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Is it better to vent out a side wall or thru the chimney?

alkah3st Member Posts: 1
So I'm in MA and plan to replace oil-fired boiler with natural gas. I have a 1.5 story Cape with a half-finished basement and heated garage. Radiators in ground floor and some baseboards. 1952 construction. 1.25 bathrooms, 2000 sq ft. Plan to have 1 to 2 extra baths in the future (like 10+ years away), a gas grill, and gas stove (near future). Currently have dryer on gas. I got several estimates in anticipation of financing through Mass Save. However the most detailed quote and also the most expensive is the only one suggesting venting thru the chimney (see below). He said it would keep the boiler quiet and that other customers complained about that in the past. This would require a new chimney liner because ours is falling down (confirmed separately by a chimney guy).

This is the text of the most expensive quote, vented thru chimney:


We will provide and install one new Lochinvar 95+% AFUE rated modulating high efficiency sealed combustion wall mounted hot water boiler to replace the existing 80% chimney vented / oil fired atmospheric boiler. The existing boiler will be removed and disposed of properly. The new boiler will be installed in the same general location in the basement mechanical area. The new boiler shall be chimney vented via a new 3” stainless steel liner along with a new 3” aluminum liner for required direct venting combustion air. We will completely rework all of the “near boiler” piping – complete with new zone valves and a new variable drive main circulator. We repair a compromised basement baseboard heat loop (assuming that the baseboard heat itself is viable (work beyond this may require a change order)…

At least one “spare” zone shall be included in this near boiler piping in case of future needs. We will also add an additional zone to serve the domestic hot water needs. We will remove and dispose of the existing glass lined indirect fired storage tank. We will supply and install a new Lochinvar SIT050 stainless steel indirect fired domestic hot water - a “Sparco – Safety Mix” valve shall be included as an added anti-scald measure beyond the digital controls included with the new boiler.

Wiring and a new zone control relay system shall be included.


The other quotes are about 5k cheaper overall:

Burnham Alpine 105k ALP105BW-4T02
Superstore Water Tank with designated zone off boiler

Install new Rinnai E110C hot water boiler on basement wall

Lochinvar Noble NKC150 wall hung, gas fired, combination forced hot water boiler and
water heater at 95% AFU (cheapest quote--it was 8k cheaper than the detailed one)


Installation of Lochinvar WHB155 wall hung, gas fired, forced hot water boiler at 95% AFUE
Installation of Lochinvar Squire SIT050 50 gallon indirect fired water heater

Any advice on the best quote here and why the most expensive one suggests venting out into the chimney?


  • PRR
    PRR Member Posts: 68
    I have no idea why the one guy wants to go up the stack. Did you ask?
    My chimney has had wood, coal, oil, and now gas. When I bought the place it had a tired oil furnace, 85kBTU, fairly loud. I converted to 40kBTU gas and took it out in PVC over the foundation (through rimboard) then up above snow and face, then out to clear siding and soffit. I can hear it. I don't think it is loud (it is comforting). I don't think it is louder than the 1.5ton compresser in the same area. My neighbor is 200 feet away so no complaints from him.
    The furnace makes small sound in cellar (less than the old beast) perhaps comparable to the vent outside. Being hot-air, some of that sound comes up the ducts, but dominated by the house-air blower. (I re-did much duct to keep that down.)
    Your BTU is 2.5X bigger than mine. I do not know how much louder that is; or how your neighbors may feel. But are they really standing around in zero weather to grouse about your smoke-roar?
    Given a place where your vent does not throw acid-water on your neighbor's best plantings, or shoot the sound to their bedroom, I'd really rather vent out a wall than a chimney.
  • SuperTech
    SuperTech Member Posts: 1,857
    I've seen it done before, but I don't prefer it. Many manufacturers require the exhaust and combustion air intake to be on the same outside wall. Just make sure they are far apart enough from each other to prevent flue gas from being pulled into the air intake, it's a problem I've seen happen with a few boilers during low fire operation.
  • Jolly Bodger
    Jolly Bodger Member Posts: 209
    Vented a ton of gas equipment out the side with a low profile vent kit. Never a complaint about sound. And if using an approved vent termination instead of built up, short cycling the exhaust gas is a non-issue.
  • PRR
    PRR Member Posts: 68
    edited December 2019
    > exhaust and combustion air intake to be on the same outside wall. ...far apart enough from each other to prevent flue gas from being pulled into the air intake

    They have to be "on the same wall, close" so wind pressure doesn't cause a difference more than the fire fan can handle.

    This suggests "the same hole", or two 3" holes in the same joist-bay.

    IIRC the installation guide said a 2-pipe job must have the ends a foot apart (this was 2" pipe). I went up and then 12" out with the smoke pipe to throw smoke and drip away from the house. The intake I bent parallel to the siding for a foot, then down to avoid rain/snow. This put the two ends 17" apart and going different directions. Observations in various wind and cold confirm that essentially none of the smoke comes near the intake.

    Yes, the Approved single vent kit is a for-sure solution. And neat. In this case it would want more complex plumbing to do the bend-up (to clear snow-drift), and I did not begrudge the wall-space (or explain it to a client). All basic fittings. (Plus a couple 99-cent bird-feeders to keep our small squirrels out of the pipe; the one for smoke rotted and I found a SS screen on Amazon. When I did that I also took the smoke out another foot cuz it tended to rise into the soffit vent and I feared it was dampening the attic insulation)