etc 2 stage ranco t-stat control 100% mua

[anthonyac1]

So im looking to install this 2 stage ranco stat in the supply air stream. This unit is a 2 stage split unit. During low loads but the building still requires cooling the unit runs and freezes up. This is not the correct unit for this application however i am trying to control it by space temp and supply air temp. I am having trouble grasping the set up concept with the 2 stage cooling feature with this ranco stat.
on a standard stat when a cooling demand is needed it brings on stage one. If the cooling setpoint is still not achieved then stage 2 comes on. But looking at the drawing provided its not making sense to me. Here is a link. On page 2 it shows the 2 stage setup.
http://controltrends.org/wp-content/.../Ranco-ETC.pdf

so these are just rough numbers warmer than 70 degrees supply air cooling needs to comes on. Stage one will come on. Above 55 degrees supply air stage 2 will come on. Once the supply air drops to about 45 degrees compressor 2 kicks back out. Once i see how to stagger my numbers i can then play with the actual numbers.
The part that is not making sense to me is they show stage 2 on the higher side of the temp scale with stage 2 on the lower end. I would picture it being backwards. looking for your input.

[pecmsg]

Explain Low Loads and freezing up?

[crazzycajun]

They show it as stage two being the higher set point,but if you read the destructions it states stage one as being the higher set point in all examples but one. My guess is it’s a typo and stage one is first or the higher temp. Sorry I have the single stage on my smoker so I’ve never used two stage,but it won’t let you interlace your settings so you will know in a hurry if I’m right

[pecmsg]

In cool mode stage 2 is higher.

In heat stage 2 is lower

[anthonyac1]

In cool mode stage 2 is higher.

In heat stage 2 is lower

In a way 2 stage being higher makes sense being that it will drop out first as the temp approaches set point. But if I want cooling to come on at 70 degrees. Any temp above 70 degrees will bring on both stages of cooling. But stage 2 will kick out first once it reaches the stage 2 set point. For example 5 degrees differential. Stage 2 will drop out at 65 degrees. Stage 1 will continue to run until it reaches its cut out temp. Is this correct? This way will make sense.

[crazzycajun]

So hook it up and see what happens just like three phase it’s 50/50

[pecmsg]

In a way 2 stage being higher makes sense being that it will drop out first as the temp approaches set point. But if I want cooling to come on at 70 degrees. Any temp above 70 degrees will bring on both stages of cooling. But stage 2 will kick out first once it reaches the stage 2 set point. For example 5 degrees differential. Stage 2 will drop out at 65 degrees. Stage 1 will continue to run until it reaches its cut out temp. Is this correct? This way will make sense.

1st things 1st

What and why is it freezing up?

[anthonyac1]

1st things 1st

What and why is it freezing up?

This is basically the wrong unit for the application. It’s a straight cool unit with no hot gas bypass. It’s a 100% outside air unit. The unit runs fine for 1-2 weeks. Then on a cool night it freezes up then eventually locks out on low pressure. Once you reset the lock out it will run fine for another week or two. I arrived at 6am the other morning to check on the unit. Found it was locked out on low pressure. Low pressure switch opens at 50 psig. R410a unit. That is the factory pressure switch. At 6am the outside air temp was 66 degrees f. The suction pressure hangs right above freezing. Low ambient on condenser also it’s a low load on the evap coil. On warmer days like in the mid 70’s / 80’s she runs happy. With 2 compressors in tandem the suction pressure drops just below freezing. But if I run only 1 compressor with the 66 degree ambient my suction saturation is about 36 degrees.

With 1 compressor during low loads it cools good. This way if I can control the supply air temp dropping out 2nd stage once the supply air gets too cold.

[anthonyac1]

So hook it up and see what happens just like three phase it’s 50/50

My local distributor does not stock the 2 stage Ranco. I had them transfer one in for me. Once I have it in my hands I can test it out then see how it works. It will arrive Friday. I just was trying to get some info on it. I never used the 2 stage model. I was not comprehending the instructions to much.

[pecmsg]

Freeze Stats
Low Ambient Kits

[anthonyac1]

Freeze Stats
Low Ambient Kits

The freeze stat will work. But for the low ambient kit I am assuming your referring to something cycling the condenser fan. I’ve tried disengaging 1 fan manually and monitoring head pressure. But being that the load is low on the evap coil it looks like the txv is closing down trying to maintain superheat. My head pressure starts to climb. But my suction pressure does not really come up proportionately. I’d rather go with freeze stats or supply air temp stat.

[beenthere]

Might want to use the ETC to control staging by outdoor temp. Above 65 1 stage can operate, above 85 second stage can operate. You can use the current thermostat to control whether or not any cooling is needed, But have the ETC control staging.

[pecmsg]

Trying to run with 70°F entering air is going to be a problem.

Can you get a Pre heat Coil in there?

RANCO A12-701 Consent Cut In Control. Feed the cap tube thru the coil. It will shut off the compressor when the coil gets to about 20 -25°f and restart at 41°F. you'll still be moving air!

[anthonyac1]

Trying to run with 70°F entering air is going to be a problem.

Can you get a Pre heat Coil in there?

RANCO A12-701 Consent Cut In Control. Feed the cap tube thru the coil. It will shut off the compressor when the coil gets to about 20 -25°f and restart at 41°F. you'll still be moving air!

A preheat coil would be a real pain to get in. But I won’t say it’s impossible. With a saw zall and a few tools almost anything is possible.
Originally my plan was to Run the cooling signal from my space thermostat in series through the etc control. Then out to the condenser. During normal conditions the 2 stages will run and be happy. But on the nights the outdoor temp dips down it causes an issue. I can run the constant cut in stat to break stage 2 compressor only. It’s a trial n error.

[anthonyac1]

Might want to use the ETC to control staging by outdoor temp. Above 65 1 stage can operate, above 85 second stage can operate. You can use the current thermostat to control whether or not any cooling is needed, But have the ETC control staging.

That was my exact plan except I was going to drop the sensor from the etc in the supply air stream instead of the return/outside section.

[pecmsg]

I would break the 1st stage and let the coil defrost then come back on 10 mins is about all it will be off!

Remember you'll still be drawing in 60° air so some cooling

[Astrobright]

"Above 55 degrees supply air stage 2 will come on. Once the supply air drops to about 45 degrees compressor 2 kicks back out."

Supply air

[gonefsihing]

I do a lot of make up air work. Research facility to old age home to condos.

The way to control that is with outdoor ambient temp. Figure out your temp rise or drop for heating or cooling. If you have a modulating burner then you van use discharge air. On/off stages it's best to use ambient. Same for cooling. 70f requires no heating or cooling. That's a deadband temp. Most makeup do nothing between 65-75+/- maybe able to bring it down to6 degrees deadband. If you need to hold 70 dead nuts then you need electric reheat that can rapidly cycle to hold the temp. We do this in FDA approved research facilities where everything is logged(temp/humidity) for study validity.

I these cases I have used a Honeywell t775 with 4 output reallys

[71CHOPS]

I don't understand why you need a 55* DA setpoint with a makeup air unit at 70* OAT. Like gonefishing said, 65-75, your unit shouldn't really be doing anything except blowing in fresh air....of course enthalpy will come into play, but if you're just looking at DB, it is what it is.....

have you measured your CFM on this unit? you may be lacking.....

also, I think you definitely need a speed drive on your condenser fan motor....a P66 or equivalent. if you can't control your head pressure, your pressure drop across your TXV can't be controlled, and it will lose control.....then you lose control of the suction. you've got to maintain head pressure.....

measure airflow, install a condenser fan control (speed, not on/off), set first stage for 70-75 start.

[ammoniadog]

I agree with those who say to forget about indoor air temperature.

This is a make up air unit. It is not designed to heat or cool the space. It is designed to heat or cool the outside air, to try to keep it blowing out of the vents around 70°.

Other heating/cooling units in the building should then take over to do rest.

Every MUA with cooling that I've ever seen has been controlled by outdoor air temperature. Stage 1 kicks in around 75° or 80°, with stage 2 kicking in around 90° OUTDOOR AIR TEMP. You can adjust those temps as needed, but I wouldn't adjust them any cooler than that.

If you try to control it off of indoor air temp then the compressors might have short run times and cycle on and off all of the time. This is not only bad for them, but it will allow for periods of dry cooled air blowing into the space, followed by hot humid air. Over and over again. People generally won't be too happy with that situation.