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

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Rear venting Bosch 125 tankless water heater (vent height requirement) ian 07-11-2006
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Posted by ian on July 15, 2006, 10:50 am
jfarch@copper.net wrote:

> 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.

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.


Posted by on July 15, 2006, 11:44 pm


> 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.

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


Posted by Alec on July 17, 2006, 11:31 am
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:
> 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=B0 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

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:
> 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=B0 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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