R290 ASHP problems

[jpadie]

Hello all 
I have a small mini-split R290 heat pump in Surrey.  No heat being expelled at the indoor unit but the larger hose leading from the outdoor unit is warming.  Probably to c 35C - I will measure today.  I don't often use the heat pump after commissioning before the Summer (it worked then) but have needed some heat the last few days! 
 

I'm not sure what the pressures should be with R290.  Compressor off I am measuring 5bar at ambient 9C.  Compressor on after 5 mins, the pressure is 5.2bar.  After 60 minutes the pressure increases to 8.3 bar.  

the vendor support has been less than marvellous and ducks the questions about target pressures, recharge steps (i.e. into vacuum or not) etc.  the total charge is 0.32Kg for a 4.5m hose run (each way).  

can anyone help with the diagnostics?  My guess is low charge as one of the flare joints is greasy (the propane odorant perhaps) - however the flare joint is supposed not to be torqued up too heavily and it is pretty tight (no movement with a. 10" wrench and moderate force). 

and if this is a low charge scenario can anyone help with target pressures at ambient and other info useful for recharging?  I could vent the propane and recharge based on weight of propane but that seems unnecessarily wasteful.

 

lastly (for a non HVAC engineer) I'd appreciate some guidance on how to "unleak" flare joints!  These are factory made joints by the way.  

 

thanks in advance

Justin 

[RobLe]

I suggest to liberally spray suspect joints with a water/washing up liquid mix.  If there is a leak, you generally get a bubble forming.

 

Our R290 system generally runs at around 13bar (hot), 3bar(cold) - of course the actual values depend on the temperatures involved - the temperatures should be slightly above the saturated values in the snip below at the measured pressures - the compressor in and out always has gas in it, never liquid.  Snip from a handy free danfoss ‘ref tools’ app.  

My gut feeling from what you’ve said though is that the compressor is suspect or the metering device is too open - I would expect pressure post compressor(guess that’s what you measured- normally you would measure both ports simultaneously, and also temperatures at the compressor) to increase rapidly after switch on.  5bar at 9C ambient is I think the expected pressure….I am at the limit of my knowledge I’m afraid though.

I assume you know r290=propane….  so don’t even think of getting a spark / flame anywhere near it.

[jpadie]

thanks for that.   v. interesting.

 

when you say 13bar (hot) does this mean when the heat pump is set to heat mode?  If so then the pressures I am seeing look more or less what would be expected.  I will have a look at the compressor again but from memory the inlet valve has no service port at which I could test, so outlet pressure was all I had. 

 

appreciate that propane is flammable.  the concentration has to be quite specific though: above/below the range it is not ordinarily flammable. 

 

from you last para do I take it that there's not likely to be awt wrong with the indoor unit?  the outdoor unit is warming the discharge hose. feels like high 30sC 

 

cheers!

 

 

[jpadie]

have been running the compressor for an hour as a test.

outlet temp is 30C.  inlet temp is 15C.  

but nothing useful coming out of the indoor unit.  

so it seems the outdoor unit is doing "something". but perhaps not enough. 
and the indoor unit ... not sure what it is achieving!  

[dpmiller]

leave it all to go stone cold. Start it up and monitor the hot pipe. If it immediately heats but then cools back to warm, then that's low gas for sure.

[jpadie]

5 hours ago, dpmiller said:

leave it all to go stone cold. Start it up and monitor the hot pipe. If it immediately heats but then cools back to warm, then that's low gas for sure.

thanks dpmiller.  there is no immediate surge of heat.  it's a gradual warm up of the outflow pipe and nothing from the indoor unit.  

[John Carroll]

On 07/11/2024 at 12:04, jpadie said:

Hello all 
I have a small mini-split R290 heat pump in Surrey.  No heat being expelled at the indoor unit but the larger hose leading from the outdoor unit is warming.  Probably to c 35C - I will measure today.  I don't often use the heat pump after commissioning before the Summer (it worked then) but have needed some heat the last few days! 
 

I'm not sure what the pressures should be with R290.  Compressor off I am measuring 5bar at ambient 9C.  Compressor on after 5 mins, the pressure is 5.2bar.  After 60 minutes the pressure increases to 8.3 bar.  

the vendor support has been less than marvellous and ducks the questions about target pressures, recharge steps (i.e. into vacuum or not) etc.  the total charge is 0.32Kg for a 4.5m hose run (each way).  

can anyone help with the diagnostics?  My guess is low charge as one of the flare joints is greasy (the propane odorant perhaps) - however the flare joint is supposed not to be torqued up too heavily and it is pretty tight (no movement with a. 10" wrench and moderate force). 

and if this is a low charge scenario can anyone help with target pressures at ambient and other info useful for recharging?  I could vent the propane and recharge based on weight of propane but that seems unnecessarily wasteful.

 

lastly (for a non HVAC engineer) I'd appreciate some guidance on how to "unleak" flare joints!  These are factory made joints by the way.  

 

thanks in advance

Justin 

 

Looking at the R290 properties, 5.0Bar = 9C &  8.3bar = 24C.  And 3.0bar = -6C & 12.54bar = 39.5C.

The pressures in the attachment look like  absolute, so add 1.0 to your pressures when (if) reading them.

Refrigerant R290 Properties.pdf

[jpadie]

Thank you. 

 

I'm never really sure what those saturation charts mean nor how to use them.  Is the relationship with the temperature of the gas or the temperature of the compressor?

I assume it is the gas (ie ambient temperature when everything is off).  

 

It's also not clear to me whether this is like r12a and needs to be recharged via the circulation pressure rather than into a vacuum.  It may not matter much given the boiling point is so low.   

 

I did try adding some r12a (isobutane +propane) using the same method as I use for that refrigerant (compressor on). However this just refilled the can!  

 

The vendor is unhelpful.

 

 

[RobLe]

When propane is cold and/or compressed, it will be a liquid.  When it’s hot and or at low pressure it will be a gas.  The chart shows the transition from gas to liquid, which is at a certain pressure at every temperature.  Tbh it’s all quite non intuitive, but that phase transition is where most of the energy is stored enabling heatpumps to operate.

 

Generally heatpumps are refilled at the cold low pressure side, when they are on, so that the low pressure there allows the fluid to be pushed into the system.

[SteamyTea]

16 hours ago, RobLe said:

Tbh it’s all quite non intuitive, but that phase transition is where most of the energy is stored enabling heatpumps to operate.

It is a bit, but it comes down to the Ideal Gas Law and Fluid density changes.

Actually it is entropy, the state of disorder.

A liquid has lower entropy than a gas: it is more ordered.

All that means is that when a fluid is in the gaseous state, it is easier (takes less energy) to increase the temperate, mainly because the colder molecules, or atom in an ideal gas, are slower moving, so easier for the photons (which are the particle that transfer the energy) to hit (raise the energy level of some of the electrons).

 

(that is a crude 'mechanical' model, but easier to understand than the quantum model, well the sums are easier)

Edited November 11 by SteamyTea

[jpadie]

18 hours ago, RobLe said:

When propane is cold and/or compressed, it will be a liquid.  When it’s hot and or at low pressure it will be a gas.  The chart shows the transition from gas to liquid, which is at a certain pressure at every temperature.  Tbh it’s all quite non intuitive, but that phase transition is where most of the energy is stored enabling heatpumps to operate.

 

Generally heatpumps are refilled at the cold low pressure side, when they are on, so that the low pressure there allows the fluid to be pushed into the system.

 

thank you, I am used to recharging on the low side (for my GSHPs and car aircon)  but in this case the only service port is on the high side (the low side does have an odd vent port too, you take the cap off and there's an allen screw underneath that opens the valve, like the back of a brake.  No schrader here and too big a hole to fit one, it looks like).  

So my plan is to pull a vacuum and vent the current propane charge completely and then charge by weight.  Annoyingly I only have long hoses, spanning 3m in total.  

 

[jpadie]

2 hours ago, SteamyTea said:

It is a bit, but it comes down to the Ideal Gas Law and Fluid density changes.

Actually it is entropy, the state of disorder.

A liquid has lower entropy than a gas: it is more ordered.

All that means is that when a fluid is in the gaseous state, it is easier (takes less energy) to increase the temperate, mainly because the colder molecules, or atom in an ideal gas, are slower moving, so easier for the photons (which are the particle that transfer the energy) to hit (raise the energy level of some of the electrons).

 

(that is a crude 'mechanical' model, but easier to understand than the quantum model, well the sums are easier)

 

 

I remember the gas laws.  Boyle and Charles ?  A level physics nearly 40 years ago so I'm probably wrong. 

 

[SteamyTea]

11 minutes ago, jpadie said:

A level physics nearly 40 years ago so I'm probably wrong. 

Only if it not 40 years ago.

[jpadie]

it was 1986 so not quite 40! 

[Tony L]

It's disappointing to hear the vendor is not offering sufficient help.  I'm in Surrey & I'll be shopping for an ASHP at some point during my build.  Would you share the name of your vendor, please so I can be sure to avoid them?  A message, rather than forum post would be great, if you don't want to run the risk of defaming these people on a public forum.

 

I hope you overcome your problems soon.

[dpmiller]

16 hours ago, jpadie said:

(the low side does have an odd vent port too, you take the cap off and there's an allen screw underneath that opens the valve, like the back of a brake.  No schrader here and too big a hole to fit one, it looks like).  

 

any chance there's a simple NRV under the allen screw that incoming charge pressure bypasses?

[jpadie]

On 12/11/2024 at 07:48, dpmiller said:

 

any chance there's a simple NRV under the allen screw that incoming charge pressure bypasses?

it's possible that there is a valve of sorts but if it is an NRV then it's pointing the wrong way!   the allen key is used in commissioning.  the commissioning steps require that the machine is left to sit for a while then a series of venting cycles via cracking the socket bolt. 

 

I pulled a vacuum down as low as I could last night, then turned off the valves.  It was holding for a couple of hours but over 24 hours I am almost back to zero (still below).  all the joints are as tight as I can make them and no visible loss via bubble tests.  I added some leak detector a few days ago but I suspect it all got evaporated by the vacuum.