Shower runs hot, then cold, then hot.

December 23, 2012 10:41 AM

My shower runs hot, then cold, then hot again under certain circumstances. What would cause this? Details within.

My bathroom is pretty far away from my water heater, so it might take 30-60 seconds to get hot water from the tap. Pretty common situation, I know. I don't like the idea of wasting all that cold water, so rather than starting with the shower, I'll use the sink first and try to make use of the cold water while waiting for the hot. Then I'll get in the shower and the hot water comes right out without any wait. So far so good.

After a few seconds, say 10-20, of decent hot water, it turns completely cold for another 10-20 seconds, even if I max out the control. Then it returns to normal hot water and stays that way until I'm done. So what accounts for that intermediate burst of cold water? No one else is in the house using hot water, so that's not it. I'm thinking maybe the mixing valve in the shower is faulty somehow? Or is it possible the pipes are routed in some funny way? Something else? Not a big enough problem to call a plumber about at this point.

posted by Right On Red to home & garden (13 answers total)

My guess is funny pipe routing. I would just ignore everything up until "Then it returns to normal hot water and stays that way until I'm done."
posted by humboldt32 at 10:58 AM on December 23, 2012

It could be a safety feature of the internal thermostat on the shower. It's expecting cold water to begin with, but gets hot. It's designed to act as though you're going to be scalded at this point, so it puts cold through instead. When that's cooled the thermostat down, it resets itself to give you hot again.
posted by Solomon at 11:10 AM on December 23, 2012

Interesting. I hadn't thought about a thermostat. There's another detail I didn't mention in my original question: the temperature of the shower water when I first get in is actually a lot hotter than the hot water that was coming out of the sink tap. This would seem to support the thermostat explanation, but raises a new question: why would the water in the shower pipes be significantly hotter than the sink pipes?
posted by Right On Red at 11:20 AM on December 23, 2012

There is an anti scald valve in the shower valve that works like solomon says and over time they scald up and corrode and stop working well. This almost certainly your problem. A good way to check is to flush the toilet nearest your shower or sink while the shower is running and see if the water changes temperature. If it does that is your problem. They are designed to be changed pretty easy, probably 200-300 for a plumber or if you are handy a lot less. There are how to videos all over and ask this old house did an episode on it once. Google how to change anti scald valve and see what find (the procedure is slightly different for different makes of valve and so you need to know that info for the right how to). BTW they work on both pressure and temperature and that is why you are getting a weird time lag issue.
posted by bartonlong at 11:28 AM on December 23, 2012 [2 favorites]

I have something similar, but not so fast. About a minute of warm water, then 2 minutes of cold, then my hot water shows up.

I've been assuming that there is a small reserve of hot water being used up, and that after that, the heater needs a few minutes to kick in and produce some hot water for me. It's a bit wasteful, but it means I will sometimes in cold weather turn the water on, putter around brushing my teeth for a bit, and doing other things, and then get in the shower.
posted by instead of three wishes at 11:48 AM on December 23, 2012

I have an on demand boiler abd fet this. I always assumed the hot water is what is left in the pipes from the last time hot water was pulled through the pipes. The cold water is fresh cold water pulled through before the boiler gets up to speed. The subsequent hot water is what your boiler has heated up. I am not a plumber. This explanation may well just be my own fiction.
posted by MuffinMan at 11:53 AM on December 23, 2012

Do you have a demand water heater, or does it have a tank?
posted by yohko at 12:09 PM on December 23, 2012

It's a tankless water heater.
posted by Right On Red at 12:13 PM on December 23, 2012

A friend of mine has a tankless water heater that gives out hot water straight away, but the flow very quickly dwindles down to a trickle. To get more hot, you have to turn the tap on even more and then wait while the change in pressure signals to the boiler to heat. I'd guess that the shower is doing a similar thing, but there's no corresponding drop in pressure (which maintains the temperature), so the boiler hasn't sent any hot through. The point at which it would normally trickle is removed, so the shower is pulling water through the boiler faster than the boiler can heat it up.

I'm not a plumber, this is pure conjecture.
posted by Solomon at 1:03 PM on December 23, 2012

At tankless heater requires a minimum flow of hot water to turn on, typically about 0.5 gallons per minute. If you adjust your shower control down because the water is too hot, there may not be enough hot water flow to keep the heater running. In that case, rather than turning your shower to a colder setting, you should lower the temperature setting of your heater. That way you will use more hot water in your shower adjustment. For example, if your heater is set to 120 degrees, you might try 110 degrees and see if that works better. At the lower temperature setting you will be using more hot water which should keep the heater running above its minimum required flow.
posted by JackFlash at 2:39 PM on December 23, 2012

It's called a "cold water sandwich". We just put in a tankless hot water heater, and got one with a small 2-gallon tank and recirculating pump with a timer to avoid this during peak hours.
posted by Lulu's Pink Converse at 5:22 PM on December 23, 2012 [1 favorite]

It's a tankless water heater.

Aha! I'm guessing you also have a single furnace/hot water heating system. We do, and the flow of hot water to the shower when the furnace is heating the house is miserable. As I get up first, and the thermostat is set to start heating the house when I wake up, I get cold-then-nice-then-HOT-then-lukewarm-then-cool-then-HOTHOTHOT in the shower as the furnace heats the house first and foremost, and only then sends hot water my way only as it sees fit.
posted by Slap*Happy at 8:45 PM on December 23, 2012

Thanks for all the ideas - several plausible explanations were offered here. I did test a couple of things today with these results:

To test the "corroded anti-scald valve" theory, I got the hot water going in the shower, then flushed the toilet. There was no change, so I think that rules this theory out, if I understand it right. Too bad, because the plumbing is 25 years old, and I could easily believe that this valve was no good any more.

I also tested the "cold-water sandwich" effect. As I understand it, this would occur when I turn the sink hot tap off, followed by turning the shower hot tap on, causing a re-ignition of the burner that takes 10 seconds to complete, during which time cold water is flowing through the hot pipe since the flow-sensor has been activated. I tested this by leaving the hot sink tap on when entering the shower, and presto! There was no hot-cold-hot sequence in the shower, so this looks like the answer, at least at first blush. Interestingly, I could not produce the expected corollary effect which would be a hot-cold-hot sequence in the sink after running the hot shower, turning it off, then calling for hot in the sink. I would have thought this would produce the same issue for the same reason, but I just got steady hot from the sink.

I also will try lowering the setting on the heater in the coming days. It is fully max-ed out at present, and I'll be interested to see what effect I might get by taking it down a bit. The reasoning here does make sense to me.

Happy holidays to all!
posted by Right On Red at 6:11 AM on December 24, 2012