How to properly vent 2 gas water heaters

[Mikepier]

I have a 75 gal water heater with a 4 " vent going into a chimney. If I wanted to add a second water heater, what would be the proper way to vent both into the same pipe? Could I just add a Y and vent both through the same pipe going into the chimney?

Reason I wanted to add a second water heater is that this is a multi-family house and I wanted to split the demand for HW by zoning off the apts with 2 water heaters By the way, my steam boiler vents into the chimney from the opposite side with its own separate vent pipe.

[Ed Pawlowski]

Check local codes., Many require a separate stack for each device. An alternative is a high efficient unit that just uses a pvc vent

[mancave]

to add a second water heater, what would be the proper way to vent both into the same pipe? Could I just add a Y and vent both through the same pipe going into the chimney?

amily house and I wanted to split the demand for HW by zoning off the apts with 2 water heaters

side with its own separate vent pipe.

A local tavern with 3 apartments upstairs just installed one of those "instant" hot water heaters. There is no tank, just a flat box on the wall operated by natural gas. It vents out a pvc pipe, and is supposed to furnish unlimited hot water. I find this hard to believe, but it's installed and the owner said it works great.

The tavern probably dont use much hot water, just enough to wash glasses a few times a day, but 3 apartments on one heater is significant.

I'm not trying to sell you on this, just saying what I saw.

If you get the local code people involved, prepare to be forced to get a permit, and they will likely want a second chimney, and in the end, you'll spend a fortune. Or just install a Y and keep your mouth shut. Gas water heaters dont really exhaust much of anything except getting rid of some unused heat, and any carbon monoxide and other toxins from the flame. But if you want to stay legal without a huge expense, the high efficient unit with pvc venting may be the better alternative, while you can keep those pesky building inspectors and costly permits out of the picture. The less they know about you, the better!

[nestork]

Yes, this is done all the time where you have a gas fired heating boiler and a separate gas fired hot water heater. In general, both will vent up the same chimney.

I had a problem with having both a 640,000 input BTU boiler and a

251,000 input BTU hot water heater connected to the same chimney, but the problem wasn't that they vented up the same chimney, it was that they weren't connected to the chimney in the right order. Apparantly, where I live, if there's a manifold chase feeding the chimney, the smallest BTU appliance has to be connected closest to the chimney, and the larger BTU appliances have to be connected progressively further from the chimney.

In my case, I had two flues coming off my boiler, but the water heater connected to the chase BETWEEN where the two flues connected to the chase, so that was a problem. If you have two water heaters with the same BTU output, I expect you can just connect them with a wye to the chimney.

Typically, any plumbing company that installs the water heater for you will measure your chimney and determine if it's big enough. You can find sizing tables online, like the ones on this web site:

'Chimney flue size rules: Flue diameter and height requirements for Category I Draft Hood and Fan Assisted Appliances'

If it turns out your chimney isn't large enough, there are flue blowers that can be installed that are basically flue fans that blow the flue gas up the chimney rather than rely on natural convection.

[CRNG]

On Sat, 7 Sep 2013 18:22:28 -0700 (PDT), Mikepier wrote in Re How to properly vent 2 gas water heaters:

That will work.

[trader4]

I'm not so sure about that. Appliance vents are sized and governed by the number of BTUs. So, it depends on the BTUs of those water heaters, what else, eg furnace is connected to the chimney, size of the chimney, etc. Most likely, the chimney has the extra capacity required. But I have doubts about the 4" pipe from the water heaters, ie that you can just put a "Y" in and the same 4" size that each had alone is then OK for both. Generally that 4" is just used for one. I've never seen it connected to another water heater like that. I have seen a water heater join a furnace vent before going into the chimney, but in those cases the vent size joined is much larger.

So, I would say:

If both WH's go into the chimney separately, you're OK provided the chimney is sized to handle the max btus. If they get combined into one pipe before going into the chimney, I would check to make sure that pipe is of correct size.

[nestork]

Yes, it couldn't be a 4X4X4 wye, it'd have to be a 4X4X6 inch wye connected to a 6 inch flue connector so the flue connector could handle all of the flue gas going through it when both water heaters are firing.

[bud--]

Has been covered here fairly often.

Yes, just do it. You might wind up with CO2 and CO in the building because the combined flue or chimney are too small, but how could that cause any problem.

[Tony Hwang]

Hi, Looking at our old furnace and Hot water tank set up 100K BTU furnace and 20 some K BTU water tank were vented into on chimney. I don't think those two water heaters will use as much as 100K BTU. But if the chimney was originally sized for one tank, it becomes an issue. Now our chimney is down sized with B vent insert to handle only water tank. Furnace is upgraded to 98% Hi efficiency unit with PVC side vent.

[CRNG]

On Sun, 8 Sep 2013 09:50:40 -0700, "Bob F" wrote in Re Re: How to properly vent 2 gas water heaters:

The cross sectional area of a 6" pipe is more than twice the size of 2

6" pipes, so it will handle it fine.

[bob haller]

If the home has a 90+ furnace with a abandoned furnace chimney then its probably better to install a stainless liner in the unused furnce chimney flue and connect it to the 2nd heater.....

since both chimneys are in use then get a power vent water heater for the 2nd water heater. they can vent thru the wall

[CRNG]

On Sun, 8 Sep 2013 12:45:09 -0700, "Bob F" wrote in Re Re: How to properly vent 2 gas water heaters:

Yes, but now you are grasping at straws. What will you think of next to try to salvage you argument: the difference in smoothness of the inside of the various pipes?

Or you just accept the fact that a 6" diameter pipe offers the same cross sectional air flow resistance as two 4" diameter pipes of the same length; and it didn't occur to you to check that.

[nestork]

I'm certainly no expert on chimney drafting, but for different chimney heights, there are maximum horizontal distances that appliances can be away from the chimney. The taller the chimney, the greater the horizontal distance the appliance can be from the chimney. As long as no appliance exceeds that maximum distance from the chimney based on it's height, the appliance should draft up the chimney OK.

That might be the reason why you often see really tall chimneys rising

10 to 12 feet above the roofs of houses. Perhaps they needed that height because the heating boiler is located on the opposite side of the house's basement from the chimney. I always wondered why any builder would extend the chimney so far above the roof line, and that's one possible explanation.

Often two appliances drafting up the same chimney at the same time will work better than only a single appliance because the increased amount of flue gas warms the chimney up more, and the result is better drafting from natural convection.

[Ed Pawlowski]

The chimney must be a given height above the peak of the roof or anything inside a particular distance away. I forget the exact formula, but there is one to determine minimum height.

[Stormin Mormon]

When I used to help install furnaces. If we would put in a PVC vent furnace, often we'd have to put in a chimney liner for the water heater. Boss would explain to me that there wasn't enough heat to keep the chimney warm. The flue liner would be expanding aluminum tube that went to the top of the chimney. I'm not sure what the formula for that is.

. Christ>

[krw]

YEs, and there is a difference between interior and exterior walls. Some time back, we switched our boiler from oil to gas, they said our chimney was fine because it was on the interior of the house. Had it been on an exterior wall, it would have required a liner.

[jamesgang]

I've seen some of this discussion about removing the furnace before. But what I don't get is why doesn't it matter in the summer? Furnace never runs then.

[Nate Nagel]

The water heater does however. The problem is that if the flue is too large in diameter for only the WH it may not draft properly. And yes, one would think that that would be a problem in summer.

I'm guessing stuff may have been done in the past that wouldn't pass muster today. I suspect that a gas WH would require a separate flue today and wouldn't be able to share the flue with the furnace as was sometimes done in the past. I am not familiar with that specific area of building codes however, this is just speculation.

nate

[trader4]

The issue is condensation. In the winter, the chimney without a furnace, gets cold. The gases from the water heater can condense and the condensate is acidic. Over time, it can eat away at the mortar, causing the chimney to fail. That's also why it matter if the chimney is outside the heated structure or inside.

[TomR]

You may also want to try posting this question on the

website;

in particular, on their forum called "The Main Wall" at

.

You could post your question there anyway, but the fact that you have steam heat also helps since they specialize in steam heat information and discussions.

I posted there about chimney issues and codes in the past and got great responses.