Odd problem with the hot water in our house. In order to get the hottest water from the tankless heater, we have to partially open another hot faucet somewhere. For instance: if the kitchen sink is partially open, the shower will get good and hot. If the toilet is filling, the kitchen sink or the shower will get hot. The problem is more pronounced in the winter when our incoming municipal well water is colder, however the kitchen sink trick works fine then too. The HWH is gas-fired and DOES NOT have a recirculation tank-none was indicated as required. I was guessing that it was a low-flow showerhead problem, but the same phenomenon occurs at the basement utility sink which has no restrictions whatsoever and the sink behaves the same if the flow restrictor/ aerator is removed. The temp selector on the HWH is at the highest setting. The bathroom (only one) is a recent reno. The shower and vanity pipes were completely replaced with copper, however the toilet was tied into the same old 3/8" galvanized line which appears to have some kind of early hot-cold mixing/ anti-sweat fitting on it (square metal box about the size of a deck of cards- hot & cold 3/8" lines in, single 3/8" line to toilet). Could I be getting crossover thru this? Any ideas out there?

Yes cross over at the toilet is possible I hope you don't have galvanized piping before the tankless if so the heat exchanger will be shot within a couple years even with routine flushing

The toilet anti-sweat valve has integral checks on both sides but they may be bad.

Can someone explain to me, how a crossover at the toilet could improve hot water at the shower? I can't follow the logic.

If the check valve has failed on the mixing valve for the toilet when you turn on cold water any where in tge house the pressure equalizes. The company I work for gas over 1,500 tankless water heaters installed that do the heat as well and if the check valve in the circ pump sticks open it does what the OP described

I hope you mean a combi-boiler and not a tankless water heater?

No they are not combi boilers they are AO Smith ATI 510 tankless heaters We make a manafold with a mixing valve and check valves for the hydro air coil. Our company does the same thing with tank type heaters



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There's issues with using them like that, if they are not listed as a boiler. The manufacturer won't honor the warranty, and if there is an incident with one of those units and someone is killed, they'll hang you out to dry. It doesn't have to be your fault, you'll be collateral damage.

So the way I'm seeing it I have two options. Please advise if my logic is sound and if not guidance would be appreciated. Option 1: eliminate the anti-sweat valve since it's probably redundant with the modern toilet installed and run only cold to the toilet valve. Option 2: install check valves on the input and output sides of the HWH to eliminate the chance of backflow/ mixing. Thanks much for the help!



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Is the unit firing, then shutting down while only one fixture is running water....on, off, on, off?

Installing check valves at the hw heater won't solve the problem you're having. The cross connection allowing you to draw cold water into the hot line is reducing the flow rate below the minimum required for the tankless to activate. Stop the cross connection and you restore the demand on the hot water side.

No. It stays on but the temperature is diminished unless I open another hot tap, either in the kitchen, or by letting the toilet run.

Ok, thanks much! Was hoping for the easy solution...doesn't happen often tho. Will replace the entire line feeding the toilet with copper and eliminate the mixing valve and double check for other crossovers. This is an old house (1878 w/ add ons in 1900 and around 1915).Lot of stubbed off boiler pipes from the heat conversion. Unfortunately we cannot use PEX in Cook County IL or The whole house would have been redone already. Copper's money and we're both disabled= tight budget.