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Question DetailsAsked on 12/28/2015

What causes hot water pipe banging and water flow locking up when dispensing hot water?

If only the hot water is turned on, once hot water is coming out, the water flow stutters with pipe banging heard. This last for a few seconds then the water flow locks up with no water flowing (vapor lock?). This only happens when using hot water.

To get the hot water to flow again, cold water can be turned on and turn off and on hot water again. You can hear in the wall when the hot water flow releases by a single bang. If the hot water is turned on to high at this point flow will bang and lock again.

I have had this happen on several faucets, several seasons and have turned down the water heater temp with the same results.

Any ideas on what could be causing this?

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While I suppose it is possible that a piece of loose plastic broken filler tube floating in the water heater is blocking the outlet intermittently, then gets pulled away when cold water is drawn, I would not expect that to be a very frequent thing - but happens.


My first guess is that there is a backflow preventer or an expansion tank with inlet valve (which it should not have for a conventional hot water heater) installed just before the hot water heater on the incoming cold line which is shutting off the flow - sounds more like an inlet control valve mounted on an expansion tank to me, though that is for boilers, not hot water heaters.


I would get two people - one to operate the faucets as you described, one listening in various places (may or may not need a stethoscope or metal rod to hold to pipe to hear well) to track down exactly WHERE the noise is coming from. I would start at the water heater and track outward from there. This assumes it happens regularly enough that you can reproduce it pretty much on demand.

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If you have a tankless hot water heater, that is the first place I would look - some have a control valve that does not let any or maybe just not full flow water out till it is up to temperature, so some seconds delay - if that is malfunctioning, could be slamming shut on you when you put full hot draw on it.


I guess could also be a backflow preventer (likely just before the water heater on cold line) installed backwards, which is malfunctioning and letting in normal flow rather than preventing flow at all, but clamping shut at full flow.


Vapor lock - I doubt it - water systems running at 30-70 psi don't generally vapor lock, because they have enough pressure to push through any gases or air - so you would get sputtering and gurgling if say the heataer was boiling and forming gases in the tank, but would not vapor lock in normal plumbing. That can happen with low pressure hot water baseboard systems, and in steam systems with water lock, and in high-rise recirculating systems, but I have never heard of it in a household pressure system in low-rise construction because 30 psi is enough to push through a bubble about 60 feet high.


I think this pretty much has to be a malfunctioning or improperly installed valve of some type. BTW, I would have added pressure regulator to the suspect list except that is usually on the entire incoming water supply pipe, not just hot - but if for reason they put a pressure regulator on just the cold pipe to the water heater, leaving the rest of the house unregulated, then that would be a prime suspect because that type of behavior, letting low flow through but not high, is common with malfunctioning regulators.


If you can't track this down yourself, or you do but are not up to the repairs, if you think it is a tankless water heater issue I would call a Heating and A/C contractor. If hot water heater or a backflow preventer or air tank issue usually a Plumber. If uncertain or not up to tracking it yourself, I would go with the Plumber if you have conventional storage type water heater, HVAC if tankless unless plumbers handle them in your area.

Answered 3 years ago by LCD




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