Backdrafting 2 pipe vent system

mars_6

Hello folks, I have a TT 399 boiler that is doing DHW production " TT 119 Smart tank" and snow melt, there is also a TT 250 that just runs on a snow melt call. The system was new construction install commissioned October of 2012. After almost exactly 1 year the blower motor that forces the gas air mix through the HX had a catastrophic failure and had to be replaced. The new blower lasted exactly 1 year and failed again. this system is piped 2 pipe with 20' of separation from the combustion air inlets and the vent terminations, so cross contamination is not a cause. upon further investigation the vent system for both boilers are backdrafting through the vent and heat exchanger and spilling into the Cabinet of the boiler. The plastic impellers on both of the blowers had failed and sheared off indicating they had seen some products of combustion they were not meant to see. The backdrafting issue is more prevalent after the boiler has fired but happens to both boilers. The house is slab on grade so no subfloor fans, no attic fans or whole house fans either. I installed a Panasonic 80 to 110 cfm fan in a hall adjacent to the mechanical room and ducted it into the mech room in hope of counteracting the backdraft. This had no effect. Any and all thoughts would be appreciated

Jamie Hall

How does combustion air get into the house? First thing I would check is how tight the house is.

mars_6

Sealed combustion 2 pipe venting system for the boiler.

mars_6

House is average construction .35 ach

Jamie Hall

OK... but think about it. The only way combustion products can escape from where they belong into the cabinet is if the air pressure in the house/furnace room is less than the pressure in the combustion area. Which it shouldn't be.

Find out why...

mars_6

Its not while the boiler is firing, this happens after the boiler has shut down.

mars_6

I spoke with ME and he recommended a blower door test to see if I do not have stack effect within the structure itself. I had my tech out there today and the difference in outdoor temp today indoor is significant. 30's yesterday 60's outdoor today backdrafting from the boiler was non existent today and would blow out a lighter yesterday. This leads me to believe that a blower door test is in the next step. Matt

icesailor

@Mars:

"" The new blower lasted exactly 1 year and failed again. this system is piped 2 pipe with 20' of separation from the combustion air inlets and the vent terminations, so cross contamination is not a cause. ""

In my experience and opinion, if that 20' separation means that the intake and exhaust are on different sides of the building, or their physical constraints with the 20' locations, you can get differential pressures in the venting systems from the outside wind.

Flowing air has a lower pressure than static non flowing air. The higher the wind speed, the lower the pressure. The wind even "bends" around objects and obstructions.

A sealed combustion venting system is really just another form of manometer. Measuring differential pressures.

IMO, if the intake and exhaust are too close to the ground, you can get "ground effect" wind that can disturb the draft after the blower stops, If you feel air coming out of the exhaust after the burner fan stops, you shouldn't.

Drill a 1/4" hole on the intake and exhaust of the appliance and put a draft gauge like a Bacharach MZR and read the pressures. Do it on a calm day, and do it on a windy day. They shouldn't change much. If the intake is under negative pressure and the exhaust is under positive pressure, it shouldn't be. Cover up the holes with silver tape when you're done.

IMO, if the exhaust is in the wind lee, and the intake is in the wind stream, the higher pressure in the lee will force its way down the exhaust because it wants to equalize through the intake. Maybe all those swirl plates and fan bladed croak. Backfiring.

Jack

I'm with Ice on the idea of having the intake/exhaust in the same pressure zone, or close together. To me that is the definition of "Direct Vent", or at least the best definition. Manuf. may approve the separation, but you are asking the equipment to compensate for a condition that could be avoided. You are missing the sweet spot and when you get out on the margins, you have problems and loose money.

mars_6

So the venting and the combustion air are both on the north side of the house, same pressure zone one would would assume. The combustion air is installed under a deck 10' above the walk out basement deck. The vent terminations are on an adjacent wall that is 12' above ground on a wall that is 20' away. The combustion air for the boilers is 45' the venting systems for the boilers are 20'. House is new construction and built to a .35 ACH. The part that is pissing me off of that TT is coming out with a new version of the TriMax that comes factory equiped with a check valve on the venting system, they know they have a problem with this and yet will not sign off on me putting the venting system in place. I am thinking maby Viessmann may be the boilers i go to due to this problem from now on. BTW this house is a $ 6,500'000 build. I need a soultion to this problem. TT i hope your listining. Matt

icesailor

You guys just don't understand about wind and draft. What you describe isn't the intake and exhaust as "close" together. If as I understand it, they are not.

Is this exhaust 3" or 2" If you are going to have wind issues,, you're better off with 3".

You can't see the wind. But if in the Fall of the year,, and the leaves have fallen, on a windy day, watch the wind blow the leaves in all different directions. There's all kinds of ways that wind can get bent around things.

If you have ever done any sailboat racing,, you can be racing along, thinking your Golden, and look up to windward and see someone rolling right past and over you, sailing in a completely patch of wind. Wind speed and direction. When sailing "Upwind", a sailboat can see wind osculation's of 5 to 15 degrees on a normal day. On a windy day, it can be far worse.

AKA: Chess on the Water.

mars_6

So here is more system information, the house sits in a valley both the Combustion air and the vent termination are on the same pressure plane. I cleared this installation with both my local rep as well as teck support at TT. The boilers in question are TT 399 and TT 285. They are both vented in 4" as per spec. The only discrepancy is that the combustion air vent is 45 developed feet and the vent termination is 20 feet. Again the problem is back drafting of the vent termination after a call for snow melt or DHW production, this causes the leftover products of combustion in the heat exchanger to spill into the vestibule of the boiler and in the process eat the inducer impeller from excessive humidity as well as excessive acidic leftover product's of combustion from the back drafting through the heat exchanger.

mars_6

Ice, thanks for the input. I appreciate it, this one has all of us scratching our heads. I will let all know when we come up with a solution. Matt R

wogpa67

Can the post purge fan timing be set longer? One of the few I have'nt installed or serviced yet.

Mark Eatherton

Matt, what were the results of the negative pressure test with IR camera and smoke sticks? I have seen situations where all the combined gaps around vertical piping can create a stack action from warm air rising up along these vertical chases and create significant negative pressures that could conceivably cause draft issues. To wogpa67, we've already had that conversation, and according to Matt and the local reps, purge time cannot be adjusted in the field... Seems to me they could program one special for him at the factory.

ME

wogpa67

I looked at the manual and saw no adjustment for purge timing. I''m sure they can come out with a laptop and give him more time on post-purge. Or like you said program a control at the factory and send it to him.

icesailor

mars said:

So here is more system information, the house sits in a valley both the Combustion air and the vent termination are on the same pressure plane. I cleared this installation with both my local rep as well as teck support at TT. The boilers in question are TT 399 and TT 285. They are both vented in 4" as per spec. The only discrepancy is that the combustion air vent is 45 developed feet and the vent termination is 20 feet. Again the problem is back drafting of the vent termination after a call for snow melt or DHW production, this causes the leftover products of combustion in the heat exchanger to spill into the vestibule of the boiler and in the process eat the inducer impeller from excessive humidity as well as excessive acidic leftover product's of combustion from the back drafting through the heat exchanger.

They simply don't understand. PERIOD!!!!

Look at the problem like a manometer or a water level. As a "U". If you fill up a piece of clear vinyl tubing, 50' long with water, and fill the tubing 12" from the end, hold the two ends together and the water level is equal in the tube. Both 12" from the end. Raise one end up 6". The level stays the same but the water is now 6" from the top in one tube and 18" in the other. Because gravity and atmospheric pressure cause the levels to balance out. An Analog Manometer is a "U". With no pressure on the instrument, the levels are equal. Put a slight pressure on one side and the level rises on one side and drops on the other. Take the pressure away and it then balances.

Take the water level outside and mark a level line around the house. If the hose is long enough, you can hold one side at the same point and run completely around the house. But if there is a wind influence, the opposite sides can be influenced by the different atmospheric pressures and you get a questionable measure.

In the case of the exhaust, with one side being longer than the other, one side will try to balance out the other once the fan has stopped. If one side is longer than the other, then the appliance isn't centered at the bottom of the "U", but unbalanced off to the side.

Someone once described "High's" and "Low's" weather patterns. A High Pressure area is like someone taking fine dry powder snow and pushing it into a ball and making a big mountain out of it. The higher the mountain, the higher the pressure at the bottom. When you take the barrier away, the dry snow runs down the hill like an avalanche. The "Low" is like a hole in the ground. The snow wants to fall in and fill the hole. If the hole fills with snow and the mountain goes away. the pressure is equal. The draft in the pipe is like that. When the fan is running, the higher outside pressure wants to rush in and fill the pipe. When the fan stops, it backs up to equalize the pipe.
IMO, if you switched the intake and exhaust, the problem would probably stop. I think the problem is with the exhaust flowing backwards in the longer pipe trying to balance the pressure.

Then, there's pipe size. Is 4" needed because of the BTU inlet of the appliance? Or because of the length of the vent? Pipes increase as do their squares. There's a lot more volume in a 4" pipe than in a 3" pipe, More restriction on the 3" pipe than the 4" pipe. But to oversize it can become a bad thing. Because the velocity and momentum goes down in the bigger pipe.

Back in the early days of Munchies, a 80M could be vented 2" or 3". It appeared to me that a 80M Munchie, vented with 3" would have issues when on low fire because there wasn't enough fan pressure to overcome any kind of head on wind. But the problem didn't happen as often or at all, if 2" was used. I found a few problem 3" vented ones that were resolved by putting a " X 2" P bushing into the 2" to give more back pressure and increase the velocity as the exhaust went through the 2" bushing.

If you have a hole in the exhaust for an analyzer, put the analyzer in while it is running, and note the CO. The CO should go down during post purge. If after the fan stops, the CO goes back up, the exhaust is backing down to equalize.

Zman

The backdraft damaging a non operating impeller sure seems like a long shot.
The damage you are describing sounds like the damage that occurred on the early model triangle tube units. Some of those units had a problem where the studs on the top of the heat exchanger would break allowing combustion gas to leak out of the exchanger and into the intake of the fan.
I would be looking very carefully at the gaskets and seals within the cabinet.
They may leak intermittently at certain temps or outdoor conditions...
Carl

icesailor

IMO, Munchies used to back fire into the swirl plate and do the same thing. On low fire.

IMO. Agreed with by some, denied by others.

SWEI

icesailor said:

If you have a hole in the exhaust for an analyzer, put the analyzer in while it is running, and note the CO. The CO should go down during post purge. If after the fan stops, the CO goes back up, the exhaust is backing down to equalize.

If it's backdrafting, shouldn't you see the draft go negative after shutoff?

nevele

Do you have two boilers on one sidewall venting unit. When one boiler runs does it fill the other boiler with the products of combustion .Are you saying that the products of combustion are escaping from what should be a sealed system?

mars_6

ME i have been over the major points of infiltration with a smoke stick as well as in the mechanical room with the door closed as well as open. Back drafting happens when it is cold out side and not so much when it is warm out. yes this one has my head spinning as I have had more than 200 years of experience in the field in this mech room and they all point to building performance issues. After speaking with the builder all of the floor to ceiling penetrations have been air sealed so this can be discounted as a factor. I am at a loss at this point? Guess there is a first time for everything. Matt

icesailor

SWEI said:

icesailor said:

If you have a hole in the exhaust for an analyzer, put the analyzer in while it is running, and note the CO. The CO should go down during post purge. If after the fan stops, the CO goes back up, the exhaust is backing down to equalize.

If it's backdrafting, shouldn't you see the draft go negative after shutoff?

Not if its just trying to reach an equilibrium.

The intake air should be heavier and denser, the exhaust should be hotter and lighter. The equalizer is that both terminations are on the same plane, but there is more of one weighted gas on one side than on the other.

90+ warm air furnaces have direct connected intakes. Most Mod/Con's only have the cold air ducted into the cabinet. They are not direct connected.

Its a concept.

When you connect two hot water heaters, you must carefully connect them so that there is no difference in restriction to ant heater. Or else, one will run out first.

Jack

Mars, this doesn't help in this situation, but making the vent grade up to the termination seems to me to be a problem or at least a contributing factor. I like to put the first 90 or fitting at the top of the riser off the boiler as the high point and grade to the outside. In other words, trying to make the high point of the termination below the invert of that high point 90 above the boiler. Some backdrafting problems can be solved by letting gravity work for you. I just don't like grading venting back to the appliance.

I'm with Ice on the air flow and pressure issues with the vent/intake separated. I know the manufs say it is ok and meets spec., but for whatever reason this building and its personality are biting you with the separation.

SWEI

icesailor said:

90+ warm air furnaces have direct connected intakes. Most Mod/Con's only have the cold air ducted into the cabinet. They are not direct connected.

The Lochinvar WH boilers are direct connected. There are pluses and minuses to that arrangement.

icesailor

SWEI said:

icesailor said:

90+ warm air furnaces have direct connected intakes. Most Mod/Con's only have the cold air ducted into the cabinet. They are not direct connected.

The Lochinvar WH boilers are direct connected. There are pluses and minuses to that arrangement.

That makes it worse. If it does what I say, all the products of combustion, in the longer exhaust pipe, all have to run back through the burner to get out the intake to balance the pressure.

SWEI

It also forces leaves and bugs through the blower, and from there into the HX. I'd much rather vacuum them out of the bottom of cabinet.