Perplexing Hot Water Problem

Last week we installed an electric tankless hot water system. Everything worked as expected for several days, so we know things were hooked up right. For the last couple of days there has been a strange hot water problem that no one can figure out. Here's what's happening: Hot water will only come out of the kitchen sink, but only if the bathtub is running full blast. Hot water will not come out of the kitchen sink if the tub is not running. The hot water to the kitchen sink does not cut out as long as the bath is running. Hot water will not come out of the tub valve or the valve in the bath sink at all now. Hot water will go into the clothes washer for a few seconds and then goes cold. IOW, the only hot water in the house goes to the kitchen sink, and then only when the tub is running. Other info: the bath valve is a single-control type. we tried replacing the mixing valve in the tub twice; no effect. we looked under the house to see if there is a hot water line leak, but there isn't. Does this make sense to anyone out there?
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wrote:

Not to me, but it might to others.
Perhaps the unit is defective, so contact the manufacturer for advice.
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On Tue, 08 Jul 2008 08:58:28 -0700, cyd wrote:

Do you have debris in the line before the heater that it would restrict the flow? How about checking for debris in all fixtures and aerators? Are there any pressure switches or activators that need adjusting? Have you consulted the manufacturer?
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aerators? Are there any pressure switches or activators that need adjusting? Have you consulted the manufacturer?
Yes, there could have been debris in the line. We've had lots of junk coming out of the faucets and aerators, including one pea-sized rock out of the tub valve. The plumber opened up all the lines yesterday and blew pressurized air through the pipes. Also, when the plumber was soldering one day, he used one of those dissolvable gooey beads to stop water from coming up the pipe and bothering his soldering... anyway, this slime keeps coming out of the kitchen sink, which I'm sure is that dissolved bead.
The manufacturer is sending us a replacement heater, in case that's the trouble.
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On Tue, 08 Jul 2008 08:58:28 -0700, cyd wrote:

http://www.seisco.com/pages/service-troubleshoot.html may not be your manufacturer, make or model, but might give you some leads.
The one which caught my attention: pipes reversed.
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Thanks. I printed out their trouble-shooting guide and I'll show it to my contractor when he shows up today.
The pipes aren't reversed, because it all worked fine for several days.
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wrote:

Sounds like a mis-adjusted, mis-sized, or failed flow sensor. When you have a lot of flow from both tub and sink on, the unit heats, but not when you have lower flow. A complaint with many early tankless units was that the flow sensor wouldn't detect small flows, such as when you have a sink tap only slightly open. I think this has mostly been corrected by most manufacturers, but it sounds like your sensor isn't working right.
HTH,
Paul
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That makes sense to me. Hopefully the new unit will work better. But how would that explain the fact that hot water will not come out of the tub, which has good flow?
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