Water Heater cannot maintain temp

During the course of an inspection, a newer electric “on demand” system was installed. It was noted that at one tub the water would come out at about 120 degrees max, but when two showers were activated the temp would drop to 98 degrees. This means that two people cannot take hot showers at the same time.

According to the manufacture the right system is installed. I cannot find anywhere in the Florida code indicating that a hot water temperature has to be maintained. It says the fixtures have to reach 110 degrees but it does not say simultaneously.

Is there any reference for this? I just find it odd on a $500,000 house two people cannot take a hot shower in two different showers at the same time and there is nothing written to contradict it.

Any help?

Change the shower heads.

I just attended a class on the on demand heaters and the rep stated they should be serviced once a year as the exchanger scale up fast depending on usage and when they scale up the temp falls

If different shower heads don’t solve it, look at having the pressure reduced going into the unit. It needs a certain amount of flow to operate the heater for safety but too much flow will prevent good heat transfer. Also, very cold water into the unit will drop the performance but I doubt you have that in FL. In big houses you will loose several degrees over the pipe run so setting the temperature about 5 degrees higher is needed often.

They are rated to heat water X degrees at X gallons per minute. If you exceed the gallons per minute you run out of hot water. check the rating on the unit, you’re probably pulling too much water through it.

Also need to make sure cartridges and shower faucets are set properly. Single handle, anti scald valves sometimes have a setting that can adjust max temps. Typically its set to “low” or in the middle.

BTW, you mentioned a tub, I just installed a high volume faucet on a tub that output 26gal per min. Most cheaper, “greener” faucets output 2-4 gpm.

All great points. Nice to see professionals answering questions.

Mine is equipped with a scale filter which I change annually. 3 years (well water) and no scaling problems yet.

You might want to check here to get some help.
We’re just a bunch of losers here. Remember?

#1 electric demand heaters are not as ‘capable’ as gas.

Off the top of my head this is about what your looking at:

You only get 3.41 btu/watt.
It takes 1 btu to raise water (1 gal) 1 degree F.
Based upon temperature rise requirement, times the water volume (per pound) = btu requirement to reach 110F.

A 9,000 watt heater = 30709 btu/hr
Temp rise = 110-50 = 60f

30709 / 60 = 5118 lbs/water / 8 #per gal = 639 gal/hr = max flow rate per hour (less efficiency loss and bypass factor).

Worth repeating.
I rarely see one of these (2 in hmm 300) and those have been gas, so this has been a very good thread.


I have this guide on my web site. If you have multiple bathrooms, one tank-less water heater will not be able to handle the demand. The guide above has formulas to figure before this type of unit is installed. One bath is OK for most single units. If you have two baths, dishwasher, wash machine, tubs, showers, your temps will drop with one unit in play.

Years ago tank-less units were heavily advertised, but as people installed them, they did not deliver hot water because of high demand in most American homes. They work in foreign markets, because they have small homes and apartments. I do not recommend them.

very nice

I just checked a 180k GAS Rinnai with one shower running and then two running.
I let it run awhile to clear the pipes and there was no real change at all, both stayed hot.
Also had the 2nd one running on full hot from the tub faucet and the other shower stayed hot.

I have checked one electric tankless before on a commercial property and it had 80 amp 240V (two 40 amp circuits) It was affected by cold, high pressure city water that day and turning the one inch, main water valve about 40 percent down fixed the lack of hot water without any affect at all on usable flow.

The comment about the anti-scald settings was good but these give 85-100 degrees with one shower running so you would have to have water around 135-145 to get 110 with anti scald still set to factory setting. I find this many times. Someone may have adjusted it partially where it only gets 90 percent of the hot water.

If an electric unit performs poorly and has 2 circuits, one might be dead but most likely just a flow issue.

INstall one of these:
To preheat the water going into the heater. With that much flow out the temperature exchange should be pretty good.

Do you have one or do you know any one who has one .
The reports I have seen they are not very efficient and not worth the cost.
In all our Inspections we have never seen one .


Did a report on them many years ago.

Roy, Wayne Wilson? had some good information on them on a recent post this week, says they work.

Thanks everyone…appreciate the information.

Adjusting the shower water temperature is really about changing the Rotational Limit Stop position which limits the shower handle throw to regulate the maximum flow of hot water. If this will not help you , I recommend, consult your problem with some professional plumber.

For comparison the typical gas fired on demand water heaters I see in new homes are 300,000 BTU (!!!) the equivalent of almost 90,000 watts which would require a 400 amp circuit to match if my math is right.