Rear venting Bosch 125 tankless water heater (vent height requirement)

Hi-- I d like to to install a Bosch 125HX on an outside wall, and vent through that wall. But my initial vertical height plus 45 leg back through the wall (which counts as vertical) don't quite make the 6' vent height Bosch recommends. Is it a performance or safety issue with a shorter vent? I want hot water when the grid's down, so trying to make it work without powered vents! Thanks. --ian
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ian wrote:

Ian,
Just so's I understand. Bosch wants the vent 6' above what, exactly? The heater, or the ground... and yes, I'm asking you rather than taking the two minutes it would take to either check my own files or download the manual from Bosch ;-).
If it's a rule, though, I'm thinking it's from Bosch, not the Code. I've installed a few propane direct vent appliances, and they go on and on about a MAXIMUM length of exhaust for non-power assisted vents, but not a minimum. Minimum distances from windows, doors, propane storage, all kinds of stuff like that, but not length of exhaust on the propane appliance. Might be that in a short exhaust, the incoming air doesn't have as much time to warm up from the exhaust portion of the pipe? This would degrade performance a bit.
DJ
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DJ wrote:

Hi. I checked the installation instructions again and it states ''Minimum vent size must be 5". Minimum vent height must be 6 feet, provided there are no elbows.'' It doesn't say what happens to the vent height requirement if there *are* elbows, tho. I was thinking about how chimneys work, and they do need to stay warm to draw well, maybe that's what they're on about?
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Umm...what about the power for the control electronics? Power vent or no, you still need electricty to make hot water!
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There are no electronics on a Busch water heater, so it needs no electric to make hot water.
--
JerryD(upstateNY)



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El Barto wrote:

The "HX" model uses a micro-hydro-generator to produce the electricity needed. Quote: "When a hot water faucet is opened, the water flow through the heater causes the gas valve to open. At the same time the hydro-generator activates the electronics which sends a spark to the pilot."
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El Barto wrote:

Oh dear - I don't know how I'm getting hot water then! There's no electricity involved in my Bosch tankless heater. The drawback is that it only applies a fixed temperature delta to the incoming water, rather than being able to output a fixed temperature, no matter what temperature the input is (which is variable, as it comes from a solar water heater).
--
derek

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Derek Broughton wrote:

The Bosch Aquastar 125 BS will do that. It was actually purpose designed to work with solar water heaters. And it also works without the benefit of electrical input ;-).
DJ
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DJ wrote:

Yeah, I really shouldn't have let my wife make all the decisions :-)
--
derek

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ian wrote:

Ian, Just for your information, I have a Thermar (no longer available) tankless water heater. It uses a directvent. It exists out the back of the unit with no vertical, using a zero clearance type pipe and hood. It came as an LP unit and has now been converted to natural gas. It uses no electric power to operate, except for the heater that keeps it from freezing in very cold conditions. I am disapointed to hear that their may be no equal to it available now. I bought spare parts, it has operated flowlessly for 20+ years.
Jerry Arch
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snipped-for-privacy@copper.net wrote:

Hi Jerry-- thanks for the info. The 125 series are natural venting, not direct, but the only difference is where the combustion air comes from, so I'd think they're comparable. The 125 produces 118,000 BTUs so it's a big flame with a lot of exhaust-- 5" vent!-- so I'm having trouble understanding why all that hot air wouldn't be happy to do a rear exit like your Thermar, or go up and back, or similar. It shouldn't need a long chimney and lots of draw, should it? Yet Bosch says 6' minimum vent. Hmm.
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Ian, Yes, where the combustion air comes from is the difference, and the reason Bosch wants the 6' of draw. They want to make sure the air goes in the right direction. For both your safety and to keep the unit from freezing (back draft). My Themar (100,000 btu) with the direct vent has a one pipe inside the other (zero clearance). The hot gases go out the inner one and the combusion air is drawn in from the outer one. The pipe is only about 10 inches long straight out the back, terminating in a hood that keeps the two seperated. We have a superinsulated house (very tight), drawing combustion from inside the house would be a real problem. The only means of getting outside air, is through the air to air heat exchanger. Trying to get combustion air from inside would cause a major balancing problem. Stick with the 6' requirements and be sure you have enough combustion air available. Jerry Arch
Jerry Arch
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Ian, The reason that Bosch requires the 6' is for the unit to operate safely and to not shut off. There is a sensor in the top of your 125HX and if it senses a blocked flue/overheat it will shut off. Folks that put on short stacks trip the sensor plus more importantly the unit will not draw the combustion air needed to operate at full output.
Alec ian wrote:

Ian, The reason that Bosch requires the 6' is for the unit to operate safely and to not shut off. There is a sensor in the top of your 125HX and if it senses a blocked flue/overheat it will shut off. Folks that put on short stacks trip the sensor plus more importantly the unit will not draw the combustion air needed to operate at full output.
Alec ian wrote:

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