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Using RTU as Make-up air
One of my clients just moved into a building and the WH keeps going out
I light pilot , crank her up , look at the flames and they are blowing all over the place
Put my hand near the top of WH and air is blowing out of the flu like crazy
Go open the back door , flames are perfect , no more air coming out of flu
The hood only has Natural Draft
Im thinking of slowing down the Hood Fan speed , and open the fresh air vent on RTU
But ... how much can I open the fresh air without upsetting the AC performance ?
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You have to know the heating and cooling capacity of the RTU to know how much OA it can handle. The other way is to experiment by opening the OA damper enough to solve your problem and monitor the performance.
No man can be both ignorant and free.
Thomas Jefferson
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It doesn't matter how much it affects the operation of the air conditioning. Leaving the space in a negative with that water heater could kill somebody. How big is the exhaust hood? Are we talking about a restaurant? Do they not have a make-up air unit?
If at First You Don't Succeed, Skydiving Is Not for You.
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Its an old ass mom n pop place , natural draft make up hood is like 8ft ...grease type
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Can you add a supply fan to the hood? That's the best solution if workable.
No man can be both ignorant and free.
Thomas Jefferson
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Or just add outside airnfeom somewhere....cut a vent in the closet door, leave some doors open, etc. But it needs fixed.
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"]You should NEVER Try To Balance or Adjust, a Buildings Airflow System, using a Hot Water Heaters Pilot Light Flame as a Go-NoGo Gauge. 
That Pilot should function in a plus or minus range of Static Pressure Conditions for safety sake. 
LZ....
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The make-up air should be close enough to the range that adding a supply fan should ease the negative pressure without impacting comfort conditions very much if any. The supply airflow should be slightly less than the exhaust. It should then be possible to adjust the RTU OA to provide a slightly positive building without over loading the RTU.
No man can be both ignorant and free.
Thomas Jefferson
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WHOOPS, I See Now OP was Talking about High Fire, as apposed to Pilot. My Bad, But the Same Message Applies.! 
LZ...
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Agree Izenglish it depends on how negative the building is. It sounds like the negative is excessive but a hood supply fan will lower the negative enough to allow the RTU to make the building positive which it should be.
No man can be both ignorant and free.
Thomas Jefferson
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Found a crawlspace type vent 8x16 blocked off , and its actually in the same room as the WH
Pulled the cover off and the air suddenly came blowing in like a fan was on !
Looked at the flames and they are much better , but there is STILL enough air coming through the Flu to blow out the pilot once in while during times its satisfied
Im gonna head there now and try slowing down the Hood fan , and give the RTU about 20 percent OA
After that he's gotta call someone in who knows what the hell he's doing lol
I know of a place that shut down their make up system ... and then opened all the RTU to 100 percent OA
They have to change air filters weekly because they are near a desert in California , and the AC units suction pressure is high like you would imagine ... how crazy is that ...
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Originally Posted by
Snapperhead
Found a crawlspace type vent 8x16 blocked off , and its actually in the same room as the WH
Pulled the cover off and the air suddenly came blowing in like a fan was on !
Looked at the flames and they are much better , but there is STILL enough air coming through the Flu to blow out the pilot once in while during times its satisfied
Im gonna head there now and try slowing down the Hood fan , and give the RTU about 20 percent OA
After that he's gotta call someone in who knows what the hell he's doing lol
I know of a place that shut down their make up system ... and then opened all the RTU to 100 percent OA
They have to change air filters weekly because they are near a desert in California , and the AC units suction pressure is high like you would imagine ... how crazy is that ...
What's a "natural draft make up hood"? I'm glad you fixed that combustion air vent. Also look for a dirty screen on the intake of whatever they're using for make up air. I also agree that opening the economizer a little bit is a good idea as well. Sounds like you're on the right track.
If at First You Don't Succeed, Skydiving Is Not for You.
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A natural draft hood has a makeup air source but no fan on it.
No man can be both ignorant and free.
Thomas Jefferson
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The regular RTU being converted into a makeup air unit? I think that you are limited to the CFM that the Economizer will/would allow under the normal 15-20% fresh air open position.
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Yeah I went there this morning and saw the OA vent is very small , like 8x8 , and it was set at 10 percent on this 5 ton unit
Even if I open it completely it would be pissing in the wind
The other system is a split unit.
I slowed down the hood 2 turns on the motor sheave , had to put 1 inch smaller belt , and its like I didn't even touch it
When you flip on the switch for hood , the filters go " clang" up against the rack
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We had a cobbled up restaurant mechanical room with a whole house exhaust fan in it because on WIF and WIC cond units in the room.
Water heater pilots out every day.
Put in a Cyclone power vented commercial water heater with combustion air piped in from outside, solved the 2 problems of pilots and adequate supply of hot water.
Amazingly the AO Smith tank is still going strong since 2005.
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Yeah I was entertaining that idea yesterday , or even go electric
I think its 75 gallons
He's gonna cry when he finds out the cost of those in power vent ...
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Our engineers use RTU units as MUA units all the time. Most are 100% OA, they oversize the cooling capacity and restrict the RA inlet, with a RAWAL APR control installed, they work just surprisingly well. Probably thirty of forty RTU units of all manufactures, some thirty years old are pulling 100% OA are out where I work and they all are doing fine.
Example, last month, I installed a new base model Daikin 5ton cool only unit, restrictive OA inlet plate installed in place of return air duct, about 2 to 2.5 tons of air aloud based on opening, brazed in a Rawal kit, adjusted charge. Had to revisit the unit and adjust the hot gas a tinch, but even on the record 105° day we had last week, that unit had no issues maintaining a perfect SAT to the building.
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I cant imagine being on the Humid ass East Coast that would fly very well ....
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Originally Posted by
Snapperhead
I cant imagine being on the Humid ass East Coast that would fly very well ....
That is why I rely on the engineers.
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