exact air flows that the book states... the systems vary... so that some skill and judgement is required to charge them.

We can discuss all of those factors

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Tell me how long the suction line is, we can go from there

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Customer reply replied 5 months ago
Hi Phil, thanks for your answer. Not sure I got the answer I was looking for. Perhaps I didn't phrase my question right so let me try again (by the way this is a 50 foot line to answer your question). In the situation I'm in right now, I'm getting the compressor shut down due to high refrigerant pressure. The only way I have to verify I have a correct charge is by following the values on the nameplate. I don't have much experience with HVAC repair but being an engineer I'm trying to understand the difference in the called out subcooling value. For sake of an example, at the temperature I was measuring subcooling, the design value is 30 degrees for upflow (which is what I think the system has right now), and 56 degrees for downflow. The reason I'm a bit skeptical of this is that it'll mean I need to add lots of refrigerant. It just doesn't seem right, given that I have a high pressure message. Any thoughts?

Hello, I need to very direct here.... do not charge by the sub cooling and super heating instructions given by the manufacturer in this case primarily because of the 50 foot long suction line

.

You most likely have either over charged the system with refrigerant, or not used a vacuum pump when charging it with refrigerant. If you try to mix those instructions with mine it will take a lot longer and you will have to read a few books to sort out why.

.

A bit about myself, I am 77 years old, the first 30 years in this business running my own service and manufacturing operation with 27 staff at peak, the next next 20 years consulting for major firms nationally with multi hundred million dollar projects for IBM, Texas Instruments, and various oil and food processing companies and for our nuclear weapons plants... the last 6 years answering technical questions, 8,600 so far with a 98.4 success rate.

.

Your suction line should be no smaller than 1-1/8" in diameter and have no more than 3each 90 degree elbows in it... The liquid line should be 1/2" OD copper tube. If not you will have problems

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If you have evacuated the system using a vacuum pump, and then charged the R410a refrigerant (if that is what you are using) in the liquid mode (as recommended on the box). Then fit a liquid line sight glass in the liquid line and charge slowly until you see the river of refrigerant disappear and only a few bubbles remaining.. keep charging until the bubbles dissapear.

.

Take my word for it please, but the only way in your situation to get a totally suitable charge is in cooling mode when the weather is warmer, in that case, with the house at 72F the suction pressure should be between 101 and 104 psig. the suction line should be cool or slightly sweating as it enters the compressor, and there must be NO sweat on the compressor.

.

That will be a perfect charge.

.

In the winter conditions vary too widely to do this by rote, the outside air can drop from 60 F to 30F way out of normal ranges so the manufacturers try to use sub cooling, AND THAT WORKS... but not very well, especially with unknown air flow across the indoor coil and a 50 foot suction line.

.

Let me know what you think, we can go from there

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Customer reply replied 5 months ago
Thanks for the reply.
I should say I went ahead and measured the length of the line. It's actually around 40 ft, which according to Rheem is not quite a long line application. Suction line is 7/8 and liquid line 3/8 (per manufacturer's instructions). I can confirm that we did use a vacuum pump prior to charge. I'm up in the Seattle area, so I use the system mostly in heating mode (rarely a need for cooling up here). We charged the system this summer using the weight method. I don't remember adding too much refrigerant so I'm not sure it's that much overcharged.
That's the reason why I'm trying to determine a way to figure the charge by subcooling. Even if it's not perfect, I just need to rule out that the system is not overcharged. In this situation, what could I do? All I have is my pressure manifold and pipe thermometer. In most cases this should be enough but I ran into this downflow issue so now I'm stuck.

Hello again, click here please, notice the 7/8" suction line sizes down to 3.5 tons... and not over 5 tons

click here for liquid line sizes, with a 3/8" liquid line, you will get flash gas in the liquid line...that causes an assortment of problems.

Notice 3/8" is recommended for systems 1/3 the size of your 6 ton unit system.

.

None the less if you have 7/8" tube run, for 40 feet, I will just figure in a 4 psig pressure drop when recommending gauge pressures taken at the unit. That will affect the system efficiency slightly.

.

Tell me the inside air temperature, the outside temperature, and the temperature of the air coming from the registers, and the suction and head pressures at the exact same time with the system running for longer than 15 minutes, I can assess the charge from that information. Use the same thermometer for all measurements, Tell me what type of thermometer it is please.

.

When taking pressures at the outside unit, both the liquid and liquid lines will read the same high side pressure, some units have a third 1/4" flare fitting in the area, the connects to the suction line at the compressor so that you can get a suction pressure reading when the system is in heating mode.

Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 9,402
Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial
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Customer reply replied 5 months ago
hi Phil. My bad. System is 5 tons. Both of your charts show I have the correct line sizes. That out of the way. The situation gets a bit more complicated when trying to measure pressure. Let me credit you for this question and start a new one. It seems the best way for me is to sign up for the monthly service with Just answer, but I'm not sure how you get paid in that case. Don't want to abuse your time.
Customer reply replied 4 months ago
hi Phil, I already rated you on this question. I'm planing to start a new question as soon as I'm able to take measurements. It's been freezing around here for the last few days. Thanks.

No need to start a new question, we can continue on the same dime so to speak here.

In order to check the refrigerant charge in heating mode, I need 4 things:

1. Inside house temperature

2. Pressure in the 7/8" line at the outside unit.

3. and temperature of the air coming out of the warm air vents inside the house.

4. Outside air temperature

Take all of these reads after the system has been running in heat mode for 15 minutes

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Customer reply replied 4 months ago
Hi Phil, for taking pressure measurements, would it work if I take the measurements at the liquid line service port?
The unit has also a suction line port that I could measure.The reason I wanted to start a new question is that there is more to the pressure measure: when I tried to measure subcool last time, I realized the pressure never stabilizes. I'll need to confirm this, but it seemed to me that the suction pressure did stabilize but the liquid line didn't. It would cycle within a range. As it reached its peak, I'd see the fan rotate faster until the compressor would make a noise and pressure would begin to drop and the cycle started again. This is a variable speed scroll compressor so I'm not sure this isn't normal.

Give me both pressures, liquid line and suction line, if the suction line is not in the 100 to 110 psig range then its the suction line ONLY in cooling mode... and will read head pressure in both the liquid line and the hot gas line going to the coil inside.

The pressure always changes, but it DOES stabilize under any given set of temperature conditions it takes about 10 minutes to stabilize.

.

Without needed training, you are over thinking the situation and running off in to the all weeds so to speak, best to just follow my lead for now.

It sounds like you have too much refrigerant in the system OR not enough air flow across the indoor coil, but until you get me the pressures AND temperatures that I asked for I have no way of knowing

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Customer reply replied 4 months ago
Hi Phil, I started the system around 13:00. After about 10 minutes I started taking measurements. Pressure started cycling as I described before so I stopped and waited 40 minutes and recorded the data below. The system shut down due to high refrigerant pressure after about 3 hours. All the while the cycle repeated (duration about 10 minutes). Liquid line fluctuated between 220 psi and 375 psi for most cycles. Sometimes going to 400, and even 500 psi. Suction pressure remained somewhat constant around 125 psi (127 psi at lowest liquid line pressure of 220 psi, 1115 at highest liquid line pressure of 500). Liquid line temperature went from 65 at lowest and 75 at highest pressure. Here's the data I gathered for one such cycle:Indoor temperature: 70.5 deg.
Outdoor temperature: 48.0 deg.
Vent air temperature: between 90 deg and 100 deg. (not paired to outdoor pressure measurements)Liquid line pressure reading (time 0:00 is cycle start about 42 minutes after system started):
00:00 220 psi
00:13 250
00:49 300
02:03 350
03:04 400
04:01 450
04:36 500
04:55 525 (compressor makes noise and pressure starts dropping)
07:00 350
08:12 300
09:03 275 (cycle resumes, pressure starts climbing)
09:42 300
10:05 350
10:38 400 (compressor makes noise and pressure starts dropping)
12:47 250
14:30 225 (cycle resumes, pressure starts climbing again)
15:40 250

Thanks, the system has between 6 and 18 ounces too much refrigerant in it.

remove refrigerant until the suction tube attached to the compressor itself inside the condensing unit, is sweating but the compressor is not sweating... at that point you have a 98% accurate charge.... when the warm weather returns, check the charge the same way again, with the inside of the house at 72F... suction pressure should be 101 to 105 psig, the suction line should be sweating but no sweat on the compressor.

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Customer reply replied 4 months ago
Thanks for the recommendation Phil. I usually need to wait a day or two to be able to restart the system.
I'd like to understand a couple of things for my own benefit. I've been studying quite a bit in the last few days and I think I'm getting to a decent understanding of the refrigeration cycle, so hopefully I can follow.(1) how do you explain the fact that the pressure never stabilizes?; (2) roughly, how do you arrive at your diagnostic? (3) can you explain a bit how the suction line sweating indicates adequate charge?Thanks.

1. the refrigerant moves around in the system at the start, after a while it settles in,

2. 500 to 550 psig head pressure on a 70,5F room tells me its over charged, along with the 120 psig suction pressure at 48F outside air. Its grossly over charged.

3. there there should be no refrigerant droplets in the refrigerant gas stream left by the time it gets to the compressor, the compressor motor puts out heat, enough to keep the compressor warm, unless there are droplets of liquid refrigerant getting back to the compressor.

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Customer reply replied 4 months ago
Thanks Phil. Appreciate your answers. I completely get your answer to (2). Regarding (1), pressure never stabilizes in this case. Could it be that it's so overcharged that it isn't able to? Lastly, for (3), could you help me understand why having some droplets in the suction line lead to condensation on the outside of it?

2. something like that, an over charge of refrigerant puts a system completely out of balance.

3. When droplets of refrigerant evaporate in the low pressure suction of the compressor they absorb heat, when heat is absorbed all that is left is 'cold'. Cold is merely the absence of heat....

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Customer reply replied 4 months ago
2. I see.
3. Great explanation. Thanks. To fully understand this topic: in theory, the suction line should have only superheated vapor. But I guess what you're saying is that I could have a refrigerant evaporate in the first half of the coil or so, and continue to absorb heat through the second half while still having some parts of it remain liquid?

yes,

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Customer reply replied 4 months ago
thanks for your help Phil. I'll try your suggestion and will likely start a new question in the next few days. Have a great 2018.

about 80% of the coil will have unevaporated refrigerant droplets in it, only the last 20% adds super heat,

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