Mendota losing oxygen or No combustion?

[Myst415]

When I turn my mendota DV on, I get large blue flames for about 1-2 minutes then they will be snuffed out. Running it temporarily with the glass cracked will "get it started" so to speak and the flames will orange out and it will have no problem running for the rest of the period.

This is a relatively new install from last year and after changing the control board and the thermocouple / pilot light its now working correctly. When the weather was warmer it would fire up no problem but now that it's colder I have to run continuous pilot for it even to light right.

Am I not getting enough O2 or is the combustion not escaping fast enough? It has to travel up about 15ft out a chimney.

 

[wooduser]

Sounds like you are not getting the combustion air you need to burn the gas.

Look carefully at the flames. If there is a shortage iof combustion air they will usually turn blue, then increasingly thready until they go out altogether.

Was the stove working OK last year?

The first thing to check is the vent termination cap for anything blocking or obstructing air flow around that, which might be plants, furniture or whatever. Any signs of black soot in the fireplace or vent pipe?

Next I'd take the termination cap off and inspect the inner and outwer pipes visually. for any obstructions, all the way into the connection at the stove.

Take the connection at the stove apaprt and check that foer any obstrructions or any sections that may have come loose.

A loose vent pipe will let the burned combustion gasses contaminate the fresh air needed to burn the gas, so poor pipe connections can be a problem.

Read the installation manual on how the vent pipe should be connected and check for any defects against that standard.

 

[KeithO]

Perhaps there is initially a low draft, until the chimney has heated ? Is the chimney in a cold chase or inside a heated space ? Once the chimney has heated and is sustaining, it is the motor that sucks the exhaust out and the fresh air in. I dont have a door that is open-able on my stove, but if you do you could try just heating the flue with a blowtorch for a few minutes and then close the door and see if the stove lights and sustains like normal.

 

[wooduser]

Perhaps there is initially a low draft, until the chimney has heated ? Is the chimney in a cold chase or inside a heated space ? Once the chimney has heated and is sustaining, it is the motor that sucks the exhaust out and the fresh air in. I dont have a door that is open-able on my stove, but if you do you could try just heating the flue with a blowtorch for a few minutes and then close the door and see if the stove lights and sustains like normal.

This is a direct vent gas fireplace. No warmup period to get it to vent should be required if it is installed and operating properly. There is no motor to force combustion gasses through the fireplace ---- the direct vent system is gravity operated.

It sounds like you have a different model fireplace. You might want to start a new thread and post the make and model of your fireplace along with a description of these issues ----if it's not operating properly, perhaps we can point out some ways to get it to run properly.

 

[KeithO]

Wooduser, no chimney of any description is free from the laws of physics. Be it stove pipe, class A, B vent or direct vent. If the appliance and chimney are cold, it will take some time for combustion to displace the slug of cold air in the chimney before the chimney is capable of operating by gravity. The original poster says that if he relights the stove then it will eventually sustain.

In the same way that people with woodstoves who vent through the wall and then have the vertical portion of the chimney outside in the cold have difficulty getting draft established, one will have a similar effect if the "fire" is from a natural gas or propane burner. The only difference is that the gas stoves can detect the flame going out and shut down the gas valve. Whereas a wood fire might just smolder and back puff smoke into the house.

And, just so we are clear, in any "gravity type" chimney it is the DRAFT which powers the flow of air to the fire and the release of the combustion products to the outside. Draft doesnt exist until warm air actually makes it out the end of the stack and reaches a critical temperature differential to ambient. If there is adverse pressure due to wind and roof shape, that only makes thingsntake longer.

Wooduser you need to quit with your snap judgements when you have neither read, comprehended nor understood what someone else has said. You could also reconsider the tone of your posts. Its neither helpful, nor encouraging to new members.

In the last week you have accused a user of not knowing how his stove was vented, when he stated it was vent free in his first post. It turned out you were ignorant about the VC vent free stoves but it didnt stop you getting beligerent about it.

You accused me of making so many modifications to my stove that cause could no longer be acertained, when I had made none and demonstrated that the stove operated fine on propane but wouldnt operate at the NG pressure of 3.5"wc. Not a surprise once one compares how the gas valve for propane is set up compared to NG 11"wc vs 3.5" wc. Yes perhaps a conversion kit for the valve is needed before it will work with NG. Or replace the gas valve with one that is made for NG in the first place.

If your circumstances require an outlet for negative energy, dont vent on members who come here looking for advice. There are healthier alternatives available which will not poison relationships in this community.

 

[wooduser]

Wooduser, no chimney of any description is free from the laws of physics. Be it stove pipe, class A, B vent or direct vent. If the appliance and chimney are cold, it will take some time for combustion to displace the slug of cold air in the chimney before the chimney is capable of operating by gravity. The original poster says that if he relights the stove then it will eventually sustain.

Well Keith O, you suggested that there was a motor circulating combustion gasses. There isn't. You again suggest that direct vent gas fireplaces might require the use of a blowtorch to heat up a direct vent appliance to get it to vent. These are ludicrous suggestions.

It's quite possible for direct vent appliances to fail to vent and to snuff out the flames as described in this post ----I've seen it many times in the thirteen years I largely repaired gas fireplaces.

Direct vent appliances have an inner pipe that that vents out the burned combustion gasses and an outer pipe that brings in fresh air to be burned. If that inner pipe becomes separated at a joint, the fresh air can be contaminated by the burned combustion gasses, which can cause the problem described that can happen, especially with poor installation. Plugged up termination fittings or something else preventing the free flow of gasses into and out of the direct vent can also be the causes of this kind of problem.

But perhaps Myst 415 will be able to identify the issue he's having with his equipment and let us know what the problem might be.

I invited you to post the make and model of your direct vent fireplace, because if you do I will see if I can find the installation and operating manuals for your equipment to see what they say about how the fireplace should vent. But I see you haven't done that yet. I again invite you to do so.

 

[KeithO]

I said draft was the motor, and it is. I suggested that he could do a diagnostic test with a blowtorch to educate himself whether the problem relates to a cold flue. Your mind is closed if information is not presented to you in a familiar way.

 

[wooduser]

Once the chimney has heated and is sustaining, it is the motor that sucks the exhaust out and the fresh air in.

Keith O,


Well, your comment above misled me. When you said there was a motor sucking the exhaust out, I presumed you were talking about a motor.

I can see now that you were trying to give a different explanation for gravity venting of the fireplace, but it was an explanation that I found quite misleading. So we have resolved that issue.

<< I dont have a door that is open-able on my stove, but if you do you could try just heating the flue with a blowtorch for a few minutes and then close the door and see if the stove lights and sustains like normal.


Direct vent gas fireplaces are all sealed systems, so there are no doors to open to ply the vent with a gas torch. Never heard of that as a test, and it should never be needed to make a direct vent gas fireplace work. Unsealing the sealed system changes the venting of the fireplace completely.

I understand that you are trying to be helpful and to make suggestions on how to diagnose possible problems with Myst 415's equipment. I am doing the same thing.

I am glad to have you critique my comments and advice, and I will do the same for yours. I think you can see that sometimes asking questions about suggested advice can help clarify ambiguous comments. I make such comments myself sometimes, and I'm always glad to have someone correct them. And I make just plain mistakes sometimes as well.

I've also started a new thread on direct vent gas fireplace venting, featuring a link to an installation manual pointing out some issues manufacturers think are important in venting such equipment.

 

[KeithO]

If there is a bad joint inside the vent, allowing exhaust and intake to mix, how does that problem resolve itself after a few minutes and allow the fireplace to vent perfecty for the remainder of the burn ? I ask the same question on the suggestion that the vent may be plugged in some way ?

On my stove I posted a pic of the front page of the manual in my first post, not sure what more I can add ?

PS, the OP of this thread specifically mentions he can light the stove "if he cracks the glass" open. I just know there is no simple way for me to do that with my stove. Some stoves like Jotul seem to have doors that open.

 

[wooduser]

If there is a bad joint inside the vent, allowing exhaust and intake to mix, how does that problem resolve itself after a few minutes and allow the fireplace to vent perfecty for the remainder of the burn ? I ask the same question on the suggestion that the vent may be plugged in some way ?

 

[wooduser]

<<Installing the restrictor suggested by Keith O is a likely way to solve the problem, but I'd prefer to run this issue by a manufacturer's tech rep who would have a lot more experience with such issues rather than jumping to the conclusion that the vent restrictor is the way to go. Very likely it is, but I'd prefer to get another opinion on it before making the modification.



Both excellent questions, Keith!

The short answer I have is that the description of the problem suggests that the main burner flame is starved for oxygen when the main burner turns on, and actually gets snuffed out. That shouldn't happen and suggests to me a defect in the venting system along the lines I've already described.

<< Running it temporarily with the glass cracked will "get it started" so to speak and the flames will orange out and it will have no problem running for the rest of the period.

I know what causes orange flames, which is dust in the combustion air. But I don;t know if that's what Myst 145 is describing or not. Sounds like he may be doing the equivalent of your blowtorch suggestion to get the fireplace drafting. Perhaps the flames are still being affected by lack of combustion air, even with enough to keep the main burner lit.


The bottom line though is that the fireplace should light normally and stay lit, not be snuffed out. The cause of that needs to be determined and corrected.

Good comments on your questions above, though.

 

[Myst415]

I agree that it is likely due to the vent system. I had someone else install the vent system so I could just hookup to it and the gas line and now I may be regretting that decision. I'll get the dealer to take a look.

Thank you for the responses and suggestions.

 

[KeithO]

Myst415, could you describe the flue installation ? Does it have minimal vertical rise and a lot of elbows ? Is part of it in a cold space ?

When I bought my DESA stove. it came with a bunch of venting. I finally figured out that the vent had a 4.5" ID and the outside was 7" whereas the stove really needed 4x6 DV pipe. Perhaps the real reason it was sitting out under the barn lean-to was because it never vented properly and a half inch gap at the inner flue to connector at the stove must have allowed all sorts of mixing of exhaust and fresh air.

 

[Myst415]

It's basically a concentric vent with a 4" exhaust and 3" intake, the exhaust is heat wrapped and it goes out a traditional chimney, 15ft? Shouldn't have any elbows but it does have to come up over the smoke shelf? and up through.

Found what I bought:

(broken link removed)

 

[KeithO]

Ok, so its a dual 3" and 4" side by side flex pipe arrangement. Is the conventional flue in a chase on the outside of the house ? You have one ofnthe special adapters to go from concentric at the fireplace connector to the side bybside lines ?

 

[Myst415]

Ok, so its a dual 3" and 4" side by side flex pipe arrangement. Is the conventional flue in a chase on the outside of the house ? You have one ofnthe special adapters to go from concentric at the fireplace connector to the side bybside lines ?

It's concentric at the cap, side by side on a mounting plate at the fireplace. It runs up the chase and not on the outside.

 

[KeithO]

What I mean is whether the chase is outside the heated space of the home or in the heated space (the classic chimney in the center of the house) ?

Is the chase on the outside of an exterior wall ? That is the way most fireplace chimneys are built in the US.

 

[Myst415]

Outside of the house connected to an exterior wall.

 

[KeithO]

Its what is called a "cold chase". If the stove had been off for a while, the entire chimney system cools down to outside ambient temperature. If your gas valve gradually increased output of the main burner instead of just hitting it with full power, you would probably have less of a problem getting the draft going.

Does the fireplace allow you to adjust the gas flow rate (flame height) ? If you can, set it to the lowest output and see if you get a better result on lighting.

 

[wooduser]

Does the fireplace allow you to adjust the gas flow rate (flame height) ? If you can, set it to the lowest output and see if you get a better result on lighting.

That's an idea that's worth a try!

 

[Myst415]

Its what is called a "cold chase". If the stove had been off for a while, the entire chimney system cools down to outside ambient temperature. If your gas valve gradually increased output of the main burner instead of just hitting it with full power, you would probably have less of a problem getting the draft going.

Does the fireplace allow you to adjust the gas flow rate (flame height) ? If you can, set it to the lowest output and see if you get a better result on lighting.

So I can set it on low and I can get it to stay on turning it off intermittently for 2-3 times before the box warms up.

I just touched the exhaust pipe and its cold after being on for ~30 mins on low /medium. Intake is extremely hot. Seems like the flue is going out the intake, it does not have a factory intake damper installed, would adding a damper or a backdraft damper on the intake side help

 

[KeithO]

Sounds dangerous. Is the exhaust not wrapped and the intake bare ? Is there any way the 2 lines could have been switched ?

 

[Myst415]

Sounds dangerous. Is the exhaust not wrapped and the intake bare ? Is there any way the 2 lines could have been switched ?

Exhaust is wrapped, intake is bare. Like I said, works fine when the weather is a bit warmer. I think creating the draw out the exhaust flue is what's not happening. I'll try dampening the intake so as to not overfill with O2. I'll probably be calling the dealer over this for their advice too.

 

[wooduser]

Do you have an intermittent pilot ignition system, that shuts off the pilot in between uses of the main burner? If so, some such gas fireplaces allow the pilot to be switched on so that it continues to burn all the time. If you have that feature, you might try running the pilot all the time during cold weather. That would tend to keep the fireplace and flue warm and rwady for reliable use when you switch on the main burner.

 

[Millbilly]

So it sounds. Like you have a standing pilot gas fireplace insert? Do you have a model number? I would think it would be hard to get the liners swapped when they are two different sizes.