Mr Slim not cooling well?

[thammel]

Garage specs: 32x28 with 12' ceiling. Have open stairway to attic - 14x32 with 7' ceiling. Have R19 in all walls and R38 in ceiling. Have two 10'x8' R11 garage doors and 5 dual pane windows (3 in garage and 2 in attic). The garage ceiling is also insulated so the two areas are thermally independent with the exception of the open stairway.

Just put in two high quality Mitsubishi Mr Slims. 18k btu in garage and 9k btu in attic. Garage stays at temp. Attic is not holding it.

I've bought the PAC-us444cn-1 add-on that enables use of a separate wired t-stat. With this or with the remote, the same thing happens on hot days here in MD (95 outside). The attic rises to about 85. I have styrofoam baffles and ridge vents for attic ventilation about the insulation.

Couple of possibilities:
1)unit is undersized
2)unit not functioning well (but air out of air handler feels cold - I need to check actual temp)
3)open stairway causes a problem with any warmer air from lower level (garage) rising and cooler air from attic falling to lower level. I will test this theory with a plastic sheet temporary block
4) no drywall yet....maybe this will help a lot?? (better sealing of all cracks)

Any comments on unit size or my thoughts above. This is really bugging me in that it's not doing the job.

The 9k unit is the top of the line FH series. THe 18k unit is one step down but they are both inverter units with high SEER ratings.

I greatly appreciate any of your thoughts!
Tom

 

[My Old Tools]

cold air falls, hot air rises.

 

[Ohmthis]

Couple of questions, did you have a manual j done (it's a heat load/heat gain calculation)? How did you know to use those size units?
1) possibly but with a load calc you won't know.
2) possibly, get a reading on the discharge air and intake air.
3) yes an open stair way will let the two zones meld. Hot air is coming up to add to the load of the upstairs and cold air is dropping to help downstairs.
4) that will help the air from slipping up through the floor/roof/walls. Heat (not hot air) goes to cold. So with out that seal, the heat can much easily get to the cold area. Simple plastic can help. Make sure to take it down before drywall if you are using Kraft faced insulation.

 

[My Old Tools]

Couple of questions, did you have a manual j done (it's a heat load/heat gain calculation)? How did you know to use those size units?
1) possibly but with a load calc you won't know.
2) possibly, get a reading on the discharge air and intake air.
3) yes an open stair way will let the two zones meld. Hot air is coming up to add to the load of the upstairs and cold air is dropping to help downstairs.
4) that will help the air from slipping up through the floor/roof/walls. Heat (not hot air) goes to cold. So with out that seal, the heat can much easily get to the cold area. Simple plastic can help. Make sure to take it down before drywall if you are using Kraft faced insulation.

Yes, heat flows to cold, but in this case the hot air is rising because the denser cold air is pushing it up.

 

[G-ManBart]

The open stairway is the basic problem, but I also think the 18K unit is a bit small for 900 sq ft with 12ft ceilings. My shop is 950 sq ft with 13' 4" ceilings and I was going to get a two ton unit, but noticed the slightly larger 30K BTU Mr. Slim is only a couple of hundred dollars more, so I may go with that.

 

[Jackfre]

I do think that the attic is undersized. It is heavily influenced by the first floor. Even as you cool the first floor the heat is laying against the floor of the attic and creating a radiant heat system. The 9 may be able to keep up if it is run early in the day. Walking out to pull down temps when the entire mass of the building is up is beyond the 9's capacity. Is the dehu function working? Your wall thermostat also poses a potential problem. Rather than sensing the room temp at the evaporator you are now sensing it, what 5' off the floor? I would disconnect the wall stat and see how the system does. I bet it improves. I did a prototype test for a manuf this past winter adding a wall stat to a unit that works splendidly without on and found the system worked. It did not work as well. Do you have a kink in your line set? Get a refrigeration leak detector and check carefully for leaks. I say carefully because you can check a flare fitting on one side and not see the leak that is on the opposite side. I had this happen on my daughters system. My error. What is the delta T across the evap? It should be in the 20* range.

 

[thammel]

Thanks guys for all the comments! I did a manual J calc and these units should be sized ok. I'll be checking temps of the outlet cold air tomorrow. And I will isolate the 2 rooms to see how much of a difference that makes.

The interesting thing is that the 18K BTu unit in the lower garage space does not have a problem at all keeping up.

Yes, we did a nitrogen purge and a micron gauge evacuation. I'll keep you all posted.

Thanks!
Tom

 

[LS6 Tommy]

The attic area unit sounds undersized to me. Even if it's sized for load properly, those miniplits don't move much air. It may be as simple as getting a fan to move the air around so it all gets conditioned.

Tommy

 

[tractordog]

Measure the temperature coming out of the head unit. Difference between outiside temp will give you a good idea if the unit is working

 

[thammel]

I'm getting 52-53F out of both of the units. It's about 85-90 outside. I put up plastic sheet to isolate the 2 areas and will see what that does.

Thanks!
Tom

 

[Spdstr280Z]

I'm a complete novice, just have been paying attention to the A/C threads and doing research for a few months now. Just based on that, even though you are well insulated, I'm wondering if both units are on the small side. Will be interesting to see how your experiment goes, with a 30 degree delta, they are certainly cooling. Good luck !

Jason

 

[My Old Tools]

As long as the stairway is open, both units are cooloing the lower.

 

[thammel]

My plastic sealing off the attic seems to be working well that the attic is cooling down. The next challenge is to come up with a good workable permanent solution. I started another thread on this already.

Yes, in retrospect, I probably should have bought slightly larger units but my thinking was that if I went too large I wouldn't get good dehumidification. I'm already wondering if I should pull the attic unit, sell on Craigslist and buy slightly larger. Nah....don't over-react yet.....

For the cooler months, I have a reznor 60kbtu heater installed that works great.

Tom

 

[LS6 Tommy]

If the units are inverter driven, oversizing isn't really much if an issue.

Tommy

 

[M-technik-3]

Professionally installed? Were they evacuated? We had a like unit installed in our living space above the garage, it was not so was not working up to par. Called company that installed and they came out and checked unit, turns out when the installed it the tech did not install it correctly. They fixed it free of charge or should I say they resolved the previous techiican's errors at no cost.

 

[tractordog]

If the units are inverter driven, oversizing isn't really much if an issue.

Tommy

Good point.

 

[Heel2toe]

Professionally installed? Were they evacuated? We had a like unit installed in our living space above the garage, it was not so was not working up to par. Called company that installed and they came out and checked unit, turns out when the installed it the tech did not install it correctly. They fixed it free of charge or should I say they resolved the previous techiican's errors at no cost.

What is defined as not working properly? I have a Hyperheat 18K BTU unit forget the exact model and either its working fine and undersized which I suspect is the case or there is something up with it.

This past winter I had it cranked and I could only get the temp like 20 degrees warmer than outside. And now in the summer Im getting at best a 10 degree change with the thing cranked. How can I test to ensure its working to it's fullest?

 

[M-technik-3]

I was not cooling properly, meaning it was cooling then it stopped being effective in our 9 days over 90 her in the North East. The unit had been there less than a month. I'm not an HVAC expert just sat there trying to absorb info they were willing to share.

 

[thammel]

I'm pretty sure mine are ok. With the attic and garage separated by means of plastic sheeting, both areas are ok. The problem I initially had was the open stairwell.

Tom

 

[miketyler]

I have similar issue with my garage. Mine is 1200sqft floor with 900sqft loft. I have two Mitsubishi Mr Slims, one 18k cassette for the loft and a 24k for the floor. The cassette is centrally located but I still get cold air that spills down the stairs. The 18k unit if left on will nearly cool the entire garage.

My question for you was how close is the wall unit to the stairs? I would have tried to place it as far away from the stairs as possible to avoid conditioned air loss.

 

[Heel2toe]

In my case I know my unit is undersized so Im doing what I can to make it work to the best of its ability. Im just not positive its functioning properly. Does anyone know what temps I should be seeing out of the unit with the heat cranked as well as the AC cranked assuming its not out of its 100% efficiency range?

I only have one unit but the garage has an upstairs as well. Would blocking off the stairs be beneficial or am I better off leaving it open when it comes to cooling? My thought is that maybe leaving it open gives the warmer air a place to go.

The second floor in my garage is just storage and my compressor is up there as well but I couldn't care less if its warm or cold; Im just trying to condition the downstairs space.

So I guess aside from determining if my unit is working properly, I'd like to do what I can to make it work best.

Is my thought that in the summer keeping the stairwell open will be beneficial to cool downstairs?

I plan to build a trap door for the winter time however as I know that is not helping my case in the winter.

 

[thammel]

MikeTyler - My wall unit in the attic is at the end away from the stairs. That still doesn't make much difference. I'm planning a door at the top of the steps. Have to build a wall and configure things so this can be done. My plastic sheeting now is proving that both units are ok for each room.

Tom