Hot Topics: Low Gas Fireplace Flame

gas fireplace with natural stone surround

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Original Post: Gas Fireplace Low Flame

brandonmckinney - Member

I have a direct vent gas fireplace insert. It is a Monessen Hearth BDV400, to be exact. My issue is that it has a low flame height. The tallest flames I can get are about five inches, mostly blue with an occasional yellow tip. My neighbor, who has the exact same unit, is getting much taller flames. This is the only gas appliance that I have in my home.

All the gas valves from the meter all the way to the fireplace are turned all the way on. There is nothing stuck in or blocking the outside vent. It otherwise seems to be burning clean and operating normally.

The fireplace does have a flame adjustment (low, medium, high), and that doesn't seem to influence the flame height at all. I believe the flame adjustment regulatory is working because I can hear it clicking when I turn the fireplace on or try and adjust the flame.

Any suggestions or thoughts?

NorthernPine - Member

My response below is based on this problem that has been present from installation. If it burned fine before and this is now a recent failure then I have a different path for you.

Typically low flame height is caused by one of three things. Either you have low gas pressure at the fireplace, or you need to restrict the venting to slow down the draft.

1) It is common to think that the gas pressure is fine when this only one appliance on the line or the meter is turned all the way up. But keep in mind, your incoming pressure may be perfect, but the manifold (outgoing) pressure may be low. Likely a valve problem. All gas valves will have two pressure taps, an in and an out. I would recommend purchasing a manometer and testing the pressure to both of those taps. When you perform the tests, the burner will need to be running. For NG, the gas pressure for the inlet usually ranges from 5"WC to 7"WC.

2) A direct vent fireplace is a co-axial system where you have a pipe within a pipe. The inner pipe will exhaust the gases out, creating a negative pressure resulting in pulling in fresh air from the outer pipe. Depending on how the fireplace is installed, you may need to put a restrictor in the venting or restrict the venting. When you restrict the venting, you are slowing down the draft, so your not bringing in as much fresh air.

The more air that is being pulled into the fireplace will lower your flame height and make it more blue, small fast-burning flames. The opposite of that, if you restrict it too much, then you have a very slow, tall, dark orange flame that will likely soot. So there is a middle ground that needs to be achieved.

3) Air shutter could also be open too far. Similar principal above. The air shutter determines how much air mixes with the gas at the burner orifice. If too much air is mixing, you get a smaller blue flame. Closed off too much, you get a taller orange slow flame. You may need to close down the air shutter just a bit.

brandonmckinney - Thread Starter

It seemed to burn fine/better before. In other words, this hasn't always been a problem.

NorthernPine - Member

If the flame height was acceptable before and just recently it has dropped, then I would look at the following:

1) With a manometer, test the incoming and outgoing gas pressure to rule out gas and valve.

2) inspect the burner itself, possibly a crack in it

3) log placement. if the logs have shifted, it will impact the flame height

4) inspect the cap outside if there is anything obstructing it. Snow, ice, etc.

mikephilryan - Member

The flame probably is low due to a faulty valve motor. There is a small servo motor that adjusts your flame valve. You will see the wires coming from the control center to a small black one-inch circle at the front of the valve system.

There are 2 small Phillips screws holding it in place. Remove the motor and observe its movements as you start up the fireplace. It should turn 6 or 7 revolutions on startup. Then it will turn back to setting medium after the full burner is on. When you turn off the fireplace, the motor will attempt to close the valve.

It should also respond to the 3 different height adjustments. If this little motor is not working as described, you will need to replace it.

It could also be the control center that is to blame. When the motor is removed, you can adjust the valve manually by turning the valve inside the casing behind the motor by putting your finger in the motor mount and turning the valve either clockwise or counter-clockwise.

It is not a gas flow problem. It is not a vent problem. You can do these tests without fear of compromising any gas lines. It is safe to do this.

PJmax - Group Moderator

Welcome to the forums.

Mike.... where did you get your information from, and which fireplace does it refer to?

The fireplace in question here uses a strictly mechanically adjustable flame level control... no motor.

gas fireplace diagram

mikephilryan - Member

The diagram you show is not the same as the one described by the owner. He describes a fireplace with remote 3 stage height adjustment. Your diagram? Is NOT the model he is inquiring about.

mikephilryan - Member

Note. This fireplace in question has an optional. Command Control remote system that is optional and is is not shown in the online manuals.

PJmax - Group Moderator

Thanks for the information. I didn't catch the remote flame adjustment in the original post.

mikephilryan - Member

Ok. I'm no expert. I happen to have the same fireplace with the same problem.

PJmax - Group Moderator

Since you had the same problem.... what did you end up doing? Replace the motor?

brandonmckinney - Thread Starter

I believe at least part of my problem is that my stepping motor is adjusting the flame height adjustment screw too far out. I can take the stepping motor off the valve and adjust the screw setting the flame height to high and it looks much more comparable to my brother-in-law's fireplace which is the exact make and model.

He lives across the street from me. Is there any way to adjust the stepping motor? I have tried manually setting my flame height to high and then electronically adjusting the stepping motor to think it is in the high position as well to create a baseline.

I then reinstall the stepping motor and it works for a couple of adjustments but eventually the flame height never goes back to high. Is there a different way to get the screw position and the stepping motor aligned so they operate properly?

Here is a diagram fireplace system configuration that I have as well as a close up of the valve body with stepping motor.

gas fireplace diagram

gas fireplace diagram

Here is a link to the manual.

brandonmckinney - Thread Starter

Thanks Mike, I did this and it is exactly as you described. This is my problem. It seems like my flame height could still be a little higher, at least compared to my brother-in-laws fireplace who lives across the street. I still have some "ghosting" that is going on. I will look into replacing this stepping motor. Could I still have a vent problem? The 3 internal vents are all clear and open. Is there another vent adjustment I can try to make?

PJmax - Group Moderator

I'm not exactly sure what is available for your adjustable gas valve.

I've always run my fireplace on high..... have never even used the lower settings.

SeattlePioneer - Former Member

Don't screw around with the gas input to the fireplace unless you have the instruments and experience to know how to do that task.

Guessing doesn't cut it.

So? Perhaps your brother-in-law's fireplace has already been turned up too high.

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