Commercial common venting

Henry Nichols

The manufacturer sent a reply to the engineer after I wrote out all of my concerns, stalled flue, CO, efficiency. They effectively took all responsibility.

What I do not get, I recommended venting two ipo four appliances and they said, sure, both will work but it is cheaper to stay with the common bank. I gave them a way out and they did not take it.

I called the engineer and asked, are you sure this is what you want? His response was, "it is all on them". We both have the manufacturers letters to prove it.

I do not wish ill on anyone, but this manufacturer is begging for problems.


Henry

Henry Nichols

4 x 3000

Gentlemen:

A little insight here would be appreciated. I have an engineer that is going to vent 4 x 3000 mbh modulating condensing boilers into a common 22" flue.

I have had a stack manufacturer run the numbers and with a single boiler input at 30%, a 14" diameter stack would only produce a velocity of 3.76 ft/sec, Apparently their program will not take a 22" stack into consideration.

The manufacturer agrees that exhaust on a condensing boiler is volatile, but the fan on their boiler will overcome the issues of lazy flue. Forgive me for asking, but if I turn up the fan, I believe the O2 will go up considerably and the efficiency will drop like a stone. The other option would be to force the boiler into a greater input,the net result their is short cycling.

To top it off, the local rep thinks I am just trying to get in his way since we compete on boilers. I have not told him yet, but I am trying to protect him from going ahead with a system that will not be efficient or safe (My opinion).

Am I missing something? If anyone has successfully vented multiple, large (3000 mbh) condensing boilers into a common vent, I would like to know what the flue gas velocity was and what your percentage of O2 was through the firing range.

Thanks, Henry

Jim Davis_3

If the boilers condense, the flue condenses regardless of the velocity. If the flues are positive pressure they have to be sealed at the joints or moisture will leak through and if an occupied area, flue gases will also come out. Obviously the flue must be some type of non-corrosive stainless. The biggest problem I see with the fans is if they can create that much velocity they most certainly blow flue gases out the idle boilers. Can't believe that type of installation would be legal. Certainly isn't mechanically sound.

JackFre

You might want to run this by

Tjernlund. They are making a modulating exhaust system called the Auto-Draft. It will modulate with the boilers to create the design gas flow thru the vent system. We have found that the reduction in flue size and therefore cost savings can pay for the system. they are at 800 255-4208. www.tjernlund.com.

Brad White

Other Players

There is another player not in the mix: The Manufacturer.

Some but not all manufacturers condone grouping their ModCons. Hydrotherm KN is more tolerant than say, Viessmann for example.

Viessmann has allowed me to combine two Vertomats (now Vitocrossals) in the past but that was after some investigation on their part.

Aerco Benchmarks are in-between. All situations are different.

The part I am most concerned about is the vent pressure controls and the wide gradient of pressures in a combined stack with modulation on top of that.

The manufacturer simply must chime in on this.

I agree with Jack and Jim in principle. Jim's point about condensing in a Class IV stack is well taken, what would the problem be in that regard.

Jack's suggestion about the use of Tjernlund (or Exhausto) draft inducers may be a solution but only if the manufacturer has their say.

My default design practice is individual vents all the way. Is that a possibility?

Henry Nichols

> There is another player not in the mix: The

> Manufacturer.

>

> Some but not all manufacturers

> condone grouping their ModCons. Hydrotherm KN is

> more tolerant than say, Viessmann for example.

> Viessmann has allowed me to combine two Vertomats

> (now Vitocrossals) in the past but that was after

> some investigation on their part.

>

> Aerco

> Benchmarks are in-between. All situations are

> different.

>

> The part I am most concerned about

> is the vent pressure controls and the wide

> gradient of pressures in a combined stack with

> modulation on top of that.

>

> The manufacturer

> simply must chime in on this.

>

> I agree with

> Jack and Jim in principle. Jim's point about

> condensing in a Class IV stack is well taken,

> what would the problem be in that

> regard.

>

> Jack's suggestion about the use of

> Tjernlund (or Exhausto) draft inducers may be a

> solution but only if the manufacturer has their

> say.

>

> My default design practice is individual

> vents all the way. Is that a possibility?

Henry Nichols

Common venting

The ME on the project has thrown his lot in with the manufacturer who has assured him that it will work. They are standing behind their design. I have called several other manufacturers since I have relationships with many and I cannot get anyone to say, yeah, that is how we would do it.

The part of this that I am chewing on now is that the manufacturer has told all parties to increase the size of the common air vent going to each boiler, flood it with the burner fan. I fear that along with the lazy flue they will also have carbon monoxide formation. Recipe for disaster.

Since theis is plan and spec, there is no room for the exhausto, the job is what is on the plans. I am just not comfortable with the design.

Brad White

I think you should

state your case in writing and if you feel strongly enough about it, to your attorney. Not being defensive or litigious, just due diligence.

Your ME and manufacturer stand behind it, get that in writing too.

Your own statement might say that you disagree in whatever terms and are installing it "under protest". You can always decline to do that part of the work but that raises other issues.

The lazy flue aspect for CO production? Once the burner has done it's thing and maintains it's own draft and mixture within itself and once the gasses are gone, I do not see how that can affect CO production. Happy to learn otherwise.

Paul Fredricks_3

A few years back with a previous employer we had a customer who had multiple 1MM btu gas boilers common vented. They weren't condensing unit per se, but the combustion gases would condense in the breaching and run into the boilers that weren't firing. The units would would fill up with water and shut down. We did some research and found that the install specs had been changed and a lot of repiping was required. Those in charge didn't want to spend the money. Long story short, a couple of kids went to the hospital with CO poisoning. It could have been worse. The whole thing ended in a huge lawsuit, but we were OK as we warned them in writing of the impending danger.

As I remember, those vented into a chimney were OK, but those that were direct vented were the problem. Bottom line: Go with what the manufacturer recommends and have them sign off on it.