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Dave, Plumber
Category: Plumbing
Satisfied Customers: 3
Experience:  10 years of plumbing experience. 3 years of which are commercial.
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I have a Bosch model AQ125BNG tankless hot water heater hot and cold fluctuations

Customer Question

Currently when I first turn the water on there is no issue and the water is very hot which is perfect. But with-in 5 minutes it goes cold and stays cold unless I shut the hot water off then back on again. If I have someone upstairs I can watch the tankless system turn on when the hot water goes on then shut itself off "pilot light stays on" about 5 minutes later.
Submitted: 8 years ago.
Category: Plumbing
Expert:  Dave replied 8 years ago.

Hello. What size gas line is feeding the water heater? Or I should say, what size gas line is coming out of the wall? 1/2" or 3/4"?

Customer: replied 8 years ago.
3/4 and this system has worked well for the last 1 1/2 years just started happening about 2 weeks ago and no changes where made to the system then.
Expert:  Dave replied 8 years ago.
ok. Let's eliminate a few things here first. Have you cleaned the filter which should be in line on the cold water side going into the heater? Reason I ask is because these heaters take a certain amount of water inlet pressure to work correctly. Also have you tried running just a sink faucet to see if the same thing happens?
Customer: replied 8 years ago.
Yes I have removed the cold water line from where it contects and check it did not see persay a filter. I have also just run the sink and the same thing happens it just take a little longer but not much longer. I have noticed it seems like there it a little loss of pressure
Expert:  Dave replied 8 years ago.

These heaters are all the same in the respect that they need a minimum gpm(gallon per minute) flow to make the heater work correctly. In other words if your shower head is using 2.5gpm, then your inlet flow has to be at least that. If there is no filter on the inlet cold side, you definentally need to install one. Tankless heaters are very tempermental if not kept clean internally. Do you have the manual for this water heater still? If so look up the flushing procedures. This will clean all of the internal piping within the heater. This should fix the problem.