It's a tempting idea, I agree.

However, here is one major problem with it: if the greenhouse is fairly tight, then the fresh-air intake on your device (that vent is not just venting, it takes in outdoor air to feed the combustion of the device) will start sucking in air with an insufficient level of oxygen and either the device will shut off or there may be a dangerous situation.

The second problem would be the dangerous air quality in the greenhouse for animals and humans.

You could try extending the double-wall pipe through the greenhouse, to act as a heat-exchanger, but it's likely that the blower-fan on the device will not accept the extra job and via the air-pressure sensor shut off.

You might want to post this over on the Greenhouses forum here for input from the GH experts.

Dave

Don't have to be a GH expert to know that any such maneuver will void the unit warranty if there is a problem, and unless the volume of the GH is very large indeed, there will be.

I like your thinking, but after consideration, I don't think this is a good idea.

Forgetting about the greenhouse for a moment, you don't want back-pressure on your boiler exhaust. The boiler might not function correctly. And it would almost certainly void the warranty and/or violate code to install a greenhouse as part of your exhaust system.

My boiler exits through the roof. I don't know if yours does, but you definitely do not want to re-route the boiler with any long horizontal runs or, worse yet, a downward leg, because condensation will build up in the duct. It has to have some way to drip out or drip back down into a chamber in the boiler. It is not a small amount of condensation. The exhaust contains a lot of water vapor.

Finally, the air quality in the greenhouse would be possibly dangerous, although this depends on ventilation and what kind of fuel you use and so-on. I don't think C02 will be a problem, but I would worry about carbon monoxide.

If your boiler is 96% efficient, it is already extracting about as much heat from the fuel as you can get.

Hope that helps.

--McKenzie

He says it is a wall vent. And it may be a condensing boiler, in which case condensate in the vent is near-zero.

But hopefully he understands now that this is a bad idea.

then the fresh-air intake on your device (that vent is not just venting, it takes in outdoor air to feed the combustion of the device) will start sucking in air with an insufficient level of oxygen and either the device will shut off or there may be a dangerous situation.

Actually, there are two separate pipes -- both fairly thick -- one for intake, one for exhaust. Does that change the verdict?
The second problem would be the dangerous air quality in the greenhouse for animals and humans.

Neah, not scared of this part... Will lock the door, when I'm not there and keep it open, when I am.

Doesn't matter, it's the same deal, they will both be within the greenhouse unless you put the wall in that small space between the two pipes. Chances are in any case the back-pressure will be changed enough to cause the pressure sensor to default to shut off the burner, unless the structure you build is very air-leaky, in which case there is little point to it.

Just think about it: the fresh air is sucked into the machine and blown right out again, it has to move freely. If you build a tight plastic balloon around the outlet, you are making the machine inflate a balloon every time it runs. Either the unit will not function or the power vent will fail prematurely.

doofus, call the manufacturer to see if they have any thoughts about what you are wanting to do and also you won't be guessing about the warranty issue. That might be something they might be willing to help you with. Just a thought.

CO2 sinks in relation to normal "air" and without air movement can build to rather heavy levels.

You don't want to pump a lot of it into a closed system (a greenhouse that's going to remained closed most of the time).

Passing out in there, whether it be from a lack of oxygen or an unrelated medical issue, could be very dangerous.

The only way that I can think of using the exhaust vent to heat the greenhouse and still maintain safety (breathing quality) in the greenhouse and allow the boiler to operate correctly would be to use the exhaust to heat the greenhouse indirectly.

My idea would be to extend the exhaust pipe from the house into a metal barrel that's inside the greenhouse. The barrel itself would then have an exhaust pipe leading out of the greenhouse. The exhaust would not be inside the greenhouse because of the barrel vent pipe and the natural flow of air would still be allowed to occur, thereby preventing backpressure from causing problems. The metal of the barrel would be acting like a radiator to gently heat the air inside the greenhouse. But even this idea is iffy.

Rodney

Chances are in any case the back-pressure will be changed enough to cause the pressure sensor to default to shut off the burner, unless the structure you build is very air-leaky, in which case there is little point to it.

I strongly doubt, any greenhouse can hold difference in pressure, that's higher than normal weather-induced variations, don't you? There are no airlocks to pass, when entering a typical greenhouse, and many don't even have normal double doors. Does not that mean, the boiler should be able to deal with it -- without breaking a sweat?
The only way that I can think of using the exhaust vent to heat the greenhouse and still maintain safety (breathing quality) in the greenhouse and allow the boiler to operate correctly would be to use the exhaust to heat the greenhouse indirectly.

Oh, but it is not just the heat I wish to capture -- plants like CO2 and here it is, free, escaping into the atmosphere...

Well it seems you're dead set (no pun intended) on doing it how you want. Despite everyone's legitimate concerns about safety.

"I strongly doubt, any greenhouse can hold difference in pressure, that's higher than normal weather-induced variations, don't you? There are no airlocks to pass, when entering a typical greenhouse, and many don't even have normal double doors. Does not that mean, the boiler should be able to deal with it -- without breaking a sweat?"

As pnbrown has said, a good greenhouse is a tightly enclosed space. Venting the pipe directly into the greenhouse could have the same effect as capping the pipe. Sure, there might be some minor air leaks in the greenhouse but those minor leaks will not be enough to allow excess pressure to escape. The air pressure will build in the greenhouse and the boiler could potentially choke itself out.

"Oh, but it is not just the heat I wish to capture -- plants like CO 2 and here it is, free, escaping into the atmosphere..."

Are you willing to risk your own life for the health of your plants? You say that you'll leave the door open when you are in the greenhouse but that would defeat the purpose of using the vent pipe to heat the greenhouse in the first place. Since by leaving the door open you are not only letting the CO2 escape but also all the heat you've built up. And you wouldn't be able to just walk inside whenever you want. You'd have to keep the door open for a bit beforehand to allow the CO2 to drop to a safe level before you go in. Also, how would you even be able to know when the CO2 dropped to a safe level? Unless you have an air monitor with you, then there's no real way of knowing and every time you go in will be a gamble.

Rodney

You are not taking the carbon monoxide threat seriously. If you insist on doing this, I would suggest that you put a carbon monoxide alarm inside the greenhouse.

I have no statistics, but venting any fuel burning appliance directly into living space is considered unsafe by people who write building codes. As long as everything is working correctly, I think there is little danger. I am not an expert, but I think that if the air intake is messed up or something, it is possible for the combustion stoichiometry to get screwed up and then maybe you could get carbon monoxide (or other partial combustion products). Carbon monoxide is lethal in fairly low concentrations, has no odor and is just as clear as air.

To solve the back-pressure problem, maybe you can put a flapper valve in the green house, similar to what some clothes driers use at their exit point. That way when the boiler is running, if there is positive pressure in the greenhouse (and really, there should be, because it should be well sealed to prevent drafts) the vent will open relieving the pressure. But when the boiler shuts off, the vent will close maintaining a reasonably good seal. My boiler tests for combustion pressure every time it turns on, and if there is a fault, it will abort the ignition sequence. A well sealed small room has a lot more air restriction than a 4 inch PVC pipe open to the atmosphere (which is how my unit is vented).

As far as I know, carbon dioxide is not considered a health problem. Your body can tell when CO2 is high and you will feel very uncomfortable, like you can't breathe. Then you will leave the room, and then you will be OK.

Again, I think you should not do this. But if you do, please put a carbon monoxide alarm in the greenhouse.

--McKenzie

In short, it is not a workable idea:

-- letting boiler/heater exhaust into greenhouse has more than one problem :
==malfunctioning boiler(intake and inadequate pipe height to create proper draft). Chimney stacks require a minimum length in height to function by natural convection.

== Even if you just could vent the boiler exhaust into greenhouse. you need to provide an exhaust and fresh air intake. Otherwise it will choke the burner.

==extending and passing just the exhaust pipe through the greenhouse (just to utilize the heat from the pipe), may also cause excess cooling and condensation that may end up inside the boiler. one pound of fuel generate approximately one pond of H2O in the form of vapor. If the original length is 10ft and you increase it to 20ft(to extract more heat), other than condensation problem, will also create too much pressure and thus malfunctioning of the boiler.

Doofus, you are being quite eponymous :)

Doofus, you are being quite eponymous :)Picking on usernames now? How like my numerous "eponyms"...

Carbon dioxide is indeed a toxic gas when concentrated and confined. It can kill rather quickly. That isn't to say that the exhaust would be pure CO2, but it's not a threat to ignore.

The question of the back pressure on your furnace is probably the more immediate concern.

I didn't mean to imply that carbon dioxide won't kill you if you breathe it (I think it throws off your blood pH, which will cause coma/death). I just meant that high concentrations produce extreme discomfort and distress, so you will probably leave the room before any real harm is done. Another way to look at it is this: your body produces CO2 and disposes of it through respiration. So you have response mechanisms for CO2 in the lungs.

I have never been in an environment with high CO2 concentration, so this is all based on what I have been told, not first hand experience. But it also makes sense, physiologically.

But your body has no way to deal with CO. Sometimes you don't notice anything. You just pass out. Then, of course, once you are passed out, there is a good chance you will die unless someone drags you out of the room.

--McKenzie

Go for it dude, it's a free country. Please report back though on the results.

"Go for it dude, it's a free country. Please report back though on the results."

Or ask a survivor to do so

Similar discussion here:

Here is a link that might be useful: Greenhouse heating from boiler flue