Heat pump overcharged in heating mode

Hi guys,

I've got a reverse cycle a/c running with R410a. Cooling is ok, no problems.
In heating mode, however, I have a small Evap (means Cond in heating mode) plus having to maintain low rate flow rate for noise level benefit. This made not decent heat exchange rate for the high side during heating cycle. Consequence, system show signs of overcharged in heating + high head pressure no matter how I control the captube length or TXV. High Hp is not accepted as it will exceed the working envelop in warm climate.

Do you think of any way to work around this? Could an accumulator or a receiver help?

Constraints:

• Maintain same charge for Cooling performance.
• Maintain low air flow rate for noise.
• Can not have a bigger Evap.
• Captube or TXV variation does not help.

These limitations made it too hard for me to control head pressure.

Thanks for any suggestion.

Hi Uni
I have had a few of these problems this year and the solution is to reduce the "outdoor unit" load on compressor during heating mode. Simple - on a water cooled unit I reduced the flow rate (condenser water) which reduced the load and worked. On an air cooled unit the outdoor fans were controlled to not run during heat mode which works sort of as the compressors are scrolls and can handle a little liquid and deice control for when outdoor coils ice up, my preferred option was to slow the outdoor fans during heat mode but that proved too expensive for the client.

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Depending on the make and quality of the thing the the outside heat exchanger is
split into 2/3 and 1/3, so when the thing is in cooling mode the outside cond
uses both 2/3 and 1/3 of the cond together to get total heat rejection but in heat
pump mode the outside heat exchanger (now evap) only uses the 2/3 of its capacity.

The thing that makes the cond 2/3 and one 1/3 can be either electronicaly controled
valves or just plain simple one way valves...

You need to make sure that when it is on heat pump mode, the outside heatexchanger
is ballenced correctley .

All the best

taz

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Dear Uni

If you talking about split unit air conditioner then I thing i have the goodsolution for you.

1- Check booth run and start capacitors if the ok??? If not the replacethem....

2- Install HP Switch on suction pipe just before the service valve, to cyclethe outdoor fan. So the HP switch should be normally close circuit when pressureis below 400psi. When pressure exceeds 400psi it should open circuit. And allwhat you need is just cut fan motor coming line and connect it throw the HP.

Regards

A/c king

Thanks for quick replies, gents. Nice to hear from Aussies, too.

Back to the topic,

A/C King, the Unit is manually built with selective parts so it can be sure all components are to specs. I guauged them all.

Your second suggestion is quite in line with the others, to ctrl the outdoor load. Promissing solution, guys.

As I have two speeds outdoor blower, so last week I tested this strategy by switching to low speed. However, it was unsuccessful; the change was tiny. After reading your posts, I double check and think the failure was because of the relatively "high" low speed plus oversize outdoor HX (air-cooled) plus charge amount. Looked like the load was still too much for the indoor HX. Initially I sized the HX for the benefit of Cooling COP + enlarge both Cooling & Heating envelops for the Down Under climate.

I will test this again today with a smaller charge + low fan speed to see what happen. Will update the outcomes.

Tesla, strongly agree with you. Just dont understand one point when you said the cost was up.

Cheers,

Uni

Uni the cost was up on either the controller cards for deice and simply disable the outdoor fans on heat mode or fit new dual speed outdoor motors (X3) and control wiring. This particular package unit is for full fresh air in Sydney conditions for summer and the product is live. I have also seen another pak unit wired so as the fan direction reverses reducing the airflow across the coil.

Tesla, thanks for the explanation. I've got a two speeds blower, so that is not a problem.

However, yesterday test was no good. As long as I switched the fan low, discharge temps & pressure rose up dramatically. Switched back to high after stabilisation, pressure droped straight away. My guess is because I have an air-cooled compressor. Less heat loss means higher output, I think.

I'm not sure about your case. How the operating parameters changed?

Quote:

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Originally Posted by Uni

Tesla, thanks for the explanation. I've got a two speeds blower, so that is not a problem.

However, yesterday test was no good. As long as I switched the fan low, discharge temps & pressure rose up dramatically. Switched back to high after stabilisation, pressure droped straight away. My guess is because I have an air-cooled compressor. Less heat loss means higher output, I think.

I'm not sure about your case. How the operating parameters changed?

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To be clear: Are you saying that with the system in heat mode, you reduced the speed on the outdoor fan?... or the indoor fan?

System in heat mode, outdoor fan was switched between Low-Hi. Compressor sits downstream the outdoor section. Thanks for any thought, Gary.

Another remark was that when I brazed (manually) another row onto the indoor HX, and maintain the same air flow rate. Pressure seems to ease. It makes sense. But due to limitation I mentioned regarding indoor side (space & air flow rate), I am trying to tune the outdoor as per suggestions.

Uni that sounds like an excellent direction achieving the same result from the other side of the system which would be more efficient, give better control (with a valve bypass or something) and match the rest of the system. Another thought was to bypass the suction side of the evap to reduce load but there is the disadvantage of flooding where increasing the capacity of the condenser in heat mode would work. Best I think would be the increase heat exchange on indoor coil and decrease heat exchange on outdoor coil during heat mode.

Really appreciate all of your inputs so far, gents.

I decided to stick to that bigger indoor HX and tune the air inlet to maintain the air flow rate. Another thing I may try next week is to follow Taz24 suggestion, will by pass 1/3 of the outdoor coil in heat mode. I will use check valves.

Taz24 or anyone have any experience doing so on the field? Any concern?

Btw, any of us used two (one) way TXV? Any issue or any comment regarding reliability? I have been using captube but thinking of changing.

please I have problem with compressor type Indianian manufacturing. KIRLOSKAR. the problem is abnormal vibration please hilpe me.

Quote:

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Originally Posted by Uni

Really appreciate all of your inputs so far, gents.

I decided to stick to that bigger indoor HX and tune the air inlet to maintain the air flow rate. Another thing I may try next week is to follow Taz24 suggestion, will by pass 1/3 of the outdoor coil in heat mode. I will use check valves.

Taz24 or anyone have any experience doing so on the field? Any concern?

Btw, any of us used two (one) way TXV? Any issue or any comment regarding reliability? I have been using captube but thinking of changing.

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Cutting into a condenser in the field needs to be given some serios thought,
but if done correctly is easy enough.

Just bear in mind if it has multiple passes, you will need to cut at the right point.

You will ineffect have to rebuild the supply to and away from the cond because the top
2/3's and the bottom 1/3 will need supplying. A small header will need to be made and the
same with the outlets. In the outlet of the smaller 1/3 part you will need to put a one way
valve to stop flow in that direction.

As for 2 TEV's, that is quite common. Just position them and have one way valves in the
opposite direction. That should not cause too much problems.

Remember the dryer. Dryers a directional unless you use a specific by-directional type.

All the best

taz

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