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Exhaust for Tankless Water Heater

I own a home built in 1955. I recently had a plumber install a new water heater (Navien NPE A2 natural gas tankless). The old water heater was a natural gas tank water heater. It had a Transite asbestos flue for the exhaust gas that passed through the roof. I read that the inner lining of this type of flue can over time flake off and sometimes obstruct the exhaust vent. The old water heater needed to be replaced, so I decided to switch to a condensing tankless gas water heater.

Unfortunately, the water heater is not near an exterior wall, so the plumber routed the vent pipes (2" PVC) up one level and then through the attic above the garage. The amount of condensation at the exhaust terminal is more than I expected. The horizontal run above the level of the garage initially had a slight downward slope toward the vent terminal. After reading that horizontal runs should have a 2% upward slope, I asked the plumber to adjust the PVC above the garage. He made a slight adjustment and there was some improvement. The slope is now roughly horizontal (still not a 2% upward slope). The plumber told me that this amount of condensation is to be expected. The exhaust terminal is above a brick pathway, so ice forms on the pathway when the temperature is below freezing, with some water/ice collecting near the base of the house (see picture).

It’s probably not ideal to have the exhaust terminal above a brick pathway, although the pathway does not get much use in the winter.

The length and number of turns is in the upper range of the manufacturer's specifications, but still within limits. Do you think that further adjusting the PVC would be helpful? A more upward slope would allow more of the condensation to flow back toward the water heater and out the condensate drain inside the house. Is there any problem with sloping upward more than 2%? Are there are other solutions to decrease the amount of dripping?

One option would be to send the exhaust through the roof. This would allow a shorter run but I have read this could create other problems, such as the possibility of a roof leak or potential damage to the roof due to the acidic nature of the exhaust.

I would appreciate any input. Thanks!

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  • Make/model of water heater? Dec 28, 2022 at 16:39
  • My vague (never having worked on one) understanding is that in general a condensing gas appliance (whether furnace or tanked water heater or tankless water heater) is supposed to somehow extract so much heat from the exhaust before sending it out that (a) PVC can be used instead of metal, (b) much of the water vapor in the exhaust condenses and is drained out rather than going up with the exhaust. So the question then is why so much water in the exhaust? Which makes me wonder if it is too hot, but then cools down enough when it hits the great outdoors that it icicles. Dec 28, 2022 at 16:43

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TL;DR summary: the plumber is correct that the condensation is expected, but you too are correct in wanting it to be managed better. Try adding insulation and maybe pipe heat cable.

Condensing appliances produce a lot of condensate water - I'd estimate two gallons per hour of operation for your water heater (based on typical 200,000 Btu/h sizing for a tankless water heater and the figure of 1 gallon per 100,000 Btu/h mentioned in an article). That big icicle hanging high overhead is both unsightly and a definite hazard; if it were to fall and hit somebody on that walk they could be seriously injured.

One of the jobs of the exhaust piping is to manage where that condensate water goes. It can be a tricky job: there's a decent volume of air (exhaust gas) being blown toward the outdoors, and yet we also want whatever droplets condense on the walls of the pipe to flow back toward the appliance, opposite to the direction of all that wind! The only tool we have at our disposal to ensure that counter-flow of liquid works is gravity. In other words, the pipe must slope steeply enough that gravity's pull on those water droplets is stronger than the pull of the exhaust gas flow.

So, more slope is definitely a good thing. Unfortunately for you, the holes through the wall already exist and it's not easy to move the exhaust outlet higher. The piping is also likely positioned as low as it can get where the vertical run elbows to cross the attic, so it may not be possible to drop that end to gain slope.

Condensation happens all along the length of the exhaust tube, but more condensate occurs where the tube is colder. Can we do anything to warm the exhaust tube crossing the attic? One easy thing is to insulate the tube. The exhaust gas has some heat; if we insulate the outside of the tube then the exhaust itself will warm the tube. The warm tube will collect less condensate -- more of that water will remain as vapor and you may notice more visible "cloud" blowing out of the exhaust.

If insulation alone helps but is insufficient you could try adding pipe heat cable. Normally these are used to prevent water supply pipes from freezing but I think this could also help reduce condensation in the attic portion of the exhaust pipe. There are some available that have a built-in thermostat so they consume zero power during warm weather, and the "self-regulating" or "PTC" type cable reduces its power consumption as it warms up. If the pipe heat cable is applied so that the terminal elbow is kept warm then whatever condensate does get to the end of the line will always drip to the ground, never forming an icicle hanging from the exhaust. (Consider enlarging the hole in the wall so that the tail of the heat cable can go outside and make a few turns around the pipe out there.)

Manufacturer instructions may direct not to insulate non-metallic pipe. Technically you're probably obliged to respect that. Their reasoning for such a restriction may be to avoid over-heating the pipe especially during hot weather. You might be able to switch to a metallic pipe, or could choose to disregard the direction.. Heat cable without insulation would be less efficient but also would not run contrary to such instructions.

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  • Thank you for sharing this information! There is room in the attic to increase the slope of the PVC exhaust vent. I will try that first. Adding insulation around the PVC exhaust vent makes sense. But there is a note in the manual for the water heater that states "Do not cover any non-metallic vent material with thermal insulation." I am not sure why insulation would cause a problem. Is there a certain type of insulation that would be acceptable?
    – Eric
    Dec 30, 2022 at 18:18
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The installation seems fine as long as he was able to achieve at minimum a horizontal or upward slope. There is no problem with sloping upward more than 2%; 100% optimal is a fully vertical pipe but that's not realistic in most cases.

The purpose of the slope is to mitigate noxious gas backdraft problems. That's why there is a limitation on elbows so they don't create airflow problems.

There is really nothing that will stop that ice from forming because the exhaust is simply humid, period.

You can consider placing a drip pan beneath it and swap it out with another drip pan when the first one becomes full. Defrost and drain the full one so it's ready to be swapped in again. Maybe add a "Wet Floor" sign too so that you don't accidentally kick and break it when using that path?

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If you don't like the drip pan idea then consider buying a bag of calcium chloride and sprinkle some on the walkways under the vent every few days to mitigate the ice buildup.

The biggest issue with sending the pipe through the roof is that you cannot easily inspect it and it can freeze shut with zero warning. Your picture actually concerns me; I would put a heat gun on an extension pole and melt that before it obstructs the vent completely. Be careful not to overheat the PVC though. If a ladder is viable then I would choose that route.

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I'm willing to wager that your problem is you still don't have the 2% slope. The reason a 2% slope is required is that with a lesser slope the water that condenses in that section will be pushed slowly by the flowing air towards the vent end and freeze.

A slope larger than 2% is not a problem. If it was, then vertical sections would not be allowed!

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