Draft challenges with oil furnace that is secondary to a heat pump

Atomic1

In a system with a primary heat pump and a secondary from an oil furnace, is it commonplace to need a draft inducer?

I am having issues with this type of a system that is serviced by a conventional 3-story exterior masonry chimney (chimney located on the exterior wall and the furnace is in the basement, dedicated clay-tile lined flue for the furnace). Circa 1989 construction. When the heat pump goes into defrost mode, it kicks on the oil furnace for a few minutes that establishes an acceptable draft at about -0.04" wc in the breech with a gas temp of 500 deg F while the burners are on, but when they shut down, the draft drops off and the flue gasses backdraft and stink up the basement. My thought is that the chimney never gets warm enough to induce a residual draft. This is more noticeable during cold weather obviously when the chimney masonry is cold. The chimney has been tested/inspected ok, as well as the heat exchanger. The flue draft regulator is also operating normally. I have been talking to a few folks, and while this type of furnace configuration isn't super common, they nobody seems to feel that a draft inducer would be required. Just wondering if any of you have seen this issue.

STEVEusaPA

First, make sure the smell isn't coming from a crack in the chimney and products of combustion leaking into your home. And also make sure your burner is properly cutting off and not allowing some after drip/burn.
Just running the oil burner into a cold, 3 story exterior chimney for a few minutes is a problem.
You may be better off with a control strategy that allows for pre and post purge. This would require a fuel pump with an oil valve, and a primary control capable of post purge, with maybe a one minute or more post purge.
Draft inducer is not needed, but maybe you switch to a through the wall power venter, which has an adjustable post purge

Jamie Hall

I agree with @STEVEusaPA , more firmly: you really need at least a post purge on this. You simply aren't running the burner long enough to warm up that flue at all, and the post purge will at least push the combustion gas out of the chimney.

I can't think that this control strategy -- such a short run on the boiler -- is doing the boiler any favours either.

JUGHNE

I'm thinking forced air oil furnace here Jamie, with heat pump coil on top. We call them dual fuel/fossil fuel where I am from.
The few I have been around I have adjusted the change over to the fire, (real heat) before the heat pump has to do any defrosting to speak of.
IMO this saves a lot of wear on the compressor as during defrost the head pressure gets pretty scary as the inside coil (now cooling) suddenly has a fire built under it.

So you could have change over to oil earlier ODT wise or disable the oil during defrost. Many years ago some heat pumps had no supplemental heat to utilize during defrost.

Jamie Hall

I'm sort of a single minded sort, aren't I? I'm sure you're right, @JUGHNE ! But as I said... a change in control strategy would seem to me to be reasonable. As you suggest.

Atomic1

Correct guys. This is a forced air furnace located in Pennsylvania. Great ideas. The latest tech I had on site made it sound like there was no option to disable the oil fired defrost cycle on the heat pump.

Steamhead

Also make sure the furnace room is not going into negative pressure, which would suck whatever was in the chimney back into the room.

John Mills_5

I could disable oil heat during defrost in 30 seconds As slow as oil furnaces heat up, you won't miss it. My 95% gas furnace goes through purge and ignitor warmup before lighting, by the time the HX starts to warm up, defrost is over. I shut the heat pump down at 25.

Atomic1

Edited, sealing around the furnace exhaust duct did nothing.

neilc

you have a CO detector, RIGHT ? !
go buy a CO detector,

am I reading this correctly that the exhaust vent is fed thru, and was leaking in, the heating airflow ?

captainco

Steamhead has one problem mentioned. Excess negative pressure. Most oil burners can overcome negative pressure when they are running because they have a combustion air fan in the burner . However, when the draft drops immediately after the burner shuts off that is a good sign of negative pressure.

The other scenario is a wet flue. It will act like an evaporative cooler and immediately remove the heat from the flue gases and draft will cease. If the flue does not have a good rain cap that is a problem. On one of the first jobs I was called on with this problem we found no cap on the tile flue and found it soaking wet. I had the contractor temporarily install a draft inducer on the flue and kept it running for two weeks. The flue dried out and the flue worked like a vacuum cleaner from then on. Obviously if negative pressure was a problem also the drying out the flue wouldn't have totally corrected the problem.

Something as simple as adding a supply register on the plenum can solve a negative pressure problem, especially if it is related to excess return duct leakage. We don't recommend randomly sealing return ducts until we check static and verify there is enough return in the first place.

Atomic1

Ah, scratch my last post, the problem still persists. The weather was warm enough (low 40's) and fooled me for a few days. Did some more testing. If I run the system in emergency heat, there is no exhaust smell from the oil furnace even when the temps got down toward the 5 deg F range. The problem is when the furnace fires for the short period of time during the heat pump defrost cycle.

I'm not sure it's a negative pressure problem as the basement door isn't getting sucked shut, I have an equalizer vent that connects the basement to the first level, and the problem still occurs even with the basement door open.

I do have a metal chimney cap.

In general, it seems like a very poor design to fire any furnace for a short time to defrost the heat pump. I guess I need to look into a post purge system....and convincing an HVAC tech to install one. None of the guys I've had out seem to have any great knowledge about how drafting works.

Steamhead

Maybe increase the defrost time so the furnace will run longer?

Atomic1

I have an Armstrong Air heat pump and believe it has one of those intelligent defrost systems that looks at previous defrost times and external temperature...therefore, running relatively short defrost cycles. If i'm going to force it to run defrost longer, it's probably just as economical to run it in emergency heat whenever it gets into the 20's and below. Or maybe disable the oil on defrost and live with a little cold air from the registers.

Atomic1

Just circling back for anyone else troubleshooting. The problem was solved by eliminating oil fired defrost cycles (required a tech to rewire). This therefore indicated a draft challenge for a cold exterior exposed masonry chimney was the problem.

I would suspect that a flue draft inducer would have been otherwise needed.

Another option would may have been to install an insulated flue liner.

captainco

I am so glad the manufacturers of flue liners have convinced everyone that the temperature of a flue or chimney creates draft?

Draft is created by the pressure difference of indoors to outdoors and the temperature difference of the flue gases and the outside temperature. No mention of chimney temperature.

Drive around town and look at some old commercial buildings with 50', 75', 100' tall brick chimneys exposed above the roof, I guarantee these chimneys never got warm.

Why is there more draft on a cold day than on a warm day? The chimney would be colder and yet draft is higher.


Crack the door going into the mechanical room and hold a piece of toilet paper in the crack and see which way it goes. If it sucks out you know which way the air is moving.

If you can't solve the negative air flow (not exactly a pressure, just a directional air issue) then an inducer may be necessary. But it would have to run for 5 - 8 minutes after the furnace shut off or you will still get smells.

Atomic1

The temperature of the flue affects how fast it sucks heat out of the flue gas. An insulated liner has an interior surface that is easy to heat up which promotes lingering draft vs dense clay tiles which remain cold and cause a waning exhaust column to cool and collapse.

captainco

The temperature of the flue gas determines the amount of draft not the temperature of the flue. The best flue is one that doesn't remove the heat from the flue gases.

Clay flues have the lowest thermal conductivity which means the heat of the flue gases stays in the flue gases. Aluminized steel liners have a thermal conductivity 40 times greater than clay. Stainless liners have a thermal conductivity that is 8 times greater than clay. The material that heat exchangers are made of need high conductivity not flues.

Not once in 41 years have I ever seen a flue that was too big be the cause of a problem. I know dozens of contractors that have put in liners and were quite embarrassed when they didn't work and in many cases made things worse.

I wonder why this big flue has worked for 60 years and is still working?

Jamie Hall

A miscellaneous addition... Cedric, for example, draughts through a 40 foot, 12 inch square tile lined flue. In a chimney shared with three other flues. Never had a problem with too little draught (barometric damper). In the summer, when he runs rarely, the flue is room temperature, of course. In the winter, when he runs for a portion of each hour (sometimes a good portion!) the flue is always warm -- the heat capacity of a masonry chimney is very high.

In many pre-modern age houses, the central chimney was a massive stone affair. Once you got that warmed up in the fall (which might take several days!) it gave steady, even heat through the winter...

captainco

We have a water heater in our training building. It is vented into a 6" X 10" tile lined chimney on the outside wall of the building. The chimney is 25' tall. The water heater has a 3" flue. That means the chimney is 8.5 times bigger than the water heater flue. In the summer when it is 90 degrees outside and the water heater is running, the draft never gets above -.01"W.C. In the winter, when the outside temperature is 10 degrees and the water heater is running the draft in the chimney is -.06"W.C. within 2 minutes. Why?

Draft is created by cold air displacing hot air. Camp fires outdoors have really big chimneys, but the smoke and heat still rise without heating up outdoors. Pressure difference affects draft indoors also, but not outdoors.