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The propane gas water heater vents the combustion exhaust vi a flute. The portable ac vents the hot air. If i only run the portable ac unit occasionally do I risk carbon monoxide poisoning from the negative pressure in the room created by the portable ac unit? I am going to die of a self inflicted wound if I don't get ac in this garage so please offer a work around for this. Should I just get a CO alarm and test the draft hood operation that way?

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    I assume that the propane fired water heater has a flue on top which passes through the attic and vents out the roof. Is that right? Is there an opening for combustion air to come in from the outside into the garage? Or is it assumed that the garage is sufficiently leaky say around the large door to supply combustion air? As far as the portable a/c unit, It exhausts hot air out a vent, but doesn't it also have a port for a hose to take in air to cool the condenser coil? You would not want to use air from the conditioned space to cool the condenser because that would be dumping conditioned air – Jim Stewart May 22 '17 at 10:46
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    I think you should build a closet around the hot water heater with a vent into the closet for combustion air (from outside the structure) to be drawn in and a separate vent to allow air heated from contact with the flue and the heater to pass out of the closet outside the structure. This would reduce the heat input to the garage from the water heater. – Jim Stewart May 22 '17 at 10:55
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    Any chance of installing a window (or through the wall) style of A\C? Ductless mini split units are slowly dropping in price, you might be able to find one to fit in your budget. They'd also likely offer better performance over a portable unit. – Tester101 May 22 '17 at 11:31
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    Is the water heater on or near to an exterior wall? When it is time to replace the water heater, you should consider a propane fired tankless heater on the outside of the garage wall. No waste heat is dumped into the garage, no combustion air, no flue at all. The price on tankless water heaters is dropping. – Jim Stewart May 22 '17 at 11:50
  • @Tester101 do these minisplits function the same as the window units? – rhill45 May 24 '17 at 23:29
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You definitely want a CO alarm anywhere there are people.

It is best to keep them away from appliances that can trigger false alarms.

Yes, you will get negative pressure while running the A/C, but probably not much, yet it could pull exhaust from the water heater into the room.

Portable AC units offer lots of ways to run out their vents. I'd suggest the best workaround is to place the AC unit across the room from the water heater, with yourself and the CO alarm in the middle.

Above all, think safety first.

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