VAV electric reheat not working

[MechEngNCPE]

Hey guys, just got a call that the reheat coils for a job we did are not working (when thermostat set higher, heaters not kicking in). Now these are five 8" titus single duct boxes with 2kw heaters. The heating cfm's range from 90-120cfm. I think that they are heating the air too high and tripping a high limit cutout. I did the calcs and the 90 cfm minimum box would heat the air to 122 deg, when I want it around 95. I am thinking that they can adjust the minimum damper position for heating mode. Is this correct? I will bump the minimum up to around 200 cfm.

This of course after we check that the thermostats are hooked up and everything electrically is in order.

Am I right?

 

[Drazen]

What is your outdoor DB? I would be surprised that electric heater would not have modulating, and 90 cfm does not look too low for 2kW heater.

Moreover, your assumuption would be probable if heater is controlled by some intelligent electronic controller, otherwise it should have either flow or temperature switch or both, but would trip at first.

 

[AnotherEllis]

It could be anything, even a T-stat wired wrong. The contractor needs to positively identify what is happening before you can offer a solution.

It's tricky when we get a call that something we did isn't working, and it's really easy to over think the problem and make assumptions. Whatever you do, eliminate speculation.

http://www.ellisconsultingengineers.com

 

[Waramanga]

If you set the minimum higher you will lose control during the cooling cycle? It sounds like you are using an on off controller for the reheat, is that right? I agree with Ellis that getting positive ID on the roblem is crucial. It is a dangerous area for engineers being pinged for installed mistakes. These things can go on for years and really leave a bad taste in the clients mouth so swift and thorough action is best IMHO....

 

[ChrisConley]

Ran into an odd situation (similar, not exact) where the control's contractor had installed a limit to the number of electric coils that could be on simultaneously (I'm sure there was a good reason, he just never shared what that good reason was). The limit was that only 4 coils were allowed to be on at a time, if a fifth unit called, it would have to wait until a unit released call for heat. This worked much of the time, but caused some of the most painful troubleshooting I've ever done to figure out why coils weren't responding to call for heat.

 

[MechEngNCPE]

Thanks guys, yea the installer (Trane) didn't set any setpoints correctly, flows were way down, it was ridiculous. All boxes working as they should now, just had to hold some hands.