Tankless water heater problem

15 years ago

As part of a major home renovation, we created a master bedroom suite with its own water heater. We have a conventional water heater for the rest of the house (including a basement rental unit, one major reason why we wanted a separate hot water supply). Primarily to save space, and to avoid running a gas line, we decided to use an electric tankless water heater; it supplies hot water only to this bathroom.

Our contractor is still tweaking it--the water is not as hot as my partner would like, but I think that's a minor adjustment. The bigger problem is that if we run hot water at the sink at the same time as the shower, the water turns cold. And I mean COLD, as in no hot water at all.

And now here's the weird thing: running the shower alone there is no problem; the temperature sometimes fluctuates slightly, but doesn't turn cold. But using the faucet alone, without the shower running, the water invariably turns cold after a few minutes, then turns warm again, and continues to fluctuate.

I think our contractor is stumped and I'm starting to think the heater is faulty. Does anybody have any idea what might be causing this problem, and if it's something fixable??

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Comments (6)

joed
15 years ago
The unit sounds like it is too small. It can't handle the flow rate of the two faucets.
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jake2007
15 years ago
Can you tell us the exact model of the heater. We need more than the brand, we need to know what size it is.
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We don't want to shut off the water when the remodel to the room is finished. We just want to switch the water source . For example after finishing the hall bath, can water coming from the water heater in the attic be switched via the remodel process to the new tankless water heater that will not be in the attic? But still will need the old water heater for rooms not yet remodeled and whose plumbing hasn't been upgraded to copper,etc. Can both water heaters be running at same time? For example keeping the master bath and the kitchen still using the water heater in the attic because plumbing will not be complete yet that routes them to tankless. So we will have both tankless not in the attic and the old water heater in the attic working at the same time but just servicing different rooms. Can we do that? I didn't specify since I'm not a plumber but I can't help thinking that main issue is that you need gas going to both water heaters and water from the outside being routed into both water heaters. Not sure if this is possible to do. Then after 6 months or a year when all rooms are remodeled and have their galvanized pipes replaced with copper, etc. and have plumbing routed to new location of tankless, the old water heater in the attic can be shut down because it won't be used.
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We don't have much info to go on. We don't know what your needs are, what your wants are or what your budget is. All we really know is that you want gas in the event that the electricity is out. A tankless requires a small amount of electricity to operate - it would not operate if the power was out. A generator would solve that problem and provide power for other things as well. I don't completely agree with Dan, you can size a tankless to run more than one thing at a time. I sized mine so that in the coldest part of the year I could run the shower and the bathroom faucet - in the summer I can run about twice that amount. It works fine in my house, but if I had a larger family I'd go with a larger unit. There are also 50 gal Hybrid units - They store the water but have high BTU input and very high efficiency. They can produce a certain amount of continuous hot water as well. These meet the 30% tax credit requirements and vent with PVC pipe - making them cheap to install where long vent runs are needed. I have included an example - I'm not endorsing a particular brand. Note that these would not run without a small amount of electrical power. Here is a link that might be useful: State Hybrid
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User
15 years ago
I think it sounds like two problems. Your triggering point for the heater to kick in is set too low, and it isn't sized large enough to work with high demands. It has a very narrow bandwidth of usability. If the flow rate (like a trickle from the faucet) is too low to activate the heater, you have exactly the problem you describe with the faucet cycling between hot and cold. And, because it can't keep up with demand, when you increase the flow rate beyond it's capacity, the water goes cold on both flows.
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JohnnieB
Original Author
15 years ago
Thanks everybody, I will check it tonight and get the information.

BTW the water from the sink faucet turns cold if we run it at full strength (not a trickle), and I want to stress that it turns cold even when run at ONLY this faucet (i.e., shower is off). Does a faucet draw more water than a shower head? I will try running it at half strength and see if that makes a difference. But it still doesn't make sense to me that it will heat the shower (alone) but won't heat the sink faucet (alone).

Also, does the temperature of the incoming water make any difference? Water from the main is considerably colder in the winter, so I assume the heater will not need to work as hard as the incoming water warms up with warmer weather?
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jake2007
15 years ago
Does this water heater boost the water temp of the hot water line, or is it heating the cold water from the street?

I'm not a fan of electric tankless for whole house, but it can make sense as a heat booster on a long hot water run to the master bath and to provide almost instant hot water for hand washing/shaving.

Can you tell the status of the heater when you are getting cold water out of the faucet? (In other words, does it show you it's on or not). I would be interested to see if it's active when you say that you are getting cold water.

Also, I'm assuming that you have a standard sink and a standard shower... not a multi-head shower or something like that.
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JohnnieB
Original Author
15 years ago
We actually do have 2 shower heads, but only use one at a time. I don't know about the heater showing its heating status--it's in a utility closet near the bathroom so I will check it out next time we use it!

It is fed from the cold water line. The bathroom is on the third floor (formerly an attic) and our main water heater is in the basement. We didn't think it made sense to run a hot water line up there because even on the second floor, it takes a long time to get hot water to the bathroom. We had issues with losing our hot water when our basement tenant ran his. So we didn't put it in to save energy, we did it to have a continuous source of hot water for the new master bathroom, and without having to wait 5 or 10 minutes for it to get warm. (Even with the tankless heater we still have to wait a couple of minutes for hot water, but not nearly as long as we did previously.)

One other thing I forgot to mention: we previously had the fluctuating temperature problem in the shower as well (i.e., shower would turn cold even when it was the only water running) but our contractor's plumber made some adjustment, which fixed that problem although it's still happening at the sink faucet. I'm really, really hoping this means we just need one more adjustment to get it right!
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Tankless water heater problem

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