Common Venting of Condensing Boilers

Jack

You state, "Now we know I will certainly not get a blessing from Viessmann on this one, there is nothing in there literature that but I am thinking that says I can do this, but..."

This is the perfect nightmare scenario. Maybe it will work, but is it right, long term? Build it to the manuf specs and you have their support. Do otherwise and the problems may go up and down the line. My suggestion would be to follow the manuf specs, make your profit on the job and leave with a system that you have confidence in and can defend your decisions.

Common venting is almost always a compromise even when allowed. You end up with reversion issues that may have subtle effects but long term can impact the gear. I'm assuming that the reason you want to put in two units is to stage them. If so, and the 2" is the right size for the one unit you are going to pump it into 3" which has almost 3X the cross sectional area of the approved 2". Is the single unit operating alone 80% of the time going to be happy with this?

If it works well, then good for you, but this is also how you create PITA jobs for yourself, the manuf and your client. Ask me how I know;)

Man with a question

2 Boilers 1 Vent

We have a job that originally called for 1 Munchkin 199 Boiler. We installed a 3" CPV exhaust and 3" intake.

I would like to change the spec to 2) Vitodens 100 8-30 boilers, These are 100 MBH units.

I'd like to vent the pair out of my existing 3" intake and 3" exhaust. The boilers would be vented in parallel into a single 3"

Now we know I will certainly not get a blessing from Viessmann on this one, there is nothing in there literature that but I am thinking that says I can do this, but...seeing as each boiler has its own blower and that the "standard" for a boiler of that output is usually 2" I am think I will not have much problem.

The boilers are staged and will almost never be running both at full capacity.

Anyone have any experience with this sort of thing?

The total developped length of the vent is about 40'.

Brad White_202

The issue

is usually one of pressure switches, which prove draft but also lock out the burner when interrupted. These are sensitive devices which is why dedicated venting is the standard.

Now, I have vented Viessmann Vertomats (now VitoCrossals) into a common flue but that was an exception and was engineered by Viessmann for that application.

I don't think you will fine many voices saying "go for it!" when the manufacturer says not to.

Man with a question

You've got a point

It could very well lock out on the pressure switch.

Though the draft from the first boiler will always be pushing up and out toward the least resistance and away from the second boiler.

Of course I won't really know what happens until I turn them both on and find out.

Ted_5

It is done in Germany

I wish they would do it here, why not? I have seen the commen venting system and they have like a ball in the pipe that floats to act like a check valve. The problem you would have with out a check would be exaust going into one boiler from the other. With out vent adaptation like the 200 has, I see you having issues. I have seen a manufacture that has a common venting system here in the US. That is Hamilton Engineering and the boiler is called EVO.

Ted

Paul Pollets

Proper venting

Vent the boilers per the manufacturer's instructions! If you don't, you'll likely void any warranty. Not to mention create a potential hazard. Talk to their technical reps, if you have questions.


Viessmann has an entire booklet on venting the Vitodens boilers, with all scenarios included. It's available on their website, if you don't have a copy.

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Man with a question

End of Issue

After a brief cost/benefit analysis and much good input from Wallies....Installed a new pair of vent pipes and vented the boilers according to mfgrs specifications.

Could not see any good reason to do otherwise.

Thanks All.