Water heater draft testing 

This is probably too late but typically the repeated attempts to kick on the burner is from the safety system logic when it detects unsafe conditions. Usually in gas appliances this is caused by lack of combustion flow either on the intake or more like the exhaust vent. I'm confused about not knowing where the vent is - it should be connected to the fan exhaust with rigid piping similar to old draft heaters and have a relatively straight and short run to a protrusion outside of the house. Draft venting would exit vertically through the roof or chimney while powered exhaust vents can exit the side of the house, similar to a dryer vent or high-efficiency furnace. In theory, the fan powered exhaust could remain cleaner with a one-way flap or slats like on a dryer or bathroom fan, to prevent bugs/dirt from the outside, though modern building/fire codes don't permit anything but an open pipe exit with no screens or slats, at least in the case of the intake and exhaust vents for high-efficiency gas furnaces but at least those connect to a sealed chamber in the furnace the prevents direct access for critters from outside the house. If your new heater detects poor exhaust flow, it usually won't run consistently or at all. If they have low-temp exhaust like high-efficiency gas furnaces, you have more options and ease for installing a new vent but it sounds to me like you don't actually have another vent at all for the second new unit?

No one talks about it because it’s not a problem. You are mistaken on the operation of an induced draft 80% furnace. The inducer is not there to push the flue gasses out of the furnace. It is there to overcome the restriction of the heat exchanger. In other words it is not a power vented furnace. There is no problem having a common chimney with a connector for a water heater and a connector for the 80% furnace. They are both negative pressure connectors. If it were a power vented furnace, it would need a sealed positive pressure flue. This is the case with the 90+ furnaces which use a sealed PVC flue.

​@@rodgraff1782 Mid-efficiency (80% to 82%) furnaces have no draft hood. Instead, they use fan-assisted draft with a fan located at the outlet (draft inducer) of the heat exchanger. If you have your water heater flue tied in with a cross tee you will get a positive pressure back thru the water heater side pushing gases out, mine was doing this. The issue was the tee, you have to use an off-set tee to avoid the direct exposure of the water heater flue, the offset will allow the pressure to bypass and go up the roof instead of across the tee back thru the water heater.

@@sunnyd6019 As you stated, the spill problem is due to a improper installation of the combination connector pipe, with both appliances connector pipes directly facing each other. One appliance, the furnace, should enter the bottom of the combination connector, the water heater should enter the side, at an angle facing up. I have an 80% carrier furnace I my garage, that is a 1990 model Carrier. It is common vented with the water heater. The furnace has a small square draft diverter, with a spill switch. I have seen several brands that have this. If there is any kind of positive pressure in the flue, the gasses will roll out of this opening, and trip the manual reset spill safety switch. If you hold a match to the opening, the negative draft pulls the flame in. If it goes the other way, you have a problem with the venting system, or the combustion air. The inducer motor on the furnace is designed to draw the gasses thru the heat exchanger, and deliver them to the flue collar. Right below the collar is the small draft diverter, with the spill switch, so the pressure at the collar is negative. I have seen these rollout switches bypassed, because the flue was not drawing properly. That’s a big no no. The induced draft 80% furnaces, at least the Carriers, have been around for over 30 years now. Mine is 33 years old, and still going.

@@rodgraff1782 100% on the issue, mine was vented wrong at the tee. I went ahead and removed all to the B-Vent at the ceiling. Then re-installed a new off-set tee and vented back to the furnace and water heater. Once this was done no more issues at water heater and perfect negative draft now. I only noticed the issue because I was in the room working and my arm got real hot and I felt the gasses being pushed back out the water heater, it only lasted like 15 seconds but I did not want anything coming back into the house since bedroom is close to mechanical. Sadly a lot of homeowners probably have a bad vent setup that is so simple to fix, again I would never have paid attention if not being in the right place at right time to catch it. Cheers and thanks for the info and reply.

@@sunnyd6019 Glad you got it fixed. All flues have what they call “priming time”. Which means they won’t draw right until the flue developers buoyancy from the hot gasses. This can last for up to one minute, and spillage is allowed during that time. A water heater pilot will keep the flue slightly warm and offset the priming time. On the other end, long flues can have a stack affect, which causes air to be drawn up, even if they are not hot. This is usually due to multi story chimneys. This can cause problems with appliances that do not have draft diverters. Also make sure you don’t have a negative pressure in the mechanical room. Cloths driers, kitchen fans, fire places, and bath fans can put a negative pressure in the house, if an insufficient supply of combustion air is not available. I’ve had to seal the doors on the furnace room and run large fresh air ducts into the furnace room to offset problems.

Power vented water heaters seem to require a pvc discharge rather than a metal flue. Not sure why this is as the exhaust is exactly just as HOT unlike from a high efficiency furnace.

Was about to ask same question

The air intake is probably a newer addition.

Flue connection for water heater should be sized based on btu under 40 k 3 inch over 4inch

@@sewerrat7612 Usually the draft diverter collar will be the proper size for the connector required. Why would a manufacturer put a 3” collar on a water heater that needs a 4” connector?

@@rodgraff1782 most of the ones I see work for 3 or 4 inch there essentially stepped just put over and attach with zip screw. Less parts manufacturers have to deal with