Carrier RTU

[HVACTECHND]

Have a store call that said there RTU unit is bringing on Mechanical Cooling. Im in North Dakota where it is 8* this morning. Its about 60 miles away so Im just trying to get any information I can as to why it is doing this. Not real familiar with these RTUs so any information would be helpful. Model is 48TJE008501. I know Y2 brings on mechanical cooling. But would there be any thermostats built into the unit that would lockout mechanical cooling any information would be helpful. I havent personally looked at the unit yet but would a bad blower or broken belt cause this? Is there a time delay between Y1 and Y2

[MsiHvac]

Is the customer trying to economize and mechanical cooling is coming on or trying to heat and mech cooling coming on??

[OldSchoolMech]

Bro Calm down breath
Its a store the employess are probaly grade 9 education man
My call t-stat is in cooling or something is powering the cooling terminals.
I bet its not even cooling and is not heating
Take pics Please

[HVACTECHND]

Customer is getting to hot so it is trying to economize, but he says mechanical cooling is coming on

[timebuilder]

Customer is getting to hot so it is trying to economize, but he says mechanical cooling is coming on

Okay then. If they are still hot while running mechanical cooling, then it could be a belt, motor, or plugged filters.

If they called YOU, and you are not their usual company, then they may have NO company that does maintenance. In that case, plugged filters are very likely.

Check all the items I mentioned, and make sure the economizer can actually move to stroke open, using the Honeywell info to test it.

Filters are either (2) 16x25x2, or (4) 16x20x2.

From memory....

[timebuilder]

Sounds like a classic economizer problem. There is an enthalpy sensor exposed to outside air inside the fresh air hood, and it is a DC polarized device that connects to two terminals on the economizer "logic module." + on the sensor to + on the module.

On the older Carrier RTU's (after they stopped using the Guillotine design and went to dampers) the module is a Honeywell W7459A. You can look it up on the internet, and find out how to test it. Usually, this is a sensor failure. From memory, I believe you can place a temporary jumper until you can get back there with a sensor, but do run the tests in the Honeywell sheet.

When you make the repair, whether it turns out to be the module or the sensor, look in the literature and decide which enthalpy setting for the small trim pot on the logic module will work best in your area. A setting at "A" brings on the mechanical cooling later, and near "D" it brings it on sooner. This is based on climate and the needs of the space. Around Philadelphia, I run between "C" and "D" to ensure adequate cooling and dehumidification.

If they are aware of mechanical cooling, they can probably hear the compressor coming on, but I don't know if they are getting cooling in the space. No Y2 delay built into the unit that I have seen. Y2 can be brought on by the logic module if Y1 cooling is insufficient, and that could be due to a damper linkage or motor actuator problem, since if the air can't get past the dampers, it will not get into the space.

A belt or motor issue usually comes to me as a "no cooling" call, and not a mechanical cooling call, but depending on the experience of the customer, that could be the problem (since that would also fail to bring outside air into the space) but that is only an outside chance. No pun intended.

[Dchappa21]

Somebody disconnected the economizer because its broke and but the bypass plug in it...

Or it's just calling for 2 stages.

[timebuilder]

Somebody disconnected the economizer because its broke and but the bypass plug in it...

Or it's just calling for 2 stages.

That is possible. A lot of guys have no interest in learning how an economizer works, or how to troubleshoot it.

[Dchappa21]

That is possible. A lot of guys have no interest in learning how an economizer works, or how to troubleshoot it.

Yep.. I make a lot of Money fixing economizers. Those damper motors and controllers are expensive.

[timebuilder]

Yep.. I make a lot of Money fixing economizers. Those damper motors and controllers are expensive.

They sure are. But when they work, they save a TON of money on electricity, and compressor wear, too.

[2sac]

8* with mechanical cooling? Coil is frozen unless there's a low ambient kit..

[timebuilder]

8* with mechanical cooling? Coil is frozen unless there's a low ambient kit..

Since we found out that it is still hot in the space, then yes, the coil is frozen.

In an area like that, I'd be using a mixed air sensor to modulate the dampers, too. Often, those terminals (T and T1) have a dummy resistor across them.

[timebuilder]

Update?