What are some red flag for the newer on demand water heaters?

Seeing these on demand water heaters a lot. Just dont know as much about them as boilers or furnaces so i hope to get a little more info on them.

By “on demand” are you talking about a small water heater located under a sink? If you are it’s a water heater there’s nothing special about it.

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What do you mean by “red flag”?

The best way to get info on them, and to learn, is to retrieve the manufacturer’s installation and use manual(s) and read them. Then apply that information to your area to see what you should look for.

Where are you located?


Many consumers think they’re “instant” hot water. Water heated at one location still has to travel to the spigot. The further it has to go the longer it takes. Also takes about .5 gallon per minute flow to turn on, so a trickle at the faucet may not fire it up.

The major advantage is energy (cost) savings over the long haul. Best example is a vacation. During the time you are away a conventional water heater fires repeatedly to maintain the tank temperature. A tankless WH sees no demand and remains off the entire time.

It’s debatable if the savings are worth the increased cost of purchase/installation. With a new home you usually don’t get a choice.


I agree Bob. At this time IMO, it is not a good ROI.

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seems to me they almost pay for themselves the day before they need replaced…


I switched from a gas tank to a gas tankless. Long showers are nice I suppose. For me, it freed up some space in my laundry room because I located the tankless outdoors. Energy savings have not been realized or significant.


Not if you want to keep your wife happy. :innocent:

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I have seen a few instances in the past, mostly on older homes, that new gas lines had to be run for the tankless unit. Older tank water heaters did not use a large enough gas line to supply the need for the 150,000 - 200,000 BTU tankless units.

I don’t have that problem. She is happy to let me make the call!


Yes. The new anode rod is a vinegar flush, with a service interval that depends on the water in the area. Else scale builds up internally and the heat can’t transfer to the water, and the thing is landfill before its time.

The gas line to these units must be SIGNFICANTLY bigger than with a tank water heater, because the units are not smart enough to reduce their heat output when the flame quality goes to crap. They “modulate” but at peak demand with an undersized gas line, start burning badly (there is no way to set a maximal flow rate, sadly). Gas line friction is a matter of both pipe diameter AND length, and number of pipe size transitions comes into it also.

Right, whenever I see a tankless gas water heater in an older house and the gas line is the correct size going to the heater, I check in the attic (or crawl) to see if the installer just tied a 3/4" or 1" to the old existing 1/2" gas line.

+1 on that. Also look for soot at the exhaust as an indicator of poor burn AND consider checking that meter and regulator at the front: I’ve seen 1/2" gas service from the street upsized to 1" before the heater.

“Tankless” water heaters rather in maine

Some of the new models I have seen in new construction this year are allowed to have a 1/2", but only for 24" max., I believe.

[quote=“David Wigger, CMI, post:15, topic:221850, full:true, username:dwigger”]
Some of the new models I have seen in new construction this year are allowed to have a 1/2", but only for 24" max., I believe. [/quote]

If you see a model with a software adjustable maximum BTU output, I’m all ears. The gas lines seem to outperform their table based ratings, but there’s no way to be sure, and having an adjustable model would be great.

I would make a point to look for descaling access ports - will be needed for maintenance at a later date. I believe Bosch has theirs internally under the cover. Attached are some from today. If you are not clear on the concept…this is a pretty simple video that is older but a good walkthrough.
Descale from Matt Risenger

Also, is it securely mounted?
Is the flue solidly connected above? I find gaps all of the time on these as it goes through the sidewall for a DV model.
How is the PRV terminated?
Is there a condensate line ran - They will have condensation like a HE Furnace!
** IF you run into any electric powered units (On condos sometimes), I would make a point to demo them for the buyer as they are exceptionally limited on output and often times will not cut it for a shower unless you have an exceptionally low flow shower head. However green it may or may not be - demo the performance with a showerhead on and then see how little heat gain you will get with another faucet in place. The temperature gain vs. flow rate is super low on electric units, and often a massive disappointment for use for day to day.