Will I have venting issues for 80% hybrid system?

[univmd]

I currently have an 80% furnace sharing a regular exhaust vent with a regular gas water heater. Replacing with a 90% furnace is difficult because of venting issues: orphaned water heater, installing new pvc vents, etc.

But if I change the furnace into a hybrid system by adding a heat pump, will I still have venting issues since the heat pump will take over the furnace most of the time in the winter, will the water heater still vent properly during this time?

With warm regards, James

[BaldLoonie]

During the coldest part of the season, the gas will be on. I wouldn't see a problem. I have not experienced it at home either.

[skippedover]

The basic standard for sizing a flue for an atmospheric gas appliance is that the flue AREA should not exceed the AREA of the vent entering it X 7. So if you're water heater has a 3-inch vent pipe on it, it should work just fine if the flue it will be venting into is 7-inches or less. If you have an 80% gas furnace, you would likely have a 7-inch flue based on the formula to size commonly vented gas appliances. I'm estimating your furnace to have a 6-inch flue (old, standing pilot model) and the 3-inch water heater commonly vented, you would have a 7-inch flue. If you have a fan assisted gas furnace, then you likely have a 5-inch flue. Of course, these are just educated guesses without seeing the project. But if my numbers are accurate, you could use a 90+ furnace and the water heater would be fine, unless your local code mandates otherwise.

As for running the PVC pipes, often there's a way but not always. It is an option to go straight up with the PVC and out the roof but sometimes people don't think you can do that with the equipment. You can.

With a hybrid system, you'll still have the same venting issues with a new furnace. Just because it's hybrid doesn't mean you won't use the gas furnace.

[ampulman]

During the coldest part of the season, the gas will be on. I wouldn't see a problem. I have not experienced it at home either.

What would happen during the Summer, when the (old) furnace is not firing; however, the pilot is lit but flue is not the least bit warm?

The water heater is vented into the same chimney through a 'Y'. (6" pipe into double-wall chimney for furnace; 3" pipe for WH)

The basement is quite cool. Does the cool air work against having a good draft? Just wondering, since it's been this way for the last 19 years.

If the furnace was replaced with a '90+', would the abandoned chimney properly function to vent the WH? How about with a power vent?

AM

[ga-hvac-tech]

The furnace will vent teh same regardless of the dual fuel (hybrid) installation. If it is an 80% furnace, then it will vent similarly to your existing. If you choose a 90%, then you will need to run a PVC vent (and possible fresh air also).

What part of the country are you in? Reason I ask is that in some areas 90% is not really worth it if one has to pay for the PVC flue also. In my market (greater Atlanta area), most folks that do dual-fuel opt for an 80% furnace, because it will not run as much anyhow.

Hope this helps.

[bananaboy]

If you install 90+, then run a 4" liner (good for up to 60 Gal. water heater) just to be on the safe side.

Yes, your state may not require the chimney liner, but I would do it anyway. Better be safe.

[beenthere]

What would happen during the Summer, when the (old) furnace is not firing; however, the pilot is lit but flue is not the least bit warm?



AM

The flue is warm.

Its 70 80, or 90 degrees outside, the chimney won't be cold, so the flue gases won't condense.




To the OP, is your chimney brick and motar, or does it already have a liner.

If B & M, you need a liner no matter what.

[johnl45]

Get a power vent water heater and get rid of the chimney!

[univmd]

Thank you for all your help!

To answer your questions, I live in Ellicott City, Maryland. The 75 gallon regular gas water heater and the 78% gas furnace y-connect to a metal vent that goes up through the middle/interior of the house (3 levels+1 attic) and goes right through the roof and up another 4ft. into the air.

I am currently thinking about getting a hybrid system with a 90% furnace or a hybrid system with an 80% furnace, but was worried about orphaning the water heater when the furnace is not running.

Now it sounds like I can use the current 7 inch vent with just the water heater without a liner:-)

--James