Cyclical Vibratory Rumble Wind/Cold Induced?

Henter

I have an oil furnace. It has an unused wood firebox attached to it. I have a gravity-fed exterior tank, oil here is 60/40, and I have had very few issues with the system, other than it's noisy and fuel is high.

The other day it was very windy and sub-freezing. On the morning startup, the burner kicked on and made a weird cyclical, rumbling vibration throughout my house.

The best description I can give is that it sounded similar to when the guy down the road drives by in his noisy diesel truck. As soon as the blower motor started, the rumble stopped.

The system ran a few times, then I turned it off for the day. Upon restarting later that afternoon, it did the same thing.

When I keep it running every few hours and it never completely cools, I don't have this problem. I also don't know if it was wind-related because it's now seriously cold and the heater's been running 24/7 every few hours.

I live in a really rural area. I talked to the HVAC folks, and they said it's probably metal expansion. No. It's not. That doesn't cause my house to vibrate.

I think this was torching up my chimney. Similar to when you close the door on a roaring woodstove.

I read online this can be a solenoid problem in the valve that feeds gas to the burner.

I wondered if this was caused by some sort of weird draft problem from the wind.

I'd love some additional opinions and to know if anyone has any ideas before I call for service...

Jamie Hall

How recently has the combustion been checked? How recently has the unit -- complete fireside -- been cleaned? And how recently has the chimney been swept and inspected? And last, is this flue ever used -- and I do mean ever used -- for anything else, particularly wood?

Henter

It was cleaned and serviced in the spring and has had about 2 months run time since. The chimney is clean.

I never burn wood. The previous owners did years ago.

When they serviced the furnace last, I asked why the basement is always warm. They said the firebox is piped together into the furnace so that the air in it has to heat before the blower motor comes on.

I don't plan to keep this house, and oil isn't getting cheaper, so I decided not to remove the firebox. But, it's also never posed a problem other than wasting some fuel.

The weird thing is this was cyclical. It wasn't one diesel truck driving by sounding. It was as if he would have gone around the block 4-5 times. It built in noise and vibration, went away, built, went away, etc...

Logic makes me wonder if it's possible it was burning fuel and tried to draft at the same time it took a big downdraft of freezing cold air. Perhaps that forced it to try and draft out the stove and built up pressure. What I was hearing would be it trying repeatedly to draft. Which, sounds insanely dangerous if it's even possible.

The other logical thing would be it was getting drowned in fuel and torching. Again, not great. But, why would that happen when the system is cold and not any other time?

I found someone online with this same problem and they claimed a sticky solenoid on the oil flow valve was the culprit.

I have a full understanding of HVAC systems, but not the oil burning part of them. I always had electric...

Henter

After talking to neighbors, I likely had ice buildup in my chimney, preventing proper draft.

Apparently, most don't have this problem because I live amongst a lot of elderly who keep their heaters on 24/7 and they know about this issue so don't turn their heaters off at night when it snows.

Dave Carpentier

Could be oil dripping past the solenoid, as it would build up a pool over time (those long periods when you have it completely off).
If you have a manual oil valve near the unit, maybe turn if off for the next downtime (and dont forget to turn it back on) to see if stops the rumbles.

EdTheHeaterMan

That noise is very familiar to me. Here is the scenario. Excess oil enters the combustion area whether on start-up or during down time. The excess oil does not get enough combustion air from the burner fan. so the flame searches for air from wherever it can find it. Pretty quickly the flame starts to go out as a result of the insufficient combustion air. This lowers the pressure in the combustion chamber (fire box for the oil flame) and this allows the burner fan to add more air as a result of the lower pressure. The excess oil then gets this influx of combustion air and rapidly burns increasing the pressure in the fire box. This increased pressure reduces the amount of air the burner fan can deliver and the flame begins to extinguish again and the process repeats very rapidly, over and over again until the chimney draft, the blower air pressure or the burner fan pressure can find a stable air/oil mixture.

That is your vibration. Now you need to find if the excess oil is from a drip during the OFF cycle... OR if the oil pressure, and air/oil mixture adjustment is incorrect. You will need a combustion analyzer to make the final adjustment. Call the Professional Service Tech for that adjustment.

Henter

I am going to have it serviced. I live in a rural area, and I am waiting for them to get back to me.

I haven't heard anything unusual since I had this problem the other day. It's also been at or near 0 F for the last few days, so it's been running, even at night.

Am I correct that it is an explosion danger when it's doing all that vibrating? Is there some sort of pressure relief like on a water heater? That's kinda my biggest worry at the moment.

EdTheHeaterMan

On the water side of any boiler there should be a pressure relier valve. On the fire side (or better stated the combustion flue gas side) the relief is the fact that it is connected to a vent that goes outside. If, however, you put more fuel into the combustion flue gas side that the exhaust pipe can handle, there is the possibility that the pipe will separate. If that happens, turn off the burner and wait for the repairman.

Henter

It's not a boiler, it's a forced air furnace with a heating oil fuel source. I've never noticed any sort of vent other than the chimney.

It rumbled again last night after not running for a couple of hours. Looking for someone to come look at it.

Henter

The HVAC tech who installed my system in the 80s was here.

If I understand it correctly, I have an "older style" burner that atomizes oil at 100 PSI.

We have temps below freezing with thick rine ice fog and thick fuel from being cold in the tank outside, so it can't keep the flame adequately burning. Apparently, all that vibration and roaring is the flame fighting to burn and draft.

He did something to force 140 PSI with smaller atomized particles and said the combo of that and keeping the furnace at a steady temperature all the time solves the problem.

Now we wait.

Thank you for everyone's opinions on this. I really do appreciate it.

EdTheHeaterMan

I use a tiger loop to solve cold oil problems related to outside above ground tanks.

Cold oil will not atomize the same way that warmer oil atomizes. Page 14 of this booklet explains this. https://delavan.com/wp-content/uploads/2022/06/Delavan-Total-Look-Book.pdf
There is actually more oil delivered to the combustion chamber when the oil is very cold but the fan does not deliver any extra air when the oil is cold.

So I have a question. Are there two fuel lines from the tank to the fuel pump (Two Pipe), or do you have only one pipe from the tank to the fuel pump? This can make a big difference in cold weather.

Dave Carpentier

Is there a plate on the furnace saying the specified nozzle and psi for it?

It regularly gets to -20f here, and does manage to hit -35f (or colder).
I dont remember hearing of anyone cranking up their pump pressure to maintain the furnace.

Henter

The HVAC guy that came out is great and has been working on oil furnaces since the 1960s.

From what he said, my furnace has a common problem in this area.

It's basically a combination of a tall old brick chimney, below-freezing damp air stagnation, an inability to atomize the fuel small enough at the old standard of 100 PSI, no flame stabilizer, and my own user stupidity of turning it way down/off at night.

He said for the last decade, he's modified these systems to force the oil to spray at a higher pressure, forming smaller particles and stabilizing the flame. That stops the pulsating that shakes my house. Makes sense.

EdTheHeaterMan

Still would like to know if your outside above ground tank has a two pipe or a single pipe fuel line system to your fuel pump on the oil burner.

Your oil repair guy sounds like he knows what he is doing!

Henter

I have one line from the tank into the basement. The furnace itself is fairly ancient.

When I moved here 3 years ago, the price of oil was less than half of what it is now, and there were a lot of parts available for this one. He advised me to leave it instead of going to electric because it's easy to fix and better made.

Considered going electric with the price of fuel ($5.21 a gallon...) until today when he told me that $3K quote from before looks more like $10K today and they have a 6-month waiting list. Something about inflation and democrats. lol

Seems to be operating better now. And, I have the peace of mind of knowing the vibration isn't going to kill anyone.

Dave Carpentier

Henter said:

He said for the last decade, he's modified these systems to force the oil to spray at a higher pressure...

The "modified" part might mean that in addition to raising the pressure, he also changed the nozzle to a smaller flow rate so that it ends up with the correct amount of flow.

But.. if it was excess oil dripping into the firebox due to unatomized oil, that would have been burning off during each run. Excess oil burning after a long off period, in addition to a "gravity-fed exterior tank" (by that do you mean the tank outlet is higher than the burner ?) does seem like a leaky solenoid. (Im not a pro though, just a diy with oil)

STEVEusaPA

I'd try what your seasoned oil burner tech mentioned and I would add a 30 second pre-purge.