Condensing boilers direct-vented exhaust a concern?

D107

May be installing an oil or gas MOD or gas MOD/CON boiler for my mother's house. She has expressed concern about the through-the wall-exhaust emissions. We know that there can be some smell--especially for oil-fired. Present plans call for vent to be about 9 feet from a property line (though we could run a longer flue in the basement and have it facing a different direction into our backyard.)

Buderus has been very helpful with their technical explanations, and suggests that in some cases flues are run up through the roof, which we would not want to do. I know that chimney flues for these boilers pose problems but perhaps there is a liner setup that would make it possible.

I am curious as to what the exact chemical composition of this exhaust might be. I know that these vents have long been permitted in most municipalities, but so was unfiltered car exhaust before they realized the potential dangers. I gather that exhaust from MODS or MOD/CON boilers is not considered more toxic than atmospheric boiler exhaust; the height is the issue. I also have read on the Wall that certain kinds of oil burn better but may not be easy to get.

I do know that this exhaust would be emitted during the cold seasons when people are inside, so it's not going to spoil anyone's barbecue. Is it possible to run the pvc flue pipe higher up the side of the house--have to be heavily insulated,yes? to prevent the exhaust from condensing before it leaves the pipe?

Thanks,

David

Nick L. in Vt

venting concerns

be very mindful of manufactures specific directions. avoid venting close to max specs and allow some lee way. these are class 4 appliances and there is serious potential for bad "stuff". talk to a pro.




They call me "Hot Pipe"

D107

Yes, a pro would be installing this if my post wasn't clear....

Thanks Nick. The advice I've gotten so far is that --for the modulating but not the condensing--if there is a chimney, better to line that with stainless steel and use that for the flue. Main reason being not the emissions but the fact that a direct vent requires a set of systems/relays/devices that if one of them fails the boiler cuts off. True the chimney liner will probably cost more but it seems worth it for piece of mind, and it may be required code in the town, especially for gas.

Condensing would have to be direct vented, and since the existing rads on the house are 1956 convectors and no radiant heat installation is planned, it's not clear that we would get the value of the low supply temps that result in all that savings.

Thanks,

David

Tony_23

Condensing

Get the mod/con. Design temps are hit less than 5% of the season. Contrary to some peoples' stubborn beliefs, bbd and convectors don't HAVE TO run at 180 all the time.

My bbd retrofits with W-M Ultras average 40% les usage, with one that now consumes 1/3 of their previous draw
THEY think it was worth it

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D107

so as I understand it, a mod-con's efficiency at 180 deg

supply temp is only 88% which would be in effect when it's closer to the outside design temp. So if it was warmer, let's say 40 degF then, might the boiler modulate down to maybe 140 deg, then with a Delta T of 20deg. on the returns, would that 120 degree return water be sufficiently 'cool' enough to activate the condensing feature and save on fuel usage by being around 93% efficient or higher?

Thanks,

David

Cosmo_3

please clarify

"I know that chimney flues for these boilers pose problems but perhaps there is a liner setup that would make it possible."

A chimney is exactly what the doctor ordered!

maybe that was a typo though

If using oil, maybe you should go with the Buderus direct vent. The burner on this boiler (Riello BF-5) I believe has a post purge cycle, in other words after the burner satisfies a burner call, the oil valve shuts flow to the burner to stop the flame, and the fan keeps going for a preset time (usually 90 seconds or so) to 'purge' the combustion chamber of the offensive smell.

My current office is actually located next to my mechanical room, and I always loved the smell of oil until I started spending a lot of time at my desk. So I purchased and installed a post purge control for my burner and voila...no smell!

As a side note, I personally like to avoid using direct-vented oil burning equipment if possible. An oil burner has a nozzle, which is a tiny orifice that the burner's oil pump pushes the oil into at about 140-150 psi to atomize the oil, or another way of putting it is to make millions of little oil droplets. By doing this the oil ignites much easier, and cleaner. Just like the fuel injectors in your car. Well in the winter when the air outside is cold, the cold air being blown over the nozzle spray makes it that much harder to atomize the oil in my opinion because the cold air cools the oil and raises the oil's viscosity a little (think of maple syrup, when it is hot it flows very easily, but cool the syrup in the fridge and it flows very slowly). Some boiler manufacturers like Viessmann do not sell direct vent oil burning boilers for this very reason (although I hear that they have one in the works that actually channels the incoming air around the boiler under the insulated jacket to preheat the air). I have installed Buderus direct vent boilers w/ the Riello BF-5 burner and have not really had a problem..... it is just my preference not to. so.....there you go!

A gas fired modulating/condensing boiler like the GB142 is a good step in the direction you are headed I think. This boiler is a great product, by a great company.

Pick a good contractor that follows manufacturer instructions and you will be happy with what he recommends after seeing the job, we unfortunately can not properly recommend anything specific to your job because.... we haven't been there!

Cosmo Valavanis

Cosmo_3

ok

If you use oil;
Chimney- if in good repair with a tile lined interior (terracotta), and it is sized properly you can by all means use it. If not then yes, line with the correct size stainless steel liner.

Smell- A chimney would be good because it helps create a draft in the mechanical room to change the air more often. But you can also install a direct vent boiler, which does not use a chimney, unless you route the exhaust up through an existing chimney, and bring the incoming fresh air for combustion from the side of the home, which some Manufacturers may say is not good because the exhaust vent is then not "balanced" re- the exhaust may experience different outside atmospheric pressures from the wind than the intake which may be installed on a different facing wall. This can make tuning the burner difficult because you cannot compensate for the changing wind patterns without a computer to monitor the exhaust stream and adjust the fuel/air mix with a standard oil burner. A Vitodens con/mod burner can do this however! OK, confused yet?

If you have radiators, then a good professional can give your home a once over, and calculate how much heat your home needs, and what minimum temperature is required to heat your home at design (coldest) outside temperature. From experience your radiators, barring any additions since the rads were installed, have a good chance of being a little over sized. But to tell you the truth, with radiators, I would not hesitate recommending the gas condensing/modulating boiler like the GB-142 over the oil boiler. But it depends on what else the boiler will be heating, like domestic hot water, fin tube baseboard, fan coils, etc.

Use the Find a Pro feature on this site to find a good contractor to help you!


Cosmo Valavanis

Tony_23

Yes

That's pretty much it, except it's not just the return temp that counts. ODR will take care of supply temps. You'd be surprised at how cool water can seem to be and still do the job.

The other factors are modulation and heat transfer coefficients of the HX material. Aluminum is MUCH faster than SS or CI.

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