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Question DetailsAsked on 1/29/2015

We have a 1926 house. Chimney liner replaced for $3000 and now a draft is causing furnace to go out. need help

House is steam radiators heat. Liner just replaced this oct. Our furnace man says problem new liner causing drafts
Chimney people now want to install another better liner for another $3k. What should we do. Furnace is going out every day due to drafts

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2 Answers


I am assuming nothing has been changed regarding the available airflow to the appliance in the area it is in - like weatherstripping on doors or better-insulated and sealing garage door or such ? Cutting down on available airflow to the area it is in can cause blowout of the pilot. If the room it is in has had the airflow reduced, you may need to provide added combustion air to that area. I am also assuming that liner/flue does not serve any devices on upper floors - that can cause blowouts too, and is generally prohibited. I am also assuming that you have not put in a more efficient or smaller furnace since the liner job - because the new unit might be too small for the liner size. Generally speaking, the flue sizing is to keep the gases hot enough that you do not get condensing conditions in the flue, and large enough that there is not a backup of gases in the flue due to flow restrictions from too small a pipe, but too large a flue can result in overdrafting and pilot blowout - even flame blowout or rollout (flames outside the bottom of firebox) in extreme cases. Sounds like the liner size may have been changed - liner size should have been checked against the size of boiler (BTU/hour rating) to determine the diameter to use. You can find tables of flue size versus appliance ratings (total of all on same flue) at many manufacturer websites - just google for - furnace flue size requirement - . Other possibility is they put on a new type cap, or no cap at all, so you are getting a substantial venturi effect at the top of the flue. My first approach would be to have a Heating and A/C contractor (or maybe Plumber in your area, for boiler) check the flue diameter against the capacity of the devices using it. I would also have him check the thermocouple - maybe it is not fully in the flame so it gets too cold when there is a wind blowing. If in doubt on thermocouple, or more than a year old and it is a standing pilot furnace, have it changed out (about $10-15 part) - sometimes they go bad and become overly sensitive to drafts blowing past them. It is also possible the thermocouple is mis-oriented in the housing - if the air draft blows over the thermocouple rather than blowing the pilot flame into the thermocouple that could be the problem too. Sometimes a bit larger pilot flame can solve this problem too - simple adjustment on the gas valve, and insignificant difference in fuel consumption. If not oversized (beyond code) then I would be looking at a draft-reduction hood on top of the flue - there are several designs to reduce the wind causing added draft in the flue, from multi-baffle types (lower maintenance) to weathervaning types (that commonly need periodic cleaning/ lubrication) that swivel the opening around with the wind. I am not convinced the latter type work well to reduce draft problems - they are usually better at inducing more draft in long flues, even though they are sold for use with this problem too. I would give about 10:1 odds one of the above is the problem. If it does turn out that the liner is the wrong size, then whether you have a claim against the liner contractor (from October) would be a question - especially if he did not get required building inspection, which should have caught a mis-sizing if that is the case.

Answered 5 years ago by LCD


You know, I thought about your case a bit more overnight.

Unless this going-out issue started as soon as the new liner was put in, I think that might be just a coincidence, because unless you are in an area with good winds every day it would not be blowing out due to draft on calm days. I would think back to when it started, and if that matches with the new liner, or if it matches up with any energy conservation measures installation, water heater changeout, change or weatherstripping of garage door, etc.

In troubleshooting, I would concentrate initially on several things that came to mind - of course, these are all hypotheticals, but any one could be the case:

0) make sure there is a cap on the top of the flue (should be able to see from ground level), and that it extends either about 2 feet above ridge of roof (if near ridge), or 3 or more feet above roof level if lower down on roof where it comes through. If they installed the flue short so it does not extend far enough above the roof, that could cause drafting through the flue as the airflow ovder the roof affects it.

1) be sure they installed a draft hood - the open bottom cone shaped skirt right on top of the furnace (water heater has one too), which helps initiate the draft in the flue when the appliance first kicks on. If they connected the flue ducting direct to the furnace without a draft hood, with the exception of very few units like fan-driven pulse units, that could cause backdrafting when the unit fires up, which can starve the fire for air or cause automatic sensor shutdown ofthe burners.

2) be sure the liner is not blocked by birds nest, insect nest, etc.

3) In both the above cases, and also if you are not getting enough incoming combustion air to the room/garage the furnace is in, the natural flue exhaust draft up the liner will be slow to establish itself. This can cause the "fire" to basically stay in the firebox or even "rollout" under the edges of the furnace, which is not only dangerous but with newer furnaces will cause it to shut down because they have rollout sensors that detect the rollout heat and cut off the burners. Look up under the furnace (assuming it is open firebox type, not sealed chamber pulse unit) when the unit kicks on - if the flame, after an initial unstable second or so when first igniting, significantly wavers, wobbles, "gutters", or "walks around" in the firebox rather than "standing up straight" above the burners without significant (more than 1/2 inch or so variation) lateral movement or with more than the tips of the flames being yellow, then you may have backpressure from the flue, you lack combustion air, or your air/gas mixture needs adjusting. Certainly if the flame balls up and flickers out to or under the edge of the firebox rather than going straight up that is a critical situation and needs immediate correction. If it is balling up and spreading out all over in the firebox rather than being vertical rows of flame that merge into a clean upward flow into the furnace, try opening a door or window to the outside from the room the furnace is in and see if that makes a difference. If it changes to a nice straight, vertical flame when you do that, then you need to provide makeup air - commonly with a screened ducted opening through an outside wall about 6-12 inches in diameter, depending on unit size. Technician can size it - depends on combined Btu demand of all appliances in that room. In some cases, this will occur only when the water heater is also firing at the same time, which can get frustrating, because it only blows out about once a day when both happen to be firing simultaneously.

Also, if you have a clothes dryer in that area, whenn it operates it pulls a lot of air out of the room and can create a partial vacuum, which can cuase furnace backdrafting, so the makeup air opening to the outside needs to be increased in that case too.

4) One other thing in the coincidence category which I think I mentioned before - this problem may have nothing to do with the flue, and may just be a thermocouple making poor contact in the gas control valve so it intermittently fails to turn on the gas flow to the burners, could remotely possibly be an intermittent flaw in the gas control valve itself (had that happen on mine once), or could be the the thermocouple is not located in the blue part of the pilot flame properly (especially with standing pilot) or that the piezoelectric ignitor is failing to ignite the pilot at times. I have never seen intermittent failure of a thermocouple, only good or dead, but I suppose that is a possibility too. Quite cheap to replace the thermocouple - electronic/ piezoelectric ignitor also in the $10-20 range commonly for the part, but if the ignitor control board has failed more like $20-60 parts - plus service call charge of around $75-150 labor.

5) if you have an electronic furnace, it is also possible that you have an intermittent problem withthe master control circuyit board, with a sensor not working right, or if the unit has an outlet damper or booster fan, that it is not kicking in correctly every time. In that case, with some makes the furnace will kick on, but then shut down the burners in a couple of seconds because the exhausst gas is not exhausting correctly. With elecctronic control units there are TONS of possible causes - some of them interlocking - so it can be a real problem to solve. I had one I was working on a few months ago that took 4 hours to debug - turned out when the motor-controlled damper operated it was vibrating a loose connection on the exhaust fan (on a new high-efficiency boiler), so the fan shut off right after turning on, causing the circuit board to recognize the exhaust fan was not operating so it shut down the burners. The joys of electronic controlled appliances - they add greatly to the diagnosis time and repair costs, and in my opinion rarely at to functionality or efficiency of the unit but fail far more frequently.

6) If you have an HVAC technician out, also have him look at what is feeding into the flue, and where - there should not be anything but same-fuel heating appliances at one level tied into it - not different appliances on different floors, and in general (in most code areas) not oil and gas fired appliances both, and only two inmost cases - two furnaces, or one furnace and one same-fuel water heater.

Answered 5 years ago by LCD

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