Direct Fired Make-up AHU Blower shutoff 1

[overdesigned]

Hi All,

I will preface this by saying that I am a building envelope engineer. So while I deal with mechanical equipment, it is not my specialty. I am working on a project where the flame on a direct fired make-up AHU went out, but the unit continued to blow cold air into the building. The building is in Montana and this happened when the temperature dropped to -46 F. It is a Mercury make up AHU that connects to a Honeywell burner controller. I am trying to figure out if the make up air handler should automatically turn off the blower when the flame goes out since this likely would have prevented the pipes to freeze inside the building.

If anyone has any insight into where I may be able to find this information, that would be much appreciated. I have included a picture of the product label for reference.

 

[LittleInch]

Try calling these guys and get a manual.

https://spraytech.com/makeup-air-unit/#

Address matches the name plate.
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Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Its a process safety requirement. Blower keeps running to purge out flammable gas that may have accumulated in the firebox from the time the burner flame went out.

Keep it that way. If you have a double block and bleed arrangement on gas supply to each burner, you could recommend auto stopping the blower say 5minutes after the flame is detected to have gone out, AND you have positive confirmation at the Honeywell BMS of the close status of both block valves and open status of the intermediate bleed on each burner gas supply. Am assuming the piping takeoff to the pilot gas burner is downstream of this double block and bleed.

Any change to this current safety measure will not be possible if you have only a single block and bleed on each burner gas supply.

 

[LittleInch]

This should all be in the manual.

There will be a shutdown sequence with timings to purge gases out of the burner although it is often part of the pre start sequence.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Compositepro]

I don't think the OP is referring to the combustion air fan, which would not be blowing into the building.

The fan blowing air into the building is kept on until a thermostat detects that the heater has cooled below about 130F. And it also does not turn on the fan until the heater is hot enough to supply hot air. If this thermostat fails the fan stays on off all the time. The consequences of failure is what determines what fail-safes are required. It seems that in this case there is a missing fail-safe to keep the building from freezing. I just replaced one of these thermo-switches. The part cost less than $10.

This failure would not have caused any significant problem, If the air was only drawn from inside the building