More Draft With Damper Closed

JStar

Utica MGB100HID



Boiler had a leaking section covered under warranty. While I was there, the customer had previous complaints about an "exhaust smell" so I start checking draft. With the boiler running, I have -0.005" WC. When the boiler shuts down on temperature, the vent damper closes and my draft goes up to -0.03"WC.



There is a 6" flexible liner approximately 20 feet in length. Could it be oversized? Too short? My charts have it looking acceptable. 100K boiler with a 40K water heater.



When the damper closes. is it just accelerating the draft that's there already?

Jim Davis_3

Draft problem

Anytime draft increases when an appliance shuts off is a sign that the flue is restricted or undersized.

Tim McElwain

Where are you taking

the draft reading? Is it before or after the vent damper? I can't remember but does the MG have a built in draft hood?

Jim Davis_3

MG Barometric

The MG is just a double acting barometric, no built in drafthood.  It doesn't matter wher the draft reading is taken in this case. draft should never increase when the burner cycles off.  That is one of the protocols in my class for testing for undersized flues, liners or restricted flues.

furnacefigher15

make up air

In order for draft to happen, there needs to be air to fill in the vacuum.

JStar

RE

Utica MGB has the front-style draft hood, or breech. The vent damper is right on top of the boiler. Draft readings were taken immediately after the damper. There is no barometric damper.



I get the same results whether the boiler is fired or not. I can cycle the damper open and closed by tripping the relay.

Steamhead

Might be normal

the chimney is attempting to pull flue gases up, and if the vent damper is closed nothing can move into the chimney. So the measured draft reading would be higher when the chimney is pulling against a closed damper and the draft gauge/digital analyzer.



When the damper opens, air or flue gas can enter the chimney and the measured draft reading would go down.



Are there any other appliances on this chimney?

furnacefigher15

Connector rise

How much rise is there out of the boiler before any 90's ?

Tim McElwain

Jim MG is referring to

the model of the boiler. This boiler has a draft hood with a vent damper??? I am not sure what you are talking about?

JStar

RE

I'm thinking, as well, that this is just a non-issue phenomenon. The draft isn't really rising. The perceived strength of the draft is rising. It's all relative?



There is a 90 right on top of the vent damper, one foot of pipe, two 45's, and 2 feet of pipe to the chimney. Very limited space to make any changes.



So, my initial draft is still too low. I think the chimney is on the same horizontal plane as the roof from the higher part of the house. It's a split level, and the chimney is on the outside wall of the lower (middle) level.



There is a natural-draft water heater as well. Get the same readings at the hood.

Jim Davis_3

MG

Yeah, I wasn't quite reading that correctly on the MG.   However there is still a big concern because most boilers will not vent 100% with a .005" draft. 

An additional test would be to turn on the water heater first and watch the draft while it is running and then see what that draft does when the boiler is turned on.

If the draft in the water heater goes down the flue is too small or restricted.  If the draft stays the same, there is a possible combustion air problem.  If the draft goes up, then the draft measurement of the boiler might be in a bad location.

Draft doesn't indicate venting, one way or the other.  The combustion numbers do.

If O2 and CO stay the sane after 3 minutes and don't change everything is okay.  But the slightest rise in CO after 3 minutes, even 1 ppm is a baed sign.

 

Tim McElwain

The only reason

you would need make up air is if there was mechanical exhausting taking place in the combustion zone, such as a dryer. If you meant air for combustion that is different than make up air by definition.

Charlie from wmass

I had a similar issue

a customer had a liner installed on a Buderus and I was scratching my head as the draft was not consistent, Some days it spilled others it ran fine. I even wrote in here looking for help. Turned out the 6" liner the chimney guy installed was a 5 1/2" liner. also turned out there was nothing wrong with the original chimney aside from the tiles the chimney guy broke out to install the liner. The end of the story was liner was removed, chimney was repaired, and the boiler runs fine now. I would double check the liner size.

furnacefigher15

Semantics

Tomato, Tomaaato.



Combustion air, make up air. I don't care what you call it, if it causes a vacuum, the vacuum needs to be broken by the addition of fresh air. Way, way too many people, engineers, and service professionals over look the fact that modern homes are much, much more tightly constructed then old ones.



If the chart says this size boiler and dhwh can vent properly into that chimney size at that height, the next logical step in my mind is fresh air. For draft, make up, combustion, all of the above.



Negative pressure venting operates by a simple formula. Thermal buoyancy.

furnacefigher15

Many variables

http://www.hartandcooley.com/Libraries/Product_Literature/Flexi-Liner_Sizing_Card.sflb.ashx



I took anther look at the chart with the chimney and appliance specs you provided.



If the connectors are all single wall, you don't have enough btu's going into the chimney to keep it hot.



May need to upgrade to double wall connectors

Tim McElwain

As one who teaches

I find it important to get things called by the correct terms. Make up air and combustion air are two different things both by definition and by actual air requirement. It only causes confusion in that make up air is typically a single opening (sometimes provided mechanically and also it may be heated or cooled) matched to the CFM of air removed from a building by mechanical exhausting.



Air for combustion is typically two openings (there is one formula which allows for one opening in the codes) designed specifically to provide primary, secondary and excess air along with dilution air to satisfy the needs of the equipment that along with the air required to keep the equipment cool. The determination for this combustion air is related to the rule for "confined spaces". There are some exceptions when using the KAIR method for air based on air changes per hour of the dwelling.



The air requirements also change when we have fan assited equipment.



Propane has to allow for more air than natural gas by the way.





I continue to find this process one of the most neglected things in a combustion zone and many times even though some kind of opening has been provided it is not adequate.





This business of correct terms has caused problems here on HeatingHelp in the past so I try to help out with correct terminology. Sorry if I seem a little anal.

JStar

RE

When I first tested, I believe it was a rather warm day (can't believe I'm saying that in December). I want to revisit after the new warranty boiler is in place. Hopefully the outdoor temperature is a bit cooler, and I can do some CAZ testing.



I may even take some pictures.