Venting Tankless Hot Water Heater

[John Sheehy]

I'm replacing a 40 gallon gas water heater with a tankless version (Aquastar

250sx). As the water heater and furnace are within 5 feet of eachother in my laundry room, I WAS going to just angle the existing vent that is connected to the hot water heater now. It is a 3" galvanized vent. It is connected to the vent which is used by the furnace (which looks like 4"), then the pipe going up into the ceiling and out the roof looks closer to 6". However, now that i get the new heater, it says that it needs stainless venting and can not be vented in common.

Now for my question, and i know it sounds stupid, but just wondered what everyone else is doing. Can i just use that vent or atleast that stack that goes up through the roof and change it all to stainless? Or is there any way to do this without running a new exhaust? If there isn't, what is better to do, a horizonal or vertical exhaust? The heater is being mounted on an outside wall so i believe the horizontal wouldn't be that hard, but are there any issues with my siding being vinyl? The previous owner melted some by placing the grill a little too close, i can't imagine that this thing is going to fair any better.

Finally, it has the option to use in room combustible air or have a seperate vent run for that, what are the advantages/disadvantages or doing it either way. I live in Central New Jersey if it makes a difference.

Thanks in advance for your help.

[SQLit]

Combustion air has to come from some where. If you use inside air then that air has to be replaced. If you use outside air then the home does not have to replace the combustion air. The directions and your local building department are good places to ask these questions.

[m Ransley]

It is a power vent I believe so combining 2 would force air back into your furnace or inside, not good. I vent horizontal with my bosch Aquastar 120 into a 12" stainless hood that pretty much directs geat away, as I have flamable Cedar, But I still removed the cedar around the vent any you could and use sheet metal. Outside air, what about freezing the unit and ruining it, read the manual. You are aware you need a Big gas line to feed it, is it 180000 btu, be sure you have the right size.

[BobK207]

It's always best to use outside (that is, unconditoned air) for combustion air.

Why heat or cool air just to send it up the flue after using it to combust your water heater gas?

The flues (individual & common) have to be sized correctly to handle the exhaust gases genrated by your WH & furnace.

The 6" is probably big enough to handle both, assuming the furnace at

4" was sized correctly (& it looks like it was)

The 3" might be a little small (depending on run length) to handle your tankless........consider the BTU input to the furnace & compare it to the BTU input to the furnace & the 40 gallon unit. Fues are sized based on the combustion capacity of the units that they serve.

I have a furnace & a 75 gallon WH & they both go up through the roof in a common 5" or 6"

cheers Bob

[Bubba]

Hmmmm. I guess Ive been doing it wrong all these years then? I also guess that GAMA venting tables are all wrong too? Might I suggest you follow this link:

is on page 4 of the .pdf but is numbered page 1. Look for "Fan + Nat" A little reading is in order Bubba

[m Ransley]

Fresh air is best , but if incomming air is to close to unit freezing it is a possibiity, even the exuast with a closeable hood can not be to close in -20f. Ive seen mine drop very low near the coil in cold weather apx to 45f. so I routed mine 7 ft away.

[John Sheehy]

[Don Young]

[Bubba]

[Steve Kraus]

Perhaps but it can also be that there is a right way to do it as Bubba says and typically that will require removing what's there and replacing with stainless and doing it by the book. Maybe the mfg figures it's safer to say no no no than have too many hacks or DIY's just hook it on where the old one was connected and call it a day.

[John Sheehy]

Yeah, that's what i meant. Could a installer make/modify the venting so that it could be used in common and that it is safe?

[m Ransley]

Isnt it a power vent on your Bosch [ made by Takagi ], exuast is forced out under pressure? So what keeps it from blowing furnace gas back inside if you Dual use a vent , water heater and furnace on same line, it sounds really wrong.

[Steve Kraus]

Check Bubba's reference. It can be done safely.

[Bubba]

You gotta read man. READ. It isnt wrong. First of all, the flue gasses are being pulled out of the home by the natural draft/suction of air going up and out the vent. Blow some smoke at the vent or diverter and watch witch way it goes. It goes up and out. NOT up and back down. If it does, you have a venting problem. "Swear man, Swear to god" Bubba

[Bubba]

[BobK207]

John-

Find yourself a knowledgable installer.

Just about "anything" can be done, undone, repaired, replaced or retrofitted.

I read Bubba's link, if you want to educate yourself on exhaust vent sizing, read it.

Done wrong, the installation might give you problems;

Done right it will be fine. It's all about getting the "plumbing" right.

cheers Bob

[Wayne Whitney]

Bubba's link was for venting Category I furnaces and solely for the purpose of rebutting m Ransley's comment that you can never common vent a forced draft Category I appliance with a natural draft Category I appliance (apparently not true). Tankless water heaters like the Bosch 250SX are Category III appliances, and they must be separately vented in a vapor tight stainless steel flue.

To the OP: do you live in a climate where it does not freeze? If so, consider mounting the water heater outside (cheaper and less hassle to vent). If not, then vent it horizontally with the shortest vent possible (the vent can be expensive). Do use outside air for combustion, it is safer and slightly more efficient. Also be aware that the Bosch 250SX has a high natural gas demand, usually best served by a dedicated supply line from the gas meter: 3/4" if it is under 35' or so, 1" if it is farther.

Cheers, Wayne

[m Ransley]

Wayne , Venting the exuast as close as you can poses a freeze threat to the coil in cold climates. I did 7 ft and coil still gets cold apx 45 with a closeable stainless hood.

[Bubba]

Thus, the reason I also explained that you need to not only follow the proper venting guidelines, you also need to follow the manufacturers directions. This isnt for kids, folks, but the OP did seem slightly "bent" on reusing that common vent no matter what. In the end, my equipment is running safely, properly vented and my home is nice and warm. If the OP chooses to do differently, I cant change a thing. Bubba

[waldo2384]

If it's not too late, return that Bosch heater while you still can. The Bosch heaters are very high maintenance. Maintenance costs will eat up all your gas savings and more. How do I know? I own one myself. It's nice when it works, but I do not recommend them at all.