it is a reem

I have a client who’s furnace shuts down on a regular basis . She had a heatind man look at it and tell her that being a two cycle furnace , when on the low cycle, spent gasses are not being forced up the chimney and the sensors are shutting down the whole system . She resets it and all is well for a day or two then it happens again. the heater man is telling her to put a chimney liner in the chimney and it should stop the problem. any thoughts out there.

If this is a two cycle furnace it is most likely also a 90+ condensing gas furnace. If so it shouldn’t be going up the chimney anyway. If the flue is plastic it should be run to the outside entirely in plastic. 2 or 4 inch schedule 40 pvc.

James I don’t think that is entirely correct…Just because it is 2 stage does not mean its 90% efficiency. The link below will show you a 2 stage furnace that is an 80% AFUE. And flues by negative draw.

If it’s a non condensing high efficiency furnace it should have a chimney liner anyway but that may not be the cause of the "lock out"condition.

She should ask him if he will guarntee that will fix the problem.

Furnace “lock out” problems are sometimes not easy to diagnose.

There are so many things that will lock out a modern high end furnace we don’t have enough info. All we can do from here is guess.

Almost all codes and all manufactures will tell you to install a flue liner on any 80% furnace single or two stage on a masonry chimney. The problem is compounded with a two-stage unit because on low fire the volume of flue gasses going up the chimney are 1/3 less and not do not contain enough heat to warm the masonry chimney for proper venting. This can be an interment issue depending on weather conditions. Even if a hot water heater vents to the same chimney as the furnace.

Furnaces have a pressure sensor that monitors the “pressure” venting into the chimney, when the flue gases cool part way up the chimney they create a “thermal block” it’s like closing the chimney, this will trip the pressure switch shutting down the furnace. Some furnace will restart over and over, others have a lock out designed to keep the furnace off after 3 failures.

It is always a good protection as a consumer to have the results of any work put in writing