Need some opinions!

[sapper]

Is it good to run outdoor less than 60 degree air across an R22 evap coil with mechanical cooling running? What problems can arise? Can it ice up? For the record I don't like it one bit but the boss made me hook it up this way. Am I going to be able to tell him "I told you so" late this fall?

Its a split system in a classroom that has an outdoor air duct tied into the return with dampers in both. So when you are bringing in the outdoor air the space return duct is closed. The system runs in low ambiant conditions. I wanted it to be free cooling during less than 60 degree days. My boss calls it 2 stages of cooling, the mechanical with the outdoor air. I'm not buying it, in fact I think its going to be problems.

So, what do you guys think?

[gruntly]

We used to be able to run our mechanicals (15 to 140 tons) with economizer cooling, but our systems were extensively capacity controlled with hot-gas bypass AND compressor unloaders operated by low pressure controls. Also, with banks of up to 12 CFMs controlled by head pressure switches as well. The idea was to maintain control over the supply temp and humidity.

Without hot-gas bypass or some form of compressor capacity control how can you maintain a minimum suction pressure to prevent freeze-up in a system of this nature? At the very minimum, you best think about adding a (anti) freezestat to the evap or evap outlet header, if there isn't one already, with a time-off delay to prevent the coil from freezing and compressor short cycling. It can be done with a low pressure control and time-off delay as well, but without a start-up bypass timer, quite often the pressure dips on start-up, creating nuisance lock-outs.

Is your boss a mechanic? If he is, then he should know better. If he isn't you're the guy who needs to step-up and correct the situation. Sometimes people need an explanation concerning the difference between what they want and what they need. All-in-all, the boss can't be argued with, but there is nothing wrong with showing him the light and letting him turn it on.

[sapper]

oh yeah he's a mechanic. He's done some things before that I questioned but nothing as bad as this. I've argued it with him and he thinks this is a way to get 2 stages of cooling. It's only a single stage system in the first place and it worked fine. I was just trying to give them free cooling since they run it in low ambiant conditions. They had 95% of the stuff already to do it, just no way to lock out mechanical. That was the purpose of installing a temp lock out control, so that it would shut down mechanical and open the outside air damper on a call for cooling when the outdoor temp was below 60. He said it won't freeze until the air temp is below 32 and they don't run ac when its that cold. Oh well its his account, his headache.

[gruntly]

You're right though, the usual way, around here anyway, for standard economizer is 55F ambient changeover. Anything mechanical below that requires extra controls. It's obvious you can see he's confusing air inlet with evap temp. Alzheimer's maybe?

On the other hand, some just don't get it.

[sapper]

It sucks to because he is usually a really smart guy that if you get stuck on something, he can help you. I feel like this is HVAC 101 though. I guess the only questions I ever had to ask him was about pumps and boilers. But oh well, enough dragging him through the mud. I appreciate the 2nd opinion though. Gives me more confidence for later on when all hell breaks loose.

[Skeizer]

I think you could be very right to have reservations, Sapper. I certainly would have them. If the system has no low ambient controls on the condenser, then pressures will be low to start with. Complicate that with lower evaporator load temperatures and I can see the reason for concern. Like you say though, he is the boss! Still, when I was the boss I appreciated my techs telling me if there was a flaw in my thinking; as if I could have ever flawed. Curious to see how it develops.

[genduct]

liquid back to the compressor? can we say SLUG alert

[sapper]

I've gotta say your exactly right because even though I didn't mention it before, the condenser has a head pressure control on it to cycle the condenser fan in low ambient. But that will maintain head pressure but the low evap pressure due to no load can cause slugging. I didn't even think about slugging the compressor. Good point. So next summer when I'm changing the compressor or the whole condenser then I can say "told you so"....LOL Either they are going to hang me out to dry or make me a hero....LOL. I wish I would of wrote on the ticket "against my advise, I was instructed to install it this way".

And the funny thing is, when I bring up other points to consider be it right or wrong, he always says "trust me on this".

[Texas-Tech]

If you have an adjustable fan control, and set it up right, you can somewhat limit the freezing up part. Better make sure the compressor has a crank case heater.