condensing boiler venting issue

bio_guy

Some of you may have read or responded to an earlier post regarding my father's change from a 55 YO oil boiler to a new gas boiler. Thanks for the responses. I was talking to a contractor yesterday who told me that they can not vent the flue gas out a plastic vent inserted into the existing chimney. He told me that his best installation experts told him that especially with the chimney on the exterior of the structure, condensation would be a problem. He said that they could run supply and vent out the chimney if there was room, but vent only would not work.

It does not make a bit of sense to me. How can a warm tube in a chimney cause condensation problems? Note that venting laterally out the side of the house will have aesthetic problems. The only locations available are near entries visible from the front of the house.

unclejohn

There are limits to the amount of vent pipe outside the home. Also most codes call for supporting the pipe every 5 feet. I have seen a system where you drop the flue down the chimney and somehow legs pop out when you pull up and it supports it's self. It's a polypropylene vent not pvc. For good or bad pvc will not be allowed on boilers much longer.

Zman

This seems to me to be more of an issue with a vent in an unconditioned space. The concern would be that the condensate would freeze in the vent.

SWEI

Without knowing which boiler or seeing the chimney and surrounding areas, we are just guessing. Is this contractor familiar with Centrotherm and/or DuraVent polypropylene flue systems?

bio_guy

Thanks, the proposal from them in hand is for a Rinnai RINE110CN. I've told them that they are too costly and indication is that they will come down. They are also going to price a Utica UB90. I've also asked them to reconsider the size.

The chimney is embedded in the concrete foundation, is exposed for about 7 feet in an exterior, inside corner (so two sides are exposed) and then goes through an attic before exiting the roof. The block chimney has a ceramic liner.

Swei, I would be astounded if these guys don't know about venting options. I will look at them myself and check with the installer.

SWEI

Flue condensation is normal for a condensing boiler. Assuming the flue is installed properly, the additional condensation resulting from a longer, cooler flue does not pose a problem. Is there some risk of it icing up?

The E110C is the largest model in Rinnai's less expensive boiler line. Do you actually need that much space heat?

The polypropylene flex liners are pretty slick. With no prior experience on the product, two of us routed three 3" flues through a 30 foot high ~13" square flue (one small offset) in about six hours. Another half day for the mason gave the customer a really nice finished product - including a brick vent air intake.

John Mills_5

You can use the chimney as a chase for PVC or other venting materials but you sure can just stick the PVC at the base of the chimney and let the gases go up. One of our techs saw a furnace vented that way couple weeks ago. We got it corrected quickly. The flue was a mess, damage done! The liners SWEI mentions sounds like what Richard Trethewey used last year.

bio_guy

Thanks John, venting the new boiler by sticking the the pipe into the bottom of the existing flue and using the existing flue to the top of the roof was never the plan. Using the existing flue as a chase would be the most explicit way to put it and I should have used that language. I’ll bet that old flue was a mess squared. (I have seen scary pics.) I’ve seen videos on the liners, but it has been a while since I looked at specifics.

Icing up is something that I had not considered, Swei. I suppose that the estimator might have gotten his signals crossed. He is a relatively young guy that worked in installation and service for a short time and changed positions. I think that the firm has a consultant engineer that he calls in if needed. I am pretty sure that he thinks that there will be a condensation problem in the within the original flue, outside of the new liner, but he may have gotten it wrong. I’ll have to push him. Again, the flue is in the foundation below ground level so its temp is 55 F there. It passes next to the house exterior on the ground level to 8 feet and then through the attic to the roof. I don’t know how to attack the freezing question. This is not mentioned in installation manuals for the boilers so I never considered it.

They’ve rebid the proposal with a E75CN* with a Utica UB90 as an alternative (aluminum heat exchanger). My thoughts right now are that if the Rinnai can’t run each of three zones at a low enough level, a dumber boiler with an indirect DHW tank will work better (the Utica or a Weill McClain GV90+) because they are built to turn on and off and the Rinnai (Baxi, Navien or Triangle tube which have been proposed by others) are not and will not operate at higher than 90% anyway.

My feeling is that E50 might be big enough. It seems to me that the fuel consumption and degree days over three years should be enough to get better BTU requirements than estimates based on what can be determined by examining the structure. I’ll work on that.


*Price is lower than the differential between the boilers, I’m sure. The rep said that they just had a discussion within the company and decided that they need to cut their prices to remain competitive.

SWEI

Icing up really should not be an issue. The PP flues I mentioned go from the boiler flue outlet all the way to the top of the chimney. The existing masonry does nothing other than support the top bracket and the spacers. It also has the potential to route combustion air to the appliance, but that would be up to the AHJ to approve.

icesailor

@SWEI:

"" Icing up really should not be an issue. The PP flues I mentioned go from the boiler flue outlet all the way to the top of the chimney. The existing masonry does nothing other than support the top bracket and the spacers ""

Its my understanding that the iceing issue comes up from the fact whatever the OAT is (Say zero), the inside of the flue will be 1/2 the inside of the heated space it is next to. Outside (3 sides exposed) chimneys will be zero inside the flue with no heat source. If a PP or PVC flue pipe is put in the space, and the exhaust temperature enters at 120 degrees, it doesn't have to travel very far before the flue gasses are below 32 degrees.



"" It also has the potential to route combustion air to the appliance, but that would be up to the AHJ to approve. ""

That is Definitely a ruling by the Massachsetts Board that you can't use the stray space in a chimney flue, to be used as an exhaust space, as a source of make up air.

SWEI

Different jurisdictions have different rules. Many of us are allowed to use chimneys (or even B-bent) as the outer portion of what in effect becomes a coaxial venting system. From a safety perspective, it still looks like a huge improvement over indoor air to me. I personally refuse to locate an intake port in close proximity to a flue no matter what the codes say.

Avoiding icing is part of a good design, regardless of the product(s) used.

icesailor

@SWEI:

I think that the concern is that with old,leaky, unlined chimney's being used for chases, and blocked at the top, can use the negative pressures built up inside tight houses and suck air through the bad mortar joints in chimneys. If they are left open at the bottom with the idea that they can provide make-up air, they can be sucking CO exhaust, mixed in with the make up air.

If you've spent your career working in antique cribs, and you watch a Gut Rehab, and see a 200 YO chimney and all the missing bricks, you wonder how it is that there are still any of these old cribs around that didn't burn down. If it wasn't for sealed combustion, many of these appliances wouldn't vent.

Gordy

icesailor said:

@SWEI:

"" Icing up really should not be an issue. The PP flues I mentioned go from the boiler flue outlet all the way to the top of the chimney. The existing masonry does nothing other than support the top bracket and the spacers ""

Its my understanding that the iceing issue comes up from the fact whatever the OAT is (Say zero), the inside of the flue will be 1/2 the inside of the heated space it is next to. Outside (3 sides exposed) chimneys will be zero inside the flue with no heat source. If a PP or PVC flue pipe is put in the space, and the exhaust temperature enters at 120 degrees, it doesn't have to travel very far before the flue gasses are below 32 degrees.



"" It also has the potential to route combustion air to the appliance, but that would be up to the AHJ to approve. ""

That is Definitely a ruling by the Massachsetts Board that you can't use the stray space in a chimney flue, to be used as an exhaust space, as a source of make up air.

As SWEI says it is dependent on a good design. In other words to do this particular chimney chase in international falls MN. With a 30' 3 sided chimney could prove disasterous. But in another milder winter region, or even a shorter chimney as a chase may have fantastic results.

As a reality check I just shot up the flue of one of my chimneys 3 sided,then turns into four sided after it leaves the eaves for 10' total height 20' the inside temp of the flue is 33* outdoor it is 0 tonight.

Then I went to my double flue chimney which serves the boiler in one flue, and the other flue is a fire place it's 3 sided for 9 ' then turns 4 sided for another 11' the inside of the fireplace flue is 54*, but the boiler has been burning about 6 hours today so that is why the adjacent fireplace flue is warm.

This all boils down to time, speed, distance, flue gas temp, flue chase temp, exterior temp.

Time: being how long a boiler will burn in a -20 oat scenario. Hopefully sized correctly for 100% modulation! and 100% duty cycle if matched to the load correctly at that oat temp.

Speed: being blower fan speed which again at 100% modulation at a,design oat.

Distance: length of flue being used as exhaust vent chase. Meaning venting goes all the way to the top of the flue not just dumped inside the flue.

Projected flue gas temps for the design load which involves emitter choice. Is it low temp radiant, base board, or rads. If so is there excess radiation to lower AWT.

Flue chase temp: I think once exhaust, and combustion vents are run, and flue properly capped a flue type chase would have a good chance of an above freezing interior temp,verses outdoor temp. Air itself in now a dead air space being somewhat of an insulator a 120* flue would actually promote convective currents with in the space.

Exterior temp, or OAT: again this boils down to a regional issue. What may work in a climate such as Chicago, may not work in international falls MN or the like.

icesailor

@Gordy:

All well and good for those in the know and read here. But if you give the Hackaroo's an ounce of space, well, you know. Its sort of like the camels nose under the tent flap. Once the camel sees how nice it is in the tent, they want to move in.

I wish I had photo's of all the exhaust vents I saw, installed and inspected/approved that were below grade on PT boxes because they didn't read or understand the instructions that said "a minimum of 12" or .33 meters above grade above or above the normal snow line." Last I knew, it still snows in Tennessee and Arkansas. Not just Washington County, Maine or Newport, Vermont.

SWEI

icesailor said:

I think that the concern is that with old,leaky, unlined chimney's being used for chases, and blocked at the top, can use the negative pressures built up inside tight houses and suck air through the bad mortar joints in chimneys.

Understood.

If they are left open at the bottom with the idea that they can provide make-up air, they can be sucking CO exhaust, mixed in with the make up air.

That particular configuration would not have occurred to me. I'm talking about sealing the bottom of the chimney around both the PP flue pipe(s) which go to the top and also the combustion air intake pipe(s) which have one end of a fitting and a birdscreen inside the chimney. We then open a hole in the side of the chimney for an outside air intake using a brick vent (around eye level, 10 feet or more below the exhaust.) I'm not a fan of coaxial vent terminations, at least around here with our crazy wind patterns.