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Home > Archive > Building and Construction > July 2006 > Rear venting Bosch 125 tankless water heater (vent height requirement)
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Rear venting Bosch 125 tankless water heater (vent height requirement)
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| 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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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,
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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| El Barto 2006-07-14, 8:25 pm |
| "ian" <ian@macky.net> wrote:
>I want hot water when the grid's down, so trying to
>make it work without powered vents!
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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| jfarch@copper.net 2006-07-14, 9:25 pm |
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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,
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.=20
Jerry Arch
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| JerryD\(upstateNY\) 2006-07-15, 9:25 am |
| > Umm...what about the power for the control electronics? Power vent or
> no, you still need electricty to make hot water!
There are no electronics on a Busch water heater, so it needs no electric to
make hot water.
--
JerryD(upstateNY)
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| DJ wrote:
> 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.
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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| El Barto wrote:
> "ian" <ian@macky.net> wrote:
>
> Umm...what about the power for the control electronics? Power vent or
> no, you still need electricty to make hot water!
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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| 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.
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| jfarch@copper.net 2006-07-16, 3:25 am |
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> 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
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| Derek Broughton 2006-07-16, 5:25 pm |
| El Barto wrote:
> "ian" <ian@macky.net> wrote:
>
> Umm...what about the power for the control electronics? Power vent or
> no, you still need electricty to make hot water!
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:
> 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).
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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| Derek Broughton 2006-07-17, 9:25 am |
| DJ wrote:
>
> 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 ;-).
Yeah, I really shouldn't have let my wife make all the decisions :-)
--
derek
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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:
> 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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