HVAC pros please advise!

[swimman]

I'm looking at having a multi zone heat pump service two grow rooms, one 18k the other 24k btus. I just got off the phone with someone who installs Daikin units, and I have my reservations.

Here's the pickle. I live in the NE, where it regularly gets below freezing. The Daikin unit recommended for my application, well its compressor is rated for cooling down to 14*, that could potentially be a problem (Here's the unit). The HVAC professional I spoke to on the phone suggested tucking the sensor into the compressor to trick it into running at lower temps. I have my reservations about this for a couple reasons.

I have also checked out the Mitsubishi 42k BTU unit found here. It can, with an air outlet guide, supposedly cool down to -5*. The cost is almost 2x, however and this particular HVAC company only services the Daikins, which I have indeed heard good things about, too.


So I guess my question is - can the Daikin be run below freezing effectively? Or would it be safer going with a Mitsubishi? Are there other options?

to you in the HVAC community. The variables in your line of work are insane, and those of you who know your trade through and through are worth your weight in gold.

 

[queequeg152]

jeeze. if it gets below freezing just bring in fresh air.

co2 costs much less than electricity trust me.

regarding fucking with the sensor. god do not do this.

dont hire that tech either.

among other things, the condenser sensor is there to detect icing conditions and then totalize how long the icing condition has taken place. it will then start a defrost cycle.

without the sensor in the correct place your unit will ice over and not realize it.

do not do this.

 

[swimman]

jeeze. if it gets below freezing just bring in fresh air.

co2 costs much less than electricity trust me.

regarding fucking with the sensor. god do not do this.

dont hire that tech either.

among other things, the condenser sensor is there to detect icing conditions and then totalize how long the icing condition has taken place. it will then start a defrost cycle.

without the sensor in the correct place your unit will ice over and not realize it.

do not do this.

Living in Maine, I know for a fact that people do cool their rooms with outdoor condensers year round, the question is how they do that. Also smell is a consideration and sealed room is really the only way to go imho.

I am concerned about what you described with the sensor, though. The tech assures me it has been done with zero problems that he knows of. I'm waiting until next week to get a quote from a Mitsubishi guy, but this two head 36k btu unit came in at $6,600 for everything so it's not chump change I'm willing to risk.

I believe the Mitsubishi will be more expensive, as that will have to be a 42k btu unit as their 36k doesn't allow for 130% capacity on their air handlers from what I can see. The Mitsubishi's are pretty solid units and can operate at a cooler temp than the Daikin so it may be my only safe route.

 

[brown_thumb]

Living in Maine, I know for a fact that people do cool their rooms with outdoor condensers year round, the question is how they do that. Also smell is a consideration and sealed room is really the only way to go imho.

I am concerned about what you described with the sensor, though. The tech assures me it has been done with zero problems that he knows of. I'm waiting until next week to get a quote from a Mitsubishi guy, but this two head 36k btu unit came in at $6,600 for everything so it's not chump change I'm willing to risk.

I believe the Mitsubishi will be more expensive, as that will have to be a 42k btu unit as their 36k doesn't allow for 130% capacity on their air handlers from what I can see. The Mitsubishi's are pretty solid units and can operate at a cooler temp than the Daikin so it may be my only safe route.

Couldn't you just add a carbon filter to the output air?

 

[queequeg152]

Living in Maine, I know for a fact that people do cool their rooms with outdoor condensers year round, the question is how they do that. Also smell is a consideration and sealed room is really the only way to go imho.

I am concerned about what you described with the sensor, though. The tech assures me it has been done with zero problems that he knows of. I'm waiting until next week to get a quote from a Mitsubishi guy, but this two head 36k btu unit came in at $6,600 for everything so it's not chump change I'm willing to risk.

I believe the Mitsubishi will be more expensive, as that will have to be a 42k btu unit as their 36k doesn't allow for 130% capacity on their air handlers from what I can see. The Mitsubishi's are pretty solid units and can operate at a cooler temp than the Daikin so it may be my only safe route.

oh derp.

i derped so hard. i assumed without thinking you wanted to HEAT with your unit at -15 .... sorry.

your condensing unit will freeze when in heat pump mode, but it wont really ever freeze in cooling mode... unless you have like freezing rain or something.

this is still a bad idea though.

the reason you need those sensors where they are... is to control the EEV equipment and fan speed on the condensing unit.

you usually have 2 sensors on both coils... one on the evap. and condensor coil piping, and one on the evap and condensor coil face to detect indoor and outdoor air temps.

you need the temp sensor on the condensor coil to detect the outdoor air temp to protect the compressor from insanely high head pressure and possibly to protect from liquid slugging back into the compressor.

when you have insane temp differentials from inside and outside you can get gigantic subcooling figures which result in less refrigerant being metered into the evaporator which means your liquid reciever fills and fills and fills.

because its the receiver is sized for normal conditions its likely going to be too small to hold all of the refrigerant at sub 0 temps... meaning you could convievably suck liquid into your compressor and damage it.

the low temp mitsubishi units i think can stop the fan completely, and i believe they have additional means protect from liquid suck back as well... possibly a larger reciever, or possibly a sensor on the return suction, IDK.

you can get around this to some extent by adding these baffles to the unit that basically choke off air flow from the fan... but if the unit is off for a while, i dont see why it would ever start up at sub 0 temps regardless of the baffle being there or not.

 

[swimman]

oh derp.

i derped so hard. i assumed without thinking you wanted to HEAT with your unit at -15 .... sorry.

your condensing unit will freeze when in heat pump mode, but it wont really ever freeze in cooling mode... unless you have like freezing rain or something.

this is still a bad idea though.

the reason you need those sensors where they are... is to control the EEV equipment and fan speed on the condensing unit.

you usually have 2 sensors on both coils... one on the evap. and condensor coil piping, and one on the evap and condensor coil face to detect indoor and outdoor air temps.

you need the temp sensor on the condensor coil to detect the outdoor air temp to protect the compressor from insanely high head pressure and possibly to protect from liquid slugging back into the compressor.

when you have insane temp differentials from inside and outside you can get gigantic subcooling figures which result in less refrigerant being metered into the evaporator which means your liquid reciever fills and fills and fills.

because its the receiver is sized for normal conditions its likely going to be too small to hold all of the refrigerant at sub 0 temps... meaning you could convievably suck liquid into your compressor and damage it.

the low temp mitsubishi units i think can stop the fan completely, and i believe they have additional means protect from liquid suck back as well... possibly a larger reciever, or possibly a sensor on the return suction, IDK.

you can get around this to some extent by adding these baffles to the unit that basically choke off air flow from the fan... but if the unit is off for a while, i dont see why it would ever start up at sub 0 temps regardless of the baffle being there or not.

Yeah the baffle (I was baffled it was $219!) I was told lowers its operating temp down to 5* which I think will be relatively safe, as it doesn't often get that cold for extended periods of time. I'm curious how long it would take for sub 5* temps to shut her down, I will say the guy knew right off the bat I was looking to have a grow room cooled, suggesting he's done a lot of work for growers. Given that I'm in a legal state, rec and med, I think that's a safe bet.

I dunno, when you're talking a $3000 compressor and thousands of dollars in plants, the idea of the unit shutting down or becoming damaged scares the living hell out of me. I think there's light control boxes that have temp sensors which flip the relay switch in the event the temp passes a certain point, but $$$$$$$$...

 

[MrBungle]

why not install a head pressure control? aka condenser fan control

ICM head pressure control

 

[swimman]

why not install a head pressure control? aka condenser fan control

ICM head pressure control

I'll ask my HVAC guy about that, I just read about those. I think with a wind baffle and something like that, I should be okay. It's rare where the temp is sub 5* for prolonged periods of time.

 

[queequeg152]

why not install a head pressure control? aka condenser fan control

ICM head pressure control

these fans can only go so slowly before they stop running entirely.

im guessing mitsubishi has a better fan becasue ive seen them running like 60 rpm... my guess is that the cheaper units cannot be slowed down that far.

im guessing most minisplits may not cooperate with a head pressure control unit.

the fans have RPM sensors that feed back to the control boards.

when the fan stop or slows beyond what the controller is calling for, its going to throw an error and possibly shut down.

i do know that one of the companies... i think mitsubishi or LG has a bulliten for low temp retrofits to the less sophisticated older units.

i did not read much of the bulliten, but i think it had to do with some sort of jumper setting on the control board forcing the fan to run at high speed... then installing a head presure controller inbetween the fan and controller.

no clue if you can do anything similar to the daikin... but again the fans can only slow down so much. some of these units even have static pressure sensors to detect icing or fouling.

 

[Lazyman]

If you're running a sealed room, and you should be with AC, you would use a carbon filter as a scrubber with a fan to recirculate and deodorize air within the room. No ducting needed. That big of an operation, you'll want 5 or 6 12x40 filters with 12 inch fans. Hang em from the ceiling so they're out of your way, but don't make them difficult to change as you'll need to replace the filters every 2 years.

 

[Lazyman]

Oh and Mitsubishi Mr slim mini splits are awesome, worth every penny.