WH back drafts when the furnace is on at the same time

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Margarett

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Sorry for the long post, but I don’t want to leave anything important out.

Over the weekend, I noticed that our hot water heater back drafts when the furnace is on at the same time. They both vent together to the chimney. Please see attached pictures. The WH (40 gallon – 40,000BTU input) is 4 years old. Our medium efficiency furnace (max input: 105,000 BTU, heating capacity: 86,000BTU) is being used for its 3rd winter season. They were both installed by the same plumbing company. Our previous WH lasted for about 16 years and was either a 40 or 50 gallon (I don’t remember) and our old furnace was from the 1960’s. The ductwork all dates from when the new appliances were put in.

The HW heater and furnace are located in a small utility room in the basement (approx 35 sq ft) but the door to it is ALWAYS left completely open and leads into a 200 sq foot room. Next to that room is an even larger room (which is not sealed very well despite my efforts), and the door between them is left open about halfway. We have no other appliance or fixture in the house which vents besides for the dryer which is not used for more than a couple hours a week, and has not been used since last Wednesday.

We have a CO detector in the 200 sq foot room, about 15 feet from the WH and 10 feet from the furnace. It has never gone off. Neither has the one outside our bedrooms on the 2nd floor of the house.

When I entered the utility room on Friday, I happened to look at the top of the WH and noticed some drops of water on it. I then noticed some rust as well. This covers an area about 1/4 of the circumference around the vent opening. The rest is all free of rust. (Maybe this is because the vent hood is centered imperfectly.) I read that this could be a sign of back drafting. My husband and I discovered that when the WH is on by itself, there is no issue and the air goes straight up the vent. But when the furnace is on as well, there is a back draft. My husband tested this with a lighter, and he couldn’t even light it anywhere near the area, the air would put it out. When the furnace is on by itself, everything seems fine. Certainly no air back drafts down the WH.

We see no other visible signs of a problem. The window in the room and the water pipes don’t have condensation. I don’t see rust on the top of the furnace.

Is there anything about the ductwork that looks amiss? Is it possible we don’t have enough air for combustion even though that room opens up into a much larger one?

One possible explanation for our situation, is that there may be a partial blockage in the chimney. Probably dating from when the new furnace was put in, although it could be from a couple years before that when the old one was serviced, my husband was able to see into the chimney from the basement (the plumber had detached the venting) and he said that he saw some debris inside, similar to the floor of a constructions site, but the airspace seemed unobstructed. We also had someone from a chimney company some months ago go on the roof of the house because we had had it repointed, but still occasionally when we have a rainstorm with wind from a particular direction, a small amount leaks into the attic. He doesn’t know for sure what is causing this but he did say that the chimney liner, which is made of (ceramic) tiles, has some joints where the mortar is missing. He was looking only with his eyes so of course he couldn’t see very far down.

If the furnace indeed has no problem and is operating properly, could it really be that there is enough air space in the chimney for the furnace gases but when combined with the WH gases, there is not enough room, and those back draft? My husband is of the opinion that the WH produces a lot less gas than the furnace. On the other hand, if we have had this problem since the furnace was installed 2+ years ago, would we not have even more rust on the WH?

Any help would be much appreciated. We certainly don’t want to go through the expense of having the chimney relined when it is not responsible for this issue. And if we called in the plumber, we would more than likely just be paying him to say that there is no problem and that he did everything correctly.
 

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Fitter30

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The water heater long run looks level. Needs a minimum of 1/4" per ft. The condensation seen wlth the wh flue because the flue gas is cooling off. What is the masonry flue size and length. With a the grout in need of repair or a liner is needed a chimney sweep might be the person to call or a hvac contractor.
 
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WorthFlorida

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From what little I know, I think your assumption is correct, the current setup doesn't have enough capacity to handle two appliances. I found the article that explains a few things. From it, it may be the horizontal run is too long before it gets to the chimney and it may not have enough slope. The water drops you're seeing I would say it's condensation. All that piping has to get warmed up to get a draft going. At first you have hot gases hitting cold metal. When you have the back draft problem, do open a window or door to the outside to provide that the appliances have enough combustable air. Wind is always a big factor where by if the house is drafty, air gets sucked out of the home on the down wind side and will pull air in from anywhere. Another if the clothes dryer is running, it also pushes about 100 Cu Ft/ minute of air out that creates a vacuum inside the home. There is a motor on the furnace exhaust but I'm sure it is a damper that closes to prevent cold air from dropping in from the chimney. Good move that you have CO detectors. There should be one on each floor of the home.

For some light reading; I found this that does some explanation but primary points to the build codes.
 
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Margarett

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Thanks for the responses.
The photos must not be very good, because there definitely is a slope.
As far as all the piping needing to get warmed up to get a draft going: It must have been warm because the HW had been on for quite a while alone (for 2 showers worth) and then very soon afterwards we had it on with the furnace and there was so much backdrafting from the WH. It's the backdrafting that concerns me - not the little bit of water.
 

John Gayewski

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How big is the common vent diameter and length? How big is the chimney (dimensions and length)? How big are the vent connectors length and diameter? Does the boiler have a draft inducer? To me it looks like it does.
 

WorthFlorida

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Model number of the furnace or boiler so it can be looked up if there is a draft inducer. Usually a draft inducer will be with direct vent units and yours is not one. There is a slope but sometime picture look deceiving. Assuming it 8 feet from the WH outlet to the tee is 8 ft. Therefore. 2"rise is all that is needed but at the tee is seems to flatten out. However, I do not think it the cause. The furnace is drawing at lot of air. Please take a picture of the motor on the furnace chimney, especially its label. It could be a draft inducer and not a damper. If it is a draft inducer that is the cause. The link I sent about backdraft, is shows a picture of what happens when a draft inducer turns on.
 
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WorthFlorida

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It's a natural draft unit. The manual explain combustible and ventilation requirements. I think you meet them. When you have the back draft situation, do open a door or window on a clam day to the outside for unrestricted air flow. If you feel the backdraft is too much you'll have to find someone that can figure the chimney minimum sizing. An option is install a direct fired water heater, where the boiler heats the water with its own circulator, however, it won't be cheap. Another is change the water heater to an electric unit. In Florida I've had electric WH since 1989 and they are not energy hogs as many think. You've been living with this for years and no issues. You have O2 detector's and it's not going off. Maybe a direct 4" PVC pipe from the outside to the space may be enough to provide air. As I stated in my first post, I'm no expert in this area. The first thing is try to get someone that knows this.


 

Master Plumber Mark

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You have the right sized pipe going into the tile wall and its large enough
for the whole run

just try cutting down the standpipe coming up from the water heater
take out about 4 inches or more and give the pipe some distance
to climb up to the tee that it goes into.... this would be the first thing I would
try and see what happens ...

you could even get more radical and install a 4 inch
elbow off the connection at the boiler and really send it flying downwards to the heater
and possibly put a 4 inch elbow off the heater ....adn you could even shave that elbow
down a few inches off to gain more rise off the heater

the more it has to climb up and out
from the heater to the chimney,
the better chance their will not be a back draft
 

Jeff H Young

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Margarett, I would expect a 21st century chimney inspection to include a video of it? if they charge money.
Not sure I follow Master Plumber Mark, When he says shorten standpipe Im guessing he means first piece at water heater. Im thinking that might help too providing more grade on horizontal section (But Ive heard you actually should have a minimum 1 foot piece befor going horizontal ) Though Ive put 90s right on top of water heater collar but have reconcidered my ways.
It looks like the vent pipe is 4 inch at w/h might a 3 inch draft at faster velocity? A few thoughts
 

Fitter30

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The problem is the flue size it's to large. Causes flue gas to condense and that moisture is very corrosive. Flue gas when wh first fires wh backdrafts trying to push through the volume of cold air in the flue. Flue gas further condenses and eats the mortar out of the tile joints
 
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