Backdrafting Hot Water Heater

[90_Shelby]

I live an older house with an upgraded high efficiency furnace and new mid efficiency hot water heater (3 years old). I've noticed a number of times when I've entered our utility room that the hot water heater is backdrafting and sending the exhaust into the utility room instead of up the chimney. Slightly concerning. The small exhaust pipe from the hot water heater enters the original large exhaust that was previously for the furnace. Without the heat and air movement from the furnace there seems to be insufficient draft to pull the exhaust out of the atmospherically vented water heater.

I added a CO detector in the room and it has not gone off yet but it's easy to tell when you enter the room that it's not venting. When this happens I open the back door of my house which almost immediately helps with the differential and sends the exhaust up the pipe.

With that being said, what is the best and easiest way to fix this? New power vented hot water heater? Chimney liner? Power vent retrofit? Fresh air vent into utility room?

Any recommendations?

[jeffh]

To meet code, when the furnace replacers installed the new furnace and detached the old vent from the stack vent, they should have installed a stainless liner. I assume the replacement was never inspected cause this has rookie move written all over it.

I would start by ensuring a liner is installed. Now would be a good time to do it, as the roof should be dry and not slippery.
Secondly, I would commend just going with a high efficient hot water tank if your current tank was near the end of it’s life cycle, but it’s not.

[90_Shelby]

Thanks!

It's a 4" vent from the hot water heater into the old 6" furnace ducting which vents straight out the roof. Should the liner be 4" B pipe, single wall or that fancy flex pipe?

[RealJimmyJames]

Sue the hack installers

[jeffh]

4” stainless flex pipe/fancy pipe. Don’t worry, its usually a simple job to install it, especially with 2 people.

[90_Shelby]

I had a 4" liner installed last weekend and with the temperature drop today it back-drafted again. Maybe not enough makeup air? Maybe I need a direct vent setup?

I'm leaning towards not enough make up air since it goes up the pipe once I get some fresh air in the room. Thoughts?

[Cash Money Hoes]

Did the installers remove the combustion bucket then they installed the new furnace? It is code compliant to run just your hot water tank off of ambient air if it is under 40k or 50k BTU. If this is the case you may want to look at having fresh air reinstalled. Could also get a damper installed on that fresh air intake, best of both worlds in that case.

[90_Shelby]

It's a 40k BTU hot water tank and I don't believe that there was a combustion bucket present prior to the new furnace going in, but I could be wrong, the house was built in 1962. I don't see anywhere that it would have been deleted, there are no extra fresh air intakes that I can see. The only fresh air intake is a 6" insulated duct that runs to the cold air return of the furnace. With that being said, I just cleaned a bunch of crap off of the mesh screen outside which was very likely restricting a lot of air flow. Does it make sense that this would be the only fresh air intake into the utility room even though it ran directly to the return side of the ducting? I don't see how this would supply enough air for combustion with the old furnace?

[jeffh]

I thought if the tank was under 100k btu, that you shouldn’t need to bring in combustion air. Regardless, you should have adequate air for the tank to run properly, but just have yourself a strange situation.

The outside air line you are describing that ties into your return duct, is fresh air. This lets your home get proper air changes. It is not designed to help with combustion for fuel burning appliances.

Being a somewhat older & leaky home, it is acceptable that there is enough air infiltration for the tank to run properly, unless post-construction steps were take to seal up the home(anything more than replacement windows and doors is where my concern would start). So, you might need to bring in a new combustion air line as suggested. You may need to consult a pro, if you are to go this route, as we could be wrong. I don’t want you wasting money. Maybe you could try cracking open a window near the tank, as a experimental fix.

[Maxt]

I don't have my code book handy, and you haven't mentioned the Horizontal and vertical of the chimney, but 4" sounds too large to me for a 40k Btu hot water heater.

[90_Shelby]

The vent pipe goes out of the water heater for 12” (12” vertically), almost 45* for 24” (10” vertically), horizontal 90* bend, 44” length (5” vertically), horizontal 90* bend, 36” length (1” rise), vertical 90* bend straight up for 15’-20’ out the existing chimney.

Thanks all for the help.

[Maxt]

I come up with 44,000 btu/hr for 3" in your application and 4" being good for 89,000 btu/hr, from the tables in Fundamentals of Gas Utilization.

[jeffh]

Problem is, I’m pretty sure the city outlawed 3” diameter b/c-vent several years ago.

[Maxt]

Yeah that is correct, 4" inch is minimum now, even if it doesn't work. Ruling out any plugged fresh air sources and other weirdness, you might be able to help induce the draft by keeping the area warmer.

[90_Shelby]

Update:

After the liner was installed I only caught the water heater back-draft once shortly after it was installed. Since I cleared out the fresh air intake for the return on the furnace, I haven't had it back-draft yet that I've noticed. Maybe it's fixed.