Low-ish ambient operation
So my dad asked a question and as I thought about it I'm starting to wonder if I wasted my time adding low ambient controls to my own system.
My dad has a single stage R22 3 ton split. Nothing special, I think it's a Bristol recip.
Sometimes it's warm out and then drops down to say 50-52f while he's in bed. Because of this he precools his house more so he can turn the system off before bed. He asked if this was really necessary and I'm not so sure it really matters.
Since it was running recently the compressor would still be warmish and the evaporator would definitely be cool so does it even need CCH at that point? I'm guessing it would just behave as undercharged due to low high side pressure but would it even matter? The system wouldn't run long enough to ice up so it should be totally harmless, no?
I'm not really sure what to tell him. I'm thinking it's fine for him to just let it run through the night under those conditions.
I've offered to add CCH and a fan controller for years but he never wanted me to waste time on it.
What are your thoughts?
Comments
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Isn't the risk the opposite, that the liquid won't all evaporate in the evaporator and flood back to the compressor because there is too much subcooling and not enough load on the evaporator?
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Not that I'm aware of no.
Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment0 -
What is you dad's motivation in turning the unit off at night? Does he not want the noise to disturb his sleep, or does he think he's saving energy?
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He was told to never run it when it's below 60 out.
Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment0 -
if it's 55 and 60* , , ,
just open windows ???
known to beat dead horses0 -
If it's 60 out and raining that would give you over 70%rh at 68f.
No open windows under such conditions
Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment0 -
Is it a TXV or orifice? I personally wouldn’t worry about operation down to 50, below that I might - depending on how much it runs. I thing the biggest thing is the crankcase heater to keep refrigerant from migrating to the compressor in the off cycle. Generally when I think of low ambient controls I’m thinking about a system that requires full capacity regardless of outdoor temperature, like a motor control center, server room or refrigeration application.
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I've wondered if it was a txv for years now but never remember to look when I'm there. There's always too much other stuff going on.
I'm going to assume piston just because it's a 22 system and wasn't anything special at the time. But I don't actually know.
Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment0 -
I would be concerned about flood back to the compressor. Fan cycler would probably be enough for limited low outdoor temperature operation, but a fan control would be better. CCH is cheap insurance as well. jmho
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What would cause the flood back in this scenario?
Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment1 -
can you explain why? @ChrisJ said no above but not why or why not and i don't understand quite enough to think it thought myself, at least not without thinking about it for a while.
My r22 system from about 2000 is txv but the guy that installed it sort of implied that was unusual because i bought the better condenser that was quieter and more efficient.(also he had to change it when he installed the system, probably not because it actually was bad but that he released a slug of oil in the way that he opened the service valves).
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I'll explain my thoughts and id like to hear others in case I've missed something.
When you size a txv or other metering device it's done with the capacity needed and the pressure differential between the high side and low side.
So if you size it for certain conditions and then it gets cooler outside this will cause the pressure differential to be lower and therefore the metering device will act undersized.
Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment0 -
the volume the compressor is pumping is also going to vary depending on the head it is pumping in to. if you reduce the discharge pressure by removing more heat in the condenser it is going to pump more volume.
If it is a txv system the txv will open and close until its sensing bulb senses a couple degrees of superheat(whatever it is set for) unless it is starved of refrigerant and can't maintain that superheat wide open(the max opening of the valve can be a factor in some conditions too)like i said, i don't know the answer but it is probably possible to think it through. there is also a huge difference between 50 f and 0f ambient.
it also makes me wonder what happens with the refrigerant control in multi stage compressor systems, is the evaporator just starved and not filled completely when it is running a lower speed? or does the lower airflow mean the evaporator stays full?0 -
Anyone that's had gauges on a system knows the high side drops as the outdoor temp drops.
What the compressor may or may not do I don't think counts. Less pressure differential across the expansion valve means less flow. Even a txv can only open so much and I believe they're typically sized for a minimum temp of 55-60f for normal air conditioning.
Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment0 -
it is the pressure differential that is important. if the suction pressure is also lower the reduction in pressure will be less. where you will have an issue is if the evaporator doesn't add enough heat to keep the suction temp above the saturation temp of the suction pressure/the condenser removes more heat than the evaporator adds
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I think that actually does happen.
But my thought is if the system doesn't run for very long that it won't matter.
I'm sure if it's 50 out and you're having a party with 25 people in a small house and have the oven going you'll be in for a real bad time. But if there's only a small load and it runs for 5-10 minutes I'm assuming it'll be fine.
That's kinda what I'm running up the flag pole I suppose.
Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment0 -
With colder ambient the head pressure drops. When the head drops with a txv the suction will also drop. When the evap temp drops below about 57psi the evap temp will drop below freezing. If it is severe enough the evap will start to frost /ice reducing air flow. The TXV only controls superheat. If the evap coil has reduced air flow flood back can occur.
The question is will the system ever get to this operating point is a guess. This is why a heat pump operating on cooling when it is defrosting will usually have a suction accumulator in the cond unit
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