point of use electric tankless hot water heater..used as a hot water "booster"
I have an older boiler with a tankless coil. It does fine providing hot water for the sinks, dishwasher, even the shower most of the time. Where it often comes up short is providing sufficient flow of DHW to fill the bath for our young children. I usually get a blast of hot water which quickly subsides to luke warm, then I have to kill the water, give the boiler time to heat up and get more hot water...a pretty big inconvenience when you're trying to put two kids to bed, and not always reliable. I've also had to turn up the temp on my aquastat to higher than necessary to keep up.
I had originally thought of getting a 40gal electric tank and putting it in series with my coil essentially as storage tank. My only concern with this is the cost to operate an electric tank.
So now Im thinking of getting a small (6-8kw) electric tankless heater and feeding it with the DHW line that feeds the only full bath in the house. There's no way a heater that size could keep up with the HW demand to fill the tub using a cold water supply, but since the DHW never really drops below 80F or so, I think it could work to "boost" the HW I get to my full bath. This would also use an incredibly small amount of energy as it would only run for 20-30 minutes/day.
The only thing is I cant seem to find any information on this type of setup, so I have some concerns.
-Will it be a problem for the unit if fully heated (110-120F) water goes through the heater?
-Most seem to have anti-scald/thermostatic protection built in, but I imagine they rely on the cold water supply to mix, could this be a problem? If so, could it be solved by adding a separate anti-scald valve after the heater?