ok, help me fix my heat pumps

[1201]

during this last winter storm , where our temps in DFW fell to as low as 11f, my three heat pumps went into various stages of failure, where they either didn't heat sufficiently or refused to produce any heat at all.

I'm going to do what it takes to fix them, up to and including replacing them with different models.

my questions:

1. I have a full tank of Argon and a full tank of CO2. Can I use either of these instead of Nitrogen to pressure test the lines? That would save about $200

2. I believe I know how to get the refrigerant in the lines back into the condenser. I don't know how to recover the refrigerant from here. there are many vids out there of recovering from regular units but I havent found one for mini splits.

3. since the units are operating, is it possible to recover refrigerant into a tank without a recovery machine? Or do I need a recovery machine still. I don't mind purchasing one if I have to.


I'll stop here for now to keep it simple. Thanks for any help

 

[Partimewages]

You really need a recovery machine to recover. I would not use anything but nitrogen to flush. Argon is a dry gas but I don't know if it has ever been tested with any system.

 

[RCinFLA]

Argon would be fine for a flush and pressure test, being an expensive alternative to nitrogen.

Many heat pumps fizzle out at about 15 degs F. Issue is compression ratio on compressor increases as delta temp between inside and outside gets wider temp spread. This causes the effective heating btu's to decrease dramatically at colder outside temps.

Refrigerant mass flow determines btu transfer. At cold outside temps the compressor cannot supply the refrigerant volume flow rate.

Assuming inside is relatively fixed around 70 degs F this requires a high side pressure for R410a of about 30 degs higher for heating or about 100 degs F which is about 318 psig for high side pressure.

If outside temp is 15 degs F then low side refrigerant saturation temperature needs to be at least 10 degs F lower to get any heat absorption. That is 5 degs F saturation temp for low side refrigerant which is about 55 psig for R410a.

Compression ratio is (318 psig + 14.7 psia) / (55 psig + 14.7 psia) = 4.8 compression ratio. Normal summertime cooling runs about 3 for compression ratio. Most mid to low tier compressors cannot move the refrigerant mass flow at such a high compression ratio requirement.

I should point out the heat transfer rate for any heat exchanger depends on delta temperature. That is basic thermodynamic law. 10 degs F difference between refrigerant saturation temp in outside coil and temp of outside air is not great for absorbing a lot of heat. It would be significantly better with 20 to 30 degrees F delta. For 20 degs F temp delta, the refrigerant saturation temp in outside coil would have to be -5 degs F for 15 degs outside temp. That is low side pressure of about 42 psig and a required higher compression ratio of 5.9. For a 30 deg F delta the refrigerant saturation temp required is -15 degs F with a low side pressure of 31 psig and a compression ratio of 7.3.

The 'hyper' heat model mini-splits have compressors and inverters that run rpm's up on compressor at cold outside temps. Mitsubishi Hyper units run their inverters up to 144 Hz frequency with about 8500 rpm max compressor speed to pump the volume of refrigerant at high compression ratios. They sound like a jet engine at that high rpm's.

Typical mid to low tier mini-split inverters max out about 75 Hz of AC drive to compressor which translates to about 4500 rpm's max for compressor.
.

 

[RCinFLA]

Your question 2 and 3 refer to recovery of refrigerant.

Question 2 is called 'pump down'. You block off high side via the factory service port you used to initially release refrigerant when unit was first installed and run compressor until low side pressure drops to near zero. Do not allow low side to go into a vacuum, however. The refrigerant is stored in the outside unit condenser, just as it was when you first got the new unit.

Question 3 is almost the same thing except instead of storing refrigerant in unit's condenser you are storing it in recovery tank. Make sure you evacuate recovery tank before doing it or you will contaminate the recovered refrigerant making it useless to reused. Also flush any air out of hoses before opening valves. This allows you to access both sides of system as there will be little to no refrigerant left in unit.

Again, don't allow compressor suction line side to go into a vacuum. It can cause electrical arcing inside compressor destroying it.

Any mini-split without a high side service port needs to be charged by refrigerant weight. You need a scale to weigh it in. A $15-$20 postal scale will do that can take the 30 lb. weight of tank and refrigerant.

Heating operation is more critical to proper refrigerant load than cooling mode.

Pulling a vacuum on a used system is tougher than a virgin system as oil in the lines and inside coil will absorb moisture that is much tougher to remove under vacuum than a clean oil free new system. Avoid opening up system allowing air to infiltrate lines with its humidity. When opening lines it is necessary to have a low level of inert dry gas flow, like nitrogen, to keep out humid air intrusion. Also any brazing on copper tubing needs inert gas flush to prevent copper oxide ash forming on inside of copper pipe. POE oil is a great solvent and will dislodge the ash and allow it to flow through the system clogging up expansion valve (EEV) and grinding up in the compressor.

I would say the most common failure on mini-splits is thermistors. Most are same as $1 10k @ 25 degsC ntc 3950 thermistors you can buy on Amazon. The thermistor on the hot compressor discharge port line may be different as it gets baked up to 300 degsF.

 

[1201]

Your question 2 and 3 refer to recovery of refrigerant.

Question 2 is called 'pump down'. You block off high side via the factory service port you used to initially release refrigerant when unit was first installed and run compressor until low side pressure drops to near zero. Do not allow low side to go into a vacuum, however. The refrigerant is stored in the outside unit condenser, just as it was when you first got the new unit.

Question 3 is almost the same thing except instead of storing refrigerant in unit's condenser you are storing it in recovery tank. Make sure you evacuate recovery tank before doing it or you will contaminate the recovered refrigerant making it useless for reused. Also flush any air out of hoses before opening valves. This allows you to access both sides of system as there will be little to no refrigerant left in unit.

Thanks. Do I need to do a pump down before I use the recovery machine?

Again, don't allow compressor suction line side to go into a vacuum. It can cause electrical arcing inside compressor destroying it.

Copy that.

Any mini-split without a high side service port needs to be charged by refrigerant weight. You need a scale to weigh it in.

That's my plan. I'm committed to mini splits so I'll buy the scale, micron gauges and everything I need.

Heating operation is more critical to proper refrigerant load than cooling mode.

Pulling a vacuum on a used system is tougher than a virgin system as oil in the lines and inside coil will absorb moisture that is much tougher to remove under vacuum than a clean oil free new system. Avoid opening up system allowing air to infiltrate lines with its humidity. When opening lines it is necessary to have a low level of inert dry gas flow, like nitrogen, to keep out humid air intrusion. Also any brazing on copper tubing needs inert gas flush to prevent copper oxide ash forming on inside of copper pipe. POE oil is a great solvent and will dislodge the ash and allow it to flow through the system clogging up expansion valve (EEV) and grinding up in the compressor.

My manual suggests a triple evac. Would that work?

 

[S Davis]

Mitsubishi Hyper heat units Btu ratings are conservative, most of the units I installed do about 15% better than the ratings.

 

[Partimewages]

Why are you going to open the system?

 

[RCinFLA]

Thanks. Do I need to do a pump down before I use the recovery machine?

Copy that.

That's my plan. I'm committed to mini splits so I'll buy the scale, micron gauges and everything I need.
My manual suggests a triple evac. Would that work?

If using compressor, no. I prefer to do a little recovery with compressor so the recovery machine only has to deal with gas. Putting liquid through recovery machine is rougher on it.

Most expensive test piece is micron gauge. Suggest you buy a good ball valve and put it in front of micron gauge to avoid getting refrigerant and oil into micron gauge. Make sure you inject enough refrigerant after vacuuming to relieve vacuum so you don't suck in air removing vacuum gauge while still under a vacuum.

Vacuum micro gauge works on heat transfer through a vacuum and is calibrated for nitrogen gas. It is a heated thermistor that requires less electrical heating to maintain resistance as vacuum increases retarding heat transfer to surrounding (remaining) gas. Oil that gets on the thermistor is a heat sink that slows down the response time and can cause vacuum reading to jump around when it get contaminated. You can clean them out with isopropyl alcohol. If you have moisture mixed in oil in system, the vacuum gauge can also jump around as bubbles pop out of oil.

Triple vacuuming is just using nitrogen re-introduction into system to help sweep out freed water vapor. It can help if you have a lot of moisture intrusion in system oil. The nitrogen will not help get the moisture bubbles out of the oil. They have to pop out on their own and it is like boiling molasses. Once they pop the nitrogen can help sweep them along to output port.

 

[S Davis]

Thanks. Do I need to do a pump down before I use the recovery machine?


Copy that.


That's my plan. I'm committed to mini splits so I'll buy the scale, micron gauges and everything I need.

My manual suggests a triple evac. Would that work?

You really need to use a micron gauge, I have systems that will pump down in an hour, I had a four ton Trane yesterday that took 4 hours and breaking the vacuum four times with nitrogen to get below 500microns and hold. Now I was working 100’ from the sound with a constant drizzle so thing’s were pretty wet.

 

[S Davis]

Look at your service instructions, there should be a pump down procedure to pull all the refrigerant into the outdoor unit and then close the service valves and you don’t need to reclaim the refrigerant. It depends on the unit if it has this capability.

 

[S Davis]

I am working on a project right now that had three top off the line Trane heat pump’s and two split heat pump water heaters installed for north of $150,000.00, two years old everything failed during the cold snap last year.

I went to inspect it and came up with a bunch of issues, they had a balance point of 44 degrees, under 44 the emergency heat activated. One issue was line set insulation, they had the line sets running underground 30+ feet to the outdoor units, the conduit was turned up at the units and not sealed. This is what it looked like.

The line set had a great heat exchange.

I replaced all the line sets and the systems ran through this last cold snap down to 10 degrees.

Check the installation, make sure your refrigerant charge is correct and lines are properly insulated.

That would be where I would start before investing a bunch of money, depending on age of equipment. If it is old and warrants replacing you are probably not going to reuse the line sets anyway, I wouldn’t risk new equipment.JMHO

 

[1201]

Look at your service instructions, there should be a pump down procedure to pull all the refrigerant into the outdoor unit and then close the service valves and you don’t need to evacuate the system. It depends on the unit if it has this capability.

There isn't one that I can find

 

[1201]

If using compressor, no. I prefer to do a little recovery with compressor so the recovery machine only has to deal with gas. Putting liquid through recovery machine is rougher on it.

Most expensive test piece is micron gauge. Suggest you buy a good ball valve and put it in front of micron gauge to avoid getting refrigerant and oil into micron gauge. Make sure you inject enough refrigerant after vacuuming to relieve vacuum so you don't suck in air removing vacuum gauge while still under a vacuum.

Vacuum micro gauge works on heat transfer through a vacuum and is calibrated for nitrogen gas. It is a heated thermistor that requires less electrical heating to maintain resistance as vacuum increases retarding heat transfer to surrounding (remaining) gas. Oil that gets on the thermistor is a heat sink that slows down the response time and can cause vacuum reading to jump around when it get contaminated. You can clean them out with isopropyl alcohol. If you have moisture mixed in oil in system, the vacuum gauge can also jump around as bubbles pop out of oil.

Triple vacuuming is just using nitrogen re-introduction into system to help sweep out freed water vapor. It can help if you have a lot of moisture intrusion in system oil. The nitrogen will not help get the moisture bubbles out of the oil. They have to pop out on their own and it is like boiling molasses. Once they pop the nitrogen can help sweep them along to output port.

Ok so I should use the mini split compressor to get most of the refrigerant into the recovery tank, then finish up with the machine? Copy that.


When you say vacuum micron gauge is calibrated for nitrogen- I thought you only use that under vacuum and not under pressure?

 

[upnorthandpersonal]

R410a

Is ok for cooling, but not so much for heating. In cold climates, opt for R32 or even better, R290.
@1201 Any options for heat-pumps with those refrigerants? I know TX isn't a cold climate, but it might be a solution if you keep getting cold spells there.

 

[1201]

Is ok for cooling, but not so much for heating. In cold climates, opt for R32 or even better, R290.
@1201 Any options for heat-pumps with those refrigerants? I know TX isn't a cold climate, but it might be a solution if you keep getting cold spells there.

That's something I will look into if these units fail.

 

[1201]

This is in the service manual with no explanation. Can anyone who is familiar with this kindly please tell me what this is? Is this the running pressure that would be tested with a gauge under different operating conditions?

 

[1201]

For example what would this box mean?

 

[Partimewages]

It should be 116 psi at the service port when it's 45 degrees. What isn't clear is where the 45 degrees is measured.
That temperature will likely be either outdoor or a specific place on one of the refrigerat lines.
Are the units still running and "working" now that it's warmer outside?

 

[1201]

It should be 116 psi at the service port when it's 45 degrees. What isn't clear is where the 45 degrees is measured.
That temperature will likely be either outdoor or a specific place on one of the refrigerat lines.
Are the units still running and "working" now that it's warmer outside?

Thanks, two of the units are working now that it's above freezing. One is still not working.

I found a different chart (from a completely different manual not even related to my units) that may explain the numbers a little bit better.

 

[Partimewages]

The numbers are specific to each individual unit.
Was a heating load calculation ever done? If not start with that.
You should choose equipment based on that data. It's not likely that you can "fix" the current equipment if it was not correctly designed to begin with.