# R-410a pressure, heat mode vs cooling mode



## chrisv2 (Jun 2, 2012)

I tried to research this but my researching mojo must be off, I really could not find a solid answer.

I realize that every mfgr/system is different, but speaking in general terms:

When you have an R-410a heat pump, I understand that in A/C mode the indoor coil is the evaporator and that the smaller copper line is liquid/high pressure and the larger line is vapor.

So, when it is in heat mode, that is reversed -- but are the pressures also reversed? In other words, can that larger copper line when acting as a liquid line for heat mode actually see over 300PSI, like the liquid line in A/C mode can on a hot summer day? Or since it's a larger diameter pipe and it's cold in general, are the pressures simply lower in heat mode? 

Or do I have this totally misunderstood? I guess if anyone has measured a heat pump system pressures in both summer (90 deg F or more) and in winter (20 deg F or less) - I'd be curious as to what the differences are for the copper lines.


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## BayouRunner (Feb 5, 2016)

You move your blue hose to the port off of the reversing valve to check the charge for heat. Some units have the valve relocated at another spot. Then charge according the chart inside cover in heat mode. Everything is reversed, your suction line will be hot


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## user_12345a (Nov 23, 2014)

In heating mode the entire lineset and indoor coil become the high side.

The low side is between the heating metering device and the compressor inlet in heat mode. (the coil is the low side)

There may be a separate port to check low side in heating mode.

The uninsulated line carries subcooled liquid back to the outdoor unit and the insulated pipe carries hot vapour to the indoor coil.


So hot you'll burn your hand if you touch it in heat mode.

It's not a liquid line because it carries vapour in both modes. 

For heat it's equivalent to the pipe going from the outlet of compressor to the outdoor coil in a straight cool unit.


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## chrisv2 (Jun 2, 2012)

Thanks - that helps me understand how it works.

I'm still curious though - if the high side pressure is >300PSI during a hot summer day on A/C mode, will the system also see high pressure like that during a cold winter day on heat mode? Or are the pressures lower when in heating mode?

And if the pressure is still that high, will the larger copper line see that high pressure, above 300PSI?


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## chrisv2 (Jun 2, 2012)

I think I found what I was looking for:

http://www.prospex.us/DOCS/HVAC/OEHHM.pdf

So, if the outdoor ambient temp is above 65F then there's a real possibility of the system seeing extremely high pressure, and hitting the high pressure lockout switch.

This may close the loop for me on my other thread about wiring the thermostat. I think I really need to consider getting t-stat that has lock-out for the compressor if in heat mode and either below 10F or above 65F (or thereabouts), to help protect the compressor from either constant defrost or dangerously high pressure. We'd want only AUX if outside those temp thresholds.


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## user_12345a (Nov 23, 2014)

The indoor coil is smaller than the outdoor coil.

So it will run some high pressures, the saturation point having to get up to the point where the coil can transfer as much heat as the heatpump outdoor unit is supplying.

The colder it is, the less heat there is in the refrigerant to put into the supply air. So given that the indoor coil is a fixed size, the pressure and saturation temp drop.

Running a heatpump in hot summer weather can cause damage just as running a/c in cold weather without appropriate controls can as well.


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## chrisv2 (Jun 2, 2012)

Thanks...and yeah I am finding that there is no such thing as a thermostat that will perform compressor lock-out based on HIGH outside ambient temp. I did find a mild weather kit for heat pumps, which simply turns off the outside fan when pressures get too high.

Seems odd to me. I would think we could have a thermostat that could do this:

*Heating mode:*

outside temp >65F: Run AUX only - lock out compressor (avoid high pressure)
outside temp >10F and <65F: Run compressor and AUX as needed
outside temp <10F: Run AUX only - lock out compressor (avoid defrost cycling)

*Cooling mode:*

outside temp <60F: lock out compressor, run air handler fan only

I looked at Honewell VisionPro, and even that doesn't seem to provide this kind of flexibility. I wonder why...it seems like something that would help protect the system from damage.


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## user_12345a (Nov 23, 2014)

Anything like that would have an outdoor temp sensor.

Re lock out of hp heating in mild weather, when it's too warm for heating mode, you don't need heat anyhow.

If it's a special case like someone with health problems (and or a senior) who requires a high indoor temp, simply get a stat with emergency heat mode and set it to that in warm weather.


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## beenthere (Oct 11, 2008)

chrisv2 said:


> Thanks...and yeah I am finding that there is no such thing as a thermostat that will perform compressor lock-out based on HIGH outside ambient temp. I did find a mild weather kit for heat pumps, which simply turns off the outside fan when pressures get too high.
> 
> Seems odd to me. I would think we could have a thermostat that could do this:
> 
> ...


65 is the base temp for heating. Load calcs figure you don't need heat at 65°F outdoor temp. So thermostats manufacturers see no reason for such a lock out.

Carrier/Bryant have had the most trouble with high pressure lock outs in heat mode above 60.


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## user_12345a (Nov 23, 2014)

If the house needs heat even starting a 65, it has no insulation and is leaking like a sieve. 

A decent house won't need heat above 55 or so. Unless it's long and narrow, those have high heat loss per sq ft. (bungalows also need more heat per sq ft, but not to same ex


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## beenthere (Oct 11, 2008)

Actually, it may simply mean they have a .3ACH or higher. Which even .4 is more then acceptable for a family of 3 in a 1,000 sq ft home. Might also mean they have a large amount of shading, so they get little solar heat during the shortened daylight hours. Could even mean they like it 74 in their house, instead of 68.


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## supers05 (May 23, 2015)

Some of my customers like their rooms to be +80.... That's what they want so we have no choice but to provide. It's water source, so no aux heat, and no receiver. We just have to carefully adjust the charge to match. 

For the OP, there more flex room in the system, but an outdoor ambient of 65*F and I'm in a tee shirt, lol. There's several methods to prevent issues at higher ambients, but the most popular for residential is to use a modulating system. (mini-splits or expensive multi stage forced air units.) They either come from the factory with what's needed to run at the higher ambients or it's fairly easy to add a control for most. 

OP: All of the honeywell thermostats that are compatible with Heat Pumps and an outdoor sensor will have the lockout in the menu. That's nearly the whole pro8000 series and above.

If you wanna use your existing thermostat, it's dead simple to add an outdoor lockout stat. 

Cheers!


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## user_12345a (Nov 23, 2014)

> Some of my customers like their rooms to be +80.... That's what they want so we have no choice but to provide. It's water source, so no aux heat, and no receiver. We just have to carefully adjust the charge to match.


I can't stand walking into a house or building that's even 75 in the winter.

I'm betting those who want it at 80 aren't paying the bills.


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## beenthere (Oct 11, 2008)

user_12345a said:


> I'm betting those who want it at 80 aren't paying the bills.


That is the case sometimes.

But like most HVAC guys, I've been to a good number of homes that the person keeping it at 78 or 80, is paying the bill. And doesn't care that its expensive, they just like being warm in their underwear.


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## supers05 (May 23, 2015)

user_12345a said:


> I can't stand walking into a house or building that's even 75 in the winter.
> 
> I'm betting those who want it at 80 aren't paying the bills.


Senior's homes usually. They indirectly pay the bills. Many people with newborns often like it hot. And some ethnic oriented groups seem to like it too. A few of the above don't pay the bills, most of them do.

The end result is the same, the equipment needs to perform as desired. 

Cheers!


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## chrisv2 (Jun 2, 2012)

Thanks for the dialog, guys. Mom is elderly and is always "cold". She is sharp though, so if I tell here "Only use emergency heat if outside is over 65 deg" -- she will. I think for now that will be the game plan; if it becomes a problem I'll in-line an external t-stat to keep the compressor from running if over 65 deg.


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