Tankless, gas water heaters. Sanity check and kind of a survey

I've had a Bosch LP tankless water heater for a decade now.
What's always bothered me is that the ventstack is open to the sky (If I look up under the burners I can see a bit of skylight through the works (its a straight shot from burner to outside)

Yesterday it got windy so i took a stick of incense and lit it. Sure enough the smoke gets drawn into the unit. (Of course it does right?) I thought maybe there was some invisible magictech I wasn't aware of that wouldn't allow that.

How does that jibe with being green? Did the installer screw me and not put a proper (ie airtight??) cap on the stack? I know when its in use it needs to open, but still this just seems really cross-purpose.

If that's the way its gotta be why do people think these are good appliances? Is there any solution to this?

The efficiency is, as was explained, that you are not continually maintaining a tank full of water at your selected usable temperature.

If the design was what you believe to be efficient, failed, would that continuing expense for maintenance fall within you perception of an acceptable, in the broader spectrum of its operational efficiency?

Professor Irwin Corey couldn't have said it any better..........

On a serious note. I installed a new hot water tank the other day although it has 50 gallon of heated water in it the vent stack hasn’t improved over the units from 50 years agothe stack is connected to a cone that sits above the vent pipe and has a constant draw up the chimney. The outer shell around the tank is so well insulated it always feels cold to the touch so I don’t think having that preheated water waiting is wasting much energy. The energy spec is based on how much energy it takes to heat the water. I don’t think any energy is factored in for heat loss in the house.


They do make new units that are higher efficiency and have blowers like the new furnaces that take more heat out of the exhaust gasses. They also cost a lot more and require power to them to run the blowers and such. I haven’t looked at tank less models to see if they are doing that yet or not.

The efficiency of on-demand device vs. tanked device isn't the issue -- its the hole in the roof. Yeah I'd have to say that hole is a failing. Tough to say whether its still better/more convenient than a standing tank.

When talking about tankless everyone notes the pluses (no 24/7 maintenance of hotwater etc) mitigated by the minuses (a bit of lag before there's hot water at the tap etc), but no one ever seems to mention the resulting hole in your roof and the extraordinary loss of heat during heating season. And you're right Sb, it is a perception. People want to believe its better and there's a good chance it is. Consider going away for a week. Some people will turn off their tanks. I'm guessing more than 50% don't. But you got me wondering (not that I've even looked at this point) if there's a study that looks at the relative merits in a controlled manner.

while I was writing this, Bud16415, your last resp. came through. The good thing for the 50gal babies is that they're usually in the basement or some other area not requiring ambient comfort.
and they gotta vent too, like you said, with that cone and (narrower) stack. Tankless , for convenience and (better) efficiency, are often right there in the living area with you.

At the time of install, there was guidance on if you're going to vent sideways you'd have to have a powered (electric) vent. I purposely got a unit that doesn't even require electric ignition in favor of a H20 impeller. I wasn't in going to trade that for a, obviously, more demanding powered vent.

So, the green question remains: is this the only way a hang-on the wall tankless heater can vent? From what you said it seems: Not. Maybe something mechanical like an old pull doorbell -- or fireplace damper! There ya' go.
I'd consider one of those kitchen vent flaps that birds use as front doors. I suppose another possibility is to change the doors on the closet where the tank hangs to solid, from louvered, and then weatherstrip. Oh sure, you've gotta remember to open the doors so you get proper combustion -- but after a couple times wondering where the hot water is or, what that funny smell is, you'd get in the habit... We really do have it too easy as fossil fuel guzzlers.

The oblivious answer is electric, and as all green minded people know things like electric cars make zero pollution. We know that because they often use the word zero a lot when describing them. but there is that place burning coal or gas on the other side of the mountains making the juice because God forbid the power plant be close to the consumer. We then loose a high percentage of the energy to transmission losses but again we don’t care because they are before our power meter, and we pay for it like it was made right outside our house.


I keep a hot tub with 400 gallons of water heated to 103 all year outside. The big losses are when I open the covers but there are mild losses all the time worse in the winter.


Some of the biggest advocates for energy usage live in the largest homes and heat and cool them year round when no one is there.


You are correct my water heater is in the basement and it is only heated enough to keep the pipes from freezing. The solution to all these heat loss problems with combustion is to use outside air in the combustion process. I wouldn’t suggest blocking off the exhaust flow when not in use unless you can design a very foolproof method that establishes the draw with a blower before ignition.

Interesting. The mechanical damping methods you have suggested, would automatically preclude young children and some elderly from use, robbing them of independense.

Back to the stack being open to the atmosphere. Its because of the burner volume required to efficiently provide the product, on demand.

These devices are not, usually, recommended for installation within the conditioned building envelope, unless there is a ducted combustion air supply of atmospheric air.

The other option is a high efficiency tankless water heater. These units vent through 2" PVC pipe like modern high efficiency furnaces. They also have a 2" PVC pipe to bring combustion air into the unit. Yes, they have an electric fan that runs when the unit ignites to move the exhaust gases but they also use coils to remove additional heat from the exhaust gases. Like modern furnaces they operate in the 90+ efficiency range and have an enclosed combustion area. While I have not tried the smoke test on one, the intake/exhaust circuit should not be open to the interior of the building. They cost more, but eventually return that money in operating savings.

" the intake/exhaust circuit should not be open to the interior of the building."

I'd ask the mfg. because such a system would require electronics and switchable dampers.

I looked at the Navien website, and you are correct. I was thinking the intake line was connected directly to the draft fan system. It is not, however the enclosure for the water heater is solid like the combustion area cover on a furnace so there should be very limited air flow in that space when the unit is not firing. And if there is airflow it should be limited to the area inside the cover and not cause heat loss from the interior of the home. I use Navien as an example because I have the most experience with them. They are Energy Star rated. They use electronics to vary their rate of firing to maintain temperature at a given flow.

Thanks.

I think there was some disassembly involved in the OP'S example. For me, a bridge too far.

Here is the Navian at my old house. As you can see, one of the pipes is the intake, and the other is the exhaust. Both are protected from the elements since they sit under the eaves of my home.

I know this has been knocked around a lot, but just one last thought from here.

I have never been involved in an exterior install. Everything that I have been involved in around here is interior installation. The set up is the same however. One pipe for venting exhaust and one pipe for combustion air. We just run the pipes across the basement and out the sidewall with a long sweep elbow turned down on the end to prevent moisture intrusion. I was involved with an installation on a lower efficiency unit and for that we installed a double wall flue through the attic and out through the roof. Through the roof is not bad and will work just as well if using dual pipes. However, I was not a fan of that one. If installing a tankless water heater spend the extra money for the high efficiency. Just my opinion.