Bosch Tankless fires up on cold water demand?

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Howdy ya'll,
My wife and I just built a home and moved in. We're using the Bosch pro tankless 635 water heater. I notice every time the cold water is demanded, ex- flushing any toilet or just turn on a cold water faucet, this dang water heater fires up for about 15 seconds. It shouldn't do that unless there is a demand for hot water. Is this normal???
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Doesn't sound right to me, I don't have a tankless, but did you check to see if the water going to the toilet is warm feel the supply tube at the bottom of the tank.
Tom

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On 18 Dec 2005 07:31:08 -0800, snipped-for-privacy@yahoo.com wrote:

Hey, somethings gotta keep the "shitter" warm. Bubba
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Nice... Nope, all water is cold like it should be. We do use the Manabloc system in the house, would that be contributing to the water flux? Looking at the manual, you have to have a significant amount of flow to make it turn on.
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your intelligent and worthwhile contributions to this newsgroup never cease to amaze me
wrote:

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Thank You, I cant say the same for you. If you were intelligent and worthwhile you would know how to use a "kill-filter". Sucks sometimes, eh? Bubba

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It might be triggering by the fluctuation in the water pressure.
Maybe there is some sort of adjustment for this?

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My Bosch 125 never runs on cold water, New house, probably plumbed wrong. Fix plumbing not the heater.
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m Ransley wrote:

If a drop in water presure cuases it, then putting a backflow preventer valve in may help
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On a new house properly designed and installed plumbing should not have an issue, when I open everything, drop pressure cold, the heater doesnt run, it triggers on flow not pressure. Likely toilet is hooked to the hot supply
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a check valve.
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snipped-for-privacy@yahoo.com says...

If you don't have a backflow preventer on it, then when you flush the toilet and drop the cold pressure, the water-hammer preventers on all the hot fixtures in the house push a little bit of water back down the pipes. If the Bosch turns on when it senses flow, it probably can't distinguish forward flow from backward flow.
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snipped-for-privacy@phred.org is Joshua Putnam
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Everybody talkes about backflow, but nobody commenting owns one or has installed one. It isnt even mentioned in my manual, I have no backflow device or issues.
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Its making sense from what you all have put together. The main comes in and splits. One to cold side of manabloc, the other to the Bosch - then it goes into the hot side of the manabloc. If any cold service is used, even if it's a hose bib, a drop in pressure is felt all of the way into the Bosch. That reverse pressure is enough to kick it on.
Does that sound right? So maybe I need a backflow preventer?
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Pressure doesnt trip the Bosch water flow does. It also wont trip on a drip or a trickle flow, it takes apx 1/4 gpm. I say the toilet is hooked to the HW line. I bet its only when the toilet is flushed, open a few cold sinks at the same time, I bet nothing happens to fire the Bosch, there goes your pressure theory. The volume needed to start tankless are a drawback for trying to get minimal water flow and saftey against drips and such. Try yours at a minimal HW sink flow it should not start.
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snipped-for-privacy@webtv.net says...

When you have compressed air trapped at each fixture, a pressure drop produces flow -- the compressed air expands as the pressure drops, and pushes water back down the pipe.

If the toilet were hooked to the hot water line, the heater would surely trigger for more than 15 seconds, unless he's got a toilet that fills *really* fast. My toilets sure take more than 15 seconds to fill the tank. I don't think a standard toilet stop can pass 10gpm, even if the toilet could take it that fast.
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A backflow preventer is a fairly complex piece of hardware that will cost about $125. A simple check valve is all that is needed and will cost about $15.
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Edwin Pawlowski wrote:

Check valve, that is what I meant to say earlier.
Anyway, I have seen where a mixing valve was added to add a little hot water to the cold water entering a toilet tank to prevent condensation under heavy use. Not sure if that could be it.
MC
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You are right JP 15 seconds is to short for firing to fill, but with 15 second start delay, or 30 second total it might work. A low flow tank of 3 gallon with high pressure might do it. The question is does the heater run till its full or cut out early. Tankless take usualy 3- 10 seconds to fire. It still sounds like its hooked up to the HW to me as it needs to sence 1/4-1/2 gallon moving to fire at all and that is alot of volume and movement in pipe length, more than backflow. Hooking it to the HW would be an understandable mistake, or there is another plumbing issue. but it doesn`t happen when he runs his sink? Why not.
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Okay guys, FORGET ABOUT THE TOILET.
It happens at any cold water demand - even a hose bib, and even when the icemaker pulls water.
so you think a check valve would do?
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