HVAC question inline fan and gas/baseboard furnace and water heater???

[WeetisPotPie]

Can any of you HVAC experts answer this for me? I have a 265cfm inline fan hooked up to my chimney stack, it pulls air from lights about 15 feet away, but it also vents my furnace(gas/baseboards) and my hot water heater. My HVAC guy helped me install it but he is on a sabbatical and I wanted to upgrade the inline fan to a 465cfm. Would this have an adverse effect on my furnace or water heater? They are much closer to the inline/chimney then my grow room, maybe 2-3 feet away.

 

[WeetisPotPie]

Got this answer from ARTofMAKINGfire.

Short answer, absolutely. Fuel combustion would surely be affected, making your equipment less efficient at least. Also you heat exchanger where combustion occurs would have a much shorter life span with the flame burning on the side or top and not center.

This could be remedied by restriction, but it would be a fine balance and take some technical knowledge to accomplish. Also the obvious strain on your fan.

 

[queequeg152]

yea so... if you are pumping air into this chimney, even if its wide open at the top, you are adding static pressure to this column of air.

how much is impossible to say without a manometer, but its probably enough to greatly effect a passive flue like you would have in an older water heater...

most passive flues have openings to allow for air to mix into the hot flue gas as it exits the device... if you look at your water heater you should see something like a little inverted cone shaped piece of galvanized metal.
when your inline fan is running im betting that this fitting is going to leak air to some extent. when your water heater fires up, the flue gas could then, at least partially escape back into your living space.
im not convinced that this air will measurably effect the combustion of your water heater.

you can confirm this by simply holding a candle or smoke pencil or some such thing sensitive to air flow... up to this fitting and observe if any air is being forced out.

might want to shut gas valves servicing your equipment this little experiment

its worth noting that cold air will naturally 'fall' down this pipe and into your house. how to exclude this? do a before and after i guess.

your gas furnaces will have whats called a draft inducer... its a small, like... 80 scfm blower thats designed to overcome a decent amount of static pressure.
this draft inducer in all likelyhood will not notice the additional static pressure introduced by dumping air into an open vessel like a chimney. with that being said however, id not risk it personally. not without putting a manometer on the draft inducer and verifying its within spec.

 

[WeetisPotPie]

Yup exactly. That is how I was testing with the candle at the water heater. My HVAC guy told me to not mess with it as he installed an inline fan for me years ago and set it up so it would run without any negative affects on my water heater or furnace or my family. Instead I installed the larger 465 inline fan in my grow room and will use a speed switch to control it. He said it should be no problem then. He also lectured me on not getting high and trying to fuck with shit and improve it, cause I always end up calling him for help! Always good to have a friend like that.

 

[WeetisPotPie]

Received the speed control switch for the fan. Worked like a charm. Used a doobie this morning instead of the candle trick. HVAC guy will be over to look at it this week and I should be back on track.