side-wall power vent because tiled chimney unlineable

jtk64

How realistic are the space clearances required for power venters? The power venter is for venting two older oil-to-gas converted boilers, for a combined BTU of a 660K BTU/hour input; a large boiler for steam radiators, a smaller boiler for indirect hot water heater. The plumber specified Field Control's SWG-8. The manufacturer requirements say minimum of 4 feet below and to side of windows and doors and porches. The plumber wants to seal up a large basement window, run power venter through that window, and lock nearby windows unopenable. However, house is large, old (1890), drafty, has original windows, and has many windows within 5 feet above the proposed venter location, so we are concerned about fumes seeping inside.

We are considering side-venting because recent inspection shows that some tiles in the chimney flue are spalling and flue is too small -- about 8" x 12" -- and is bent to allow a stainless steel liner. In fact, four chimney companies have determined that flue is not lineable. Boiler manufacturer says boiler can be power-vented out side of house.

STEVEusaPA

@Bob Harper will provide the best answer.
The reality is what if something tragic happens and you (installer) didn't maintain those clearances?
Locking a window doesn't qualify as un-openable, I wouldn't want to go to court to defend it.
You might need to run a metal exterior chimney.

jtk64

I should have been clearer: We are not trying to minimize clearance, but are inquiring if other people have real-world experience that the minimum specified clearances are usually realistic. The basement window space through which venter will be installed will be boarded up, then venter mounted on the board. The adjacent basement window will be bolted shut. But, yes, there are a lot of variables -- tightness of house, layout of house, weather, microclimates-- that can affect how exhaust dissipates.

The licensed plumber will be doing the work. All work will be permitted, inspected by the town.

To install a chimney up the side of the house, we would have to find a different location to exit the house because of all the windows on all the three floors above the original exit point.

Also, the house is in a historic area, so we want to try to keep the outside as original and uncluttered as possible. Of course, safety is priority, but we may have to get approval from historic commission before adding outside chimney. The house has a brick foundation, so we could brick over both windows, but that is changing exterior look of house.

pecmsg

jtk64 said:

How realistic are the space clearances required for power venters? The power venter is for venting two older oil-to-gas converted boilers, for a combined BTU of a 660K BTU/hour input; a large boiler for steam radiators, a smaller boiler for indirect hot water heater. The plumber specified Field Control's SWG-8. The manufacturer requirements say minimum of 4 feet below and to side of windows and doors and porches. The plumber wants to seal up a large basement window, run power venter through that window, and lock nearby windows unopenable. However, house is large, old (1890), drafty, has original windows, and has many windows within 5 feet above the proposed venter location, so we are concerned about fumes seeping inside.

We are considering side-venting because recent inspection shows that some tiles in the chimney flue are spalling and flue is too small -- about 8" x 12" -- and is bent to allow a stainless steel liner. In fact, four chimney companies have determined that flue is not lineable. Boiler manufacturer says boiler can be power-vented out side of house.

Not going to cut it WHEN something goes wrong!

captainco

Flue gases are heavier than air. Even CO below 30 degrees in heavier than air. There is more chance of infiltration if the vent is above a window and that clearance is only 1 foot.
If the plumber does a combustion test and sets the system up correctly there will be very little harmful fumes.

Bob Harper

I agree with Jimbo (never called him that). The lower in the wall typically the worst any negative pressure in the basement. Aside from combustion analysis, I would test the pressure gradient btw the Combustion Appliance Zone (CAZ) and outdoors. Understand any C.A. or depressurization test will be a snapshot in time. Normally, you WANT passive infiltration at the lowest level to alleviate depressurization. In this case, you'll want this side of the house tight as Tupperware near the exhaust. However, since passive MakeUp Air (MUA) was found by ASHRAE to be wholly unreliable, a powered MUA system will probably need to be installed slaved to the power vent- stuff in/ stuff out. Once the unit has been tuned using C.A., fire it and sample ambient air in the CAZ and room above for trace CO.

Meanwhile, you'll want the top of the thermal envelope sealed as tight as Tupperware to minimize or eliminate exfiltration. If there is ductwork in the attic, it MUST be rigorously sealed using UL 181 closure systems and tested. External duct leaks will exacerbate low level infiltration, which could entrain CO. Old drafty houses are more prone to CO poisoning than closed terrariums.

I would investigate the chimney again. If you put both boilers on a priority control, they can share a single liner that is properly sized for the largest. Typically, priority is given to the DHW unit. Since it's an indirect, it can charge the tank then standby while Big Daddy serves a call for heat.

I'm curious about the offset that's preventing a flexible liner. If you break out the flue tiles and install an ovalized ss liner, they can offset laterally as long as the offset does not exceed about 30 degrees and there's only one offset. I've opened many chimneys to relieve a restriction then seal it up. Without removing the flue tiles, a typical 9"x13" OD tile has an ID of about 7"x11". You should be able to get a 6"x10" liner down unless there are severe offsets or mortar protrusion in the joints. In those cases, remove the flue tiles. A 6" x 10" oval flue has a cross-sectional area of 52.26" sq or just over the equivalent of an 8" rd. If you use a smoothwall liner that does not need to be derated for corrugations, at 30ft., it should be able to handle about 460 MBH. If the big boiler is less than this, a prioritized liner might work. You wouldn't know for sure until installed and passed combustion analysis.

Sealing up the basement on an old house that leaks like a sieve is asking for trouble.
HTH

captainco

Bob,
You can call me any name you want:)

jtk64

Wow, thanks for the detailed, sobering analysis, Bob Harper.

Yes, the plumber did say that we will need to vent in make up air from outside. Currently, both boilers get make up air from the air inside basement, but basement has vents in floor to upper floors (probably to distribute heat), so there is a lot of air pulled from all over house, I guess.

Even if we prioritize one boiler over the other, I think boiler BTU sizes means a large liner is required: Steam boiler is Weil McLain 80/480 with 491 MBH input. The hot water boiler is Utica, 175 MBH input. Both were converted from oil to gas with Carlin burners around 2012. The vent pipes from each boiler have Field Controls barometric draft regulator, then merge into common stainless-steel Y which goes into the single flue.

The chimney is in interior of house, not on outside. It has three flues -- one for boilers, two for first-floor fireplaces which are in adjacent rooms. We don't use the fireplaces because their flues are unlined and they don't have the required clearance to the mantles. The chimney tech who did the video inspection confirms that that the bend is located in chimney segment just below the second floor, so we would have to break open some plaster walls.

Steamhead

jtk64 said:

The chimney tech who did the video inspection confirms that that the bend is located in chimney segment just below the second floor, so we would have to break open some plaster walls.

I'd seriously consider doing this. Once done, there would be no additional moving parts, and a SS liner should outlast everyone in this thread. Replastering isn't that complicated, you just need to find someone who's good at it.