We had a new house built in S. FLorida, and we put in an AO Smith Tankless Hot water heater. This was spec'd by the builder (or his plumbing sub). For over a year now we've been having trouble getting the unit to kick on, or sometimes when it kicks on, it goes cold after a short time. The builder says it is OK, that his plumber checked everything and it is fine.

BUT - I've been in touch with the manufacturer, and he provided me a chart and calculations showing the water temps & water flow rates needed for the heater to turn on and stay on. Turns out, that with the heater set at 122 degrees (which is the recommended temp), the flow rate needed depends on the temp coming in. The chart sent shows that if the water temp coming in is around 80 - 85, at a flow rate of 0.7 gpm, the unit will not turn on. AO Smith pointed out that, based on the calcs, I should have my tank set to 131 - that seems way too hot for domestic hot water! We do not have an issue with the unit turning on on our kitchen faucets or shower or tub, but frequently happens in our bathroom sink. If I run bathroom faucets at 0.8 gpm they splash like crazy! THat flow rate, for that faucet, is really running the water much too strong, not only because of splashing but also makes it difficult to use at that much pressure. Also - maybe related(?) - the bathroom sink is about 2 feet from the tank - it takes 45 seconds for hot water to get there. Our powder room, which is about 75 ft away, takes almost 3 minutes.

Do other makes/models of tankless units have different on/off points, and did they spec a unit that is not compatible with my plumbing selections, and florida water temperatures?

Thanks

You may want to get a small storage tank and add a recirculation system. Waiting for hot water is costing you time, money and valuable water. A newer recirculation pump may have a timer built in so the water is flowing only at peak times.

Sounds like the problem with the intermittent hot water is isolated to one fixture. well that's great news.
Do some testing of flow rates at this fixture and make changes as needed.

I have my first tankless water heater project in the rough/top phase. Final coming up soon. There had better be good flow of hot water to every fixture or else I'll have additional investigation and/or work.



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Thanks. My issue with a Recirc pump is that it seems to me that it defeats the purpose of a Tankless unit. There is no consistent time where the 3 minute wait sink is used. But even 30 - 45 seconds for the close faucet seems like way too long a wait. And the way our house was built, I can't see where we could fit a mini tank without messing up the look my wife wants in the house!

How do I go about measuring the flow rate at a faucet, and the flow rate being seen by the tank It SEEMS to flow pretty much like the others. Oh - and I am told that occasionally we have the same problem with hot water turning off at several other locations. Maybe once a month or so - as opposed to several times a week at the one bathroom faucet.

it's a mystery to the so-called plumbers who checked the system. They are not the ones who installed and did all the underslab & in wall plumbing.

a bucket with lines for gallons marked on it and a stop watch is a simple way of measuring the GPM of a faucet.
assuming there is no other water running anywhere in the house or there are no recirculating lines nor check valves, then the volume coming out of the faucet hot side is the same as flowing thru the heater coil.
if you have a single handle faucet you are testing, it is best to shut the cold stop under the sink.
one note ... I suspect the heater may be oversized BUT if other plumbers were on site doing their own testing, they would know better than I would.

It's a crazy fancy sink and I can't fit anything under the faucet much bigger than a one quart container! :-(

THe other plumbers say everything is working properly. But it's not. The builder says he will not deal with it anymore since his plumber said it is OK. A unit not turning on, or turning off once on, and a 3 minute wait for hot water on a tankless unit, is NOT OK. It is frustrating to have a water issue in a new, custom built expensive house.

I cannot imagine HOW the temperature of the incoming water would keep the heater from turning on as long as it was cooler than the heater's setting. BUT, if the heater is on and the water is NOT getting hot at that faucet quickly, I would suspect there is some other issue that the plumbers are ignoring or not checking for.

A tankless unit does not have heat loss from stored water, but it has NOTHING to do with how quickly the water gets to a faucet. That time will be the same no matter what type of heater you have.

According to the mfr, whether the unit turns on has to do with temp in, flow rate, and setting. But I agree with you that there is Something going on that the plumber is not addressing.

For measuring the flow rate of the sink, if you cannot fit a bucket under the faucet, you can either get an adapter (depending on the faucet brand and type), that you can thread into the end of the spout once the aerator is unscrewed. They have several different sizes and I use one to run a small hose from the faucet each time I need to refill the aquarium. You can then go from that adapter to a hose - short garden hose, laundry supply hose etc...to a bucket on the floor.

Option 2 is to just unscrew the hot side stop valve hose connection from the sink and run a 3/8" connection hose from there to a near by bucket. You may want to do that for another sink that doesn't have this problem and see if the flow rate varies. May be do that on the cold side just to compare.



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After I wrote my reply, I realized that the Remote COntroller from the Heater shows the flow rate. So I guess I have no reason to measure it - I know the flow from that faucet. The problem is something is causing the Heater to not turn on, or to turn off, when running at a flow rate that in theory should activate the flowrate. I wonder if it might be some kind of electrical or gas spiking? It is totally random.

"According to the mfr, whether the unit turns on has to do with temp in, flow rate, and setting."

What kind of settings are there that you can monitor or adjust that may make a difference?

Also my understanding is the flow rate on these heaters must be maintained for a minimum time duration to kick on the heater, so if the flow rate is barely at or above the threshold and there is fluctuations in the flow rate (such as turning on or off the cold side of the faucet) could affect the hot side flow rate.

Any chance you have a pressure balance cartridge in your sink faucet that regulates the hot side further and that is acting up? I would still be inclined to connect a hose to the hold side stop valve and use that to test and see if you are getting hot water if the faucet is bypassed.

The temp of the water coming out can be adjusted to various temps. 108, 122, 131. The default was 108 (maybe 112?), but we were advised to set at 122. If I am reading the chart sent to me,if the incoming water is warm (85 at this time of year), I am better off setting the temp to 131. But that creates a possible scalding problem. And if we mix cold water in, that will mean turning down the flow of the hot, and then possibly not getting enough flow to keep the water hot.

I am "blaming" the initial plumbing install, but I have no way to show that. OR - the unit spec'd is not correct for this environment.

With regards to the plumbing, at a flow rate of 1 gpm, the faucet about 50' from the heater takes 2:20 to get hot water. And then when the hot water comes, it has never been above 104 degrees, although set at 122. There is that much heat loss. I'm sure there is a way to calculate the pipe sizes from the flow rate & time to arrive at the faucet, and that sizing could be my issue. Beyond my pay grade.

Personally I would not have a tank less installed in my home. They are way too expensive, require too much maintenance, tend to lime up and burn gas inefficiently, too technical for the average homeowner, need I continue? They suck and if you do not do the required maintenance, you will replace it sooner than you want to. Way too Expensive...

Well, that's what I have, and it does seem to be too complicated - for the plumbers here!