Unit heater vent pipe condensation

Any idea on how to stop the condensation build-up and leakage in 4 hanging NG vent pipes? I had an addition put on my repair shop this summer, and all 4 unit heater vents are dripping water. The shop has radiant floor heat as the primary heating system, and the building is so tight that I rarely, if ever, use the unit heaters. Obviously I have called the HVAC contractor and they installed the drip-leg tees that you see in the pictures. They are now full of water and spilling out of the seems. I poked a hole in one and about 2 qts of water came out.

It was confirmed 100% that it is condensation and not a roof leak. The vents all go up through an unheated attic through the roof. Before the addition, the one existing unit heater pictured below never leaked, but it also ran a lot too. With the building being so tight, they very rarely run (good thing!)

Anyone dealt with this before or have any ideas?


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Adjust the heat anticipator or cycle rate so the units run longer and then are off longer. Changing the single wall out to double wall wouldn't hurt either. I would also insulate the B-vent where it passes through the attic.

I see the units are new. If they are over-sized you may not be able to easily solve the problem. You may try running only 2 of the 4 units to increase the cycle time as well.

B&H said:

... I rarely, if ever, use the unit heaters. ...
Anyone dealt with this before or have any ideas?

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That's your problem, the vents are not getting/staying hot enough and that allows for condensation. Like Mike says, make them run longer, and add insulation. They should run long enough so that you minimize the condensation which can damage the heat exchanger.

One of the best thing you can do is to replace the single wall vent connector pipe with B-vent. Protect the temperature of the exhaust gases and prevent them from falling below the dew point. I'd also check the vent caps. Is this driven rain water? I'd put a tee with drain at the units exit too. That condensate is approximately 3.2 Ph so it is corrosive and will take out those HX if it gets back there. Count on that.

Add a ****** and drain line to the bottom cap?

Charles

Thanks for the responses. I am almost certain that the pipe going through the ceiling is B pipe. How do you tell? I remember the contractor stating that this was what code was.

I really don't want to have to maintain 4 drip lines. There has to be another way?

The water is 100% condensation, not rain water or wind driven. The problem first occurred when the temp dropped late fall, and if you remove the vent caps and look down, you can see it. It's 8°F now, so no rain LOL!

There are 4 unit heaters (all Reznors). One is in the old section (pictured, installed in about 2009), one new one is in the main part of the new section and 2 new smaller ones are in the "back storage area", heating a 10'x80' storage area. There is no floor heat in the back storage area. There are 2 pass-through doors that we keep open, so most of the time the we don't actively run the unit heaters in the storage area.

I think that the main heaters are 80-100K BTUs each, and the storage area ones are around 30K BTUs each.

I like the suggestion to insulate the pipes where they pass through the cold attic. That just might do it. Is there insulation made specifically for B-pipe?

The storage area heaters may not be so easy -- the roof is a shed roof so there is no attic. Maybe we're keeping the area too cold or maybe I should insulate the pipe up to the ceiling? It would seem counterproductive to waste NG to keep condensation from forming, but I agree if the water gets back to the unit, bad things will happen.

Just to be clear, I'm not sure that we've had the need to run any of the unit heaters except for a couple of times each maybe 2 days so far this winter. I'm just outside of Syracuse, NY, and it's been a very cold start to winter so far. Altering any on/off settings of the unit heaters is moot. Even with opening four 10' bay doors all day, the radiant floor heat and the well-insulated nature of the building have turned the unit heaters in expensive water fountains!

I think I'll either recommend to the HVAC co. to insulate the pipes or just do it myself.

A shot in the dark.

A flue fan attached to the bottom of that tee. Maybe a time delay shut off ?

easiest fix would be to just turn 2 of them off

To insulate the B-vent, which is what the vertical portion is you could only use a hi temp fiberglass insulation. The type used on big steam systems. You would have to go to a commercial insulation supplier or maybe FW Webb could help you with this. I want to say emphatically that you should not have to do this.

How tall is the system from the B-vent T to the cap?

Anthony, all 4 unit heaters ARE off 99% of the time. The condensation builds up and drips when they are off. If they ran more, I'm sure there wouldn't be a problem.

Jackfre, are you saying that "I" shouldn't have to do this (aka, it should be the contractor's responsibility to find a solution), or are you saying that it is unnecessary and something else should be done to resolve the issue? I will get some approx. measurements tomorrow when we open back up. Thank You!

B&H said:

Anthony, all 4 unit heaters ARE off 99% of the time. The condensation builds up and drips when they are off. If they ran more, I'm sure there wouldn't be a problem.

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sorry b&h .. i was way vague there, beers .. i meant turn two of them completely off & use the remaining two to recover the heat from the doors opening .. that would make force the two you kept on to run longer and hotter solving your condensation issues

.. at very least it would confirm that the problem is they don't get worked hard enough

It sounds like you are experiencing condensation of room air moisture on the inside of the very cold flue pipes. Even with the gas shut off you would get this moisture correct? The issue is that the flue is always drawing and passing your conditioned air through that ice cold flue where the moisture condenses out and runs down.

You can keep the flue pipe warm which will prevent condensation. That would be by running them and wasting fuel. Heavy insulation will help as well but not a complete solution.

You can prevent air from flowing up the cold flue with a damper of some sort, or just plain plug. Be careful not to forget!

Turn off the gas to a unit and plug off the flue for a while with a cap or a bunch of rags and see if the flue stays dry.

I'm with Highbeam here. I'm sure there's some condensation when they are running, but only for a short time. Roughly under 140 degree's in a vent (flu) pipe is when you have condensation. Those are great heaters, but they aren't so efficient as to get that low of an exhaust temp by any stretch.

You're having a problem with the natural draft going up though the chimney, mother natures physics wins! lol

I think you might have to change the venting to eliminate that issue. Insulating it will help, but might not help enough. If you were to go with a horizontal vent (if you can) that would eliminate the draft going up and out. Check in the installation manual, you might be able to put a gooseneck (2 90's) on the end of the vent pipe. Might not be able to also, depends on what model it is, if the manual doesn't say you can though, don't do it.

Last option would be a vent damper installed in there. It'd be wired so it opens (and proves it's open by an end switch) before it lets the heater run. I wouldn't imagine it'd be a terribly expensive option either.

If you don't find anything else wrong ........

I'm not an hvac guy but ran into this same problem with large (520,000 and 350,000 btu) water heaters for my business. The building is sealed up so well that there's no make-up air for the proper updraft in the vent.

For air to go out, air has to come in someplace. I have to open an overhead door about 4 inches to get it working properly.

Just something to consider.

I have to agree with djjsr. I think you have negative pressure issue. Where you have cold outside air sucking down the warn pipe causing the condensation. If you had a positive pressure in the space. Warm air would draft up the flue pipe. You might get a little condensation from pipes in none condition spaces. But don't think it would be that amount of water.
It would be interesting to put a thermometer probe in that drain hole on a cold day and monitor the temp during the day.
Don't know what's going on in the shop as far as exhaust fans or makeup air. But it might be worth checking.
Your shop with radiant floor heat makes it nice for everyone working on the floor. But most shops on a cold day the unit heaters cycle quit often keeping the flue pipes rather warm. Ron

Since the warm air in the room is gradually going up the stack hitting the cold stack in the attic it is condensing and filling up. If they ran more this would not be as big a problem. Insulate the portion that runs thru the attic by wrapping non faced fiberglass batts around the 'B'-vent witch does not get flue temps high enough to burn. Have seen this many times in my career.

Here are some more pictures. Maybe someone will see something that will help.

Back storage area, 2 small unit heaters:




Main area unit heater:



Below, back of shop showing all 4 vents. The 2 vents closest are the storage area unit heaters, the one to the left is the for the unit heater in the first picture that I posted the other day, and the one to the right is the main area unit heater.

Interestingly, I poked holes in the 3 other vent pipe bottoms like I did above, and the main area one was dry and the 2 storage area ones had at least a quart of water in each. The main area unit heater has the shortest run of exhaust pipe, as seen in the pictures.

I agree that evaporating snow off of the heated floor and washing the floors is creating humidity which is drafting up the vent pipes on 3 of the units, condensing and dripping back down. We didn't use any of the heaters today, and probably won't until next week some time, looking at the predicted temperatures.

Adjust wall connection with main pipe valves and check all connected valves.

As a follow up to this dilemma, the contractor installed one motorized damper on the heater vent pipe that had the most condensation. It was normally closed and wired to automatically open when the furnace turned on. It didn't help. As it turns out, one vent pipe didn't have a condensation issue, but 3 did. So the contractor ended up adding ******* to the bottom of the "T" on the 3 leakers, and run condensation lines to the same drain line as a condensing boiler that I have. One of the 3 needed a pump, but all is good now.