No hot water in barn apartment if wash stall hot water faucet left turned on, but turned off at the end of connected hose

Question

We have a situation (along with two other new barn owners). Our new barn has a tankless water heater. Everything works great in the rest of the barn fixtures, if the wash stall hot water is turned off at the faucet. That wash stall faucet has a garden hose attached to it with a hose sprayer on the end for washing horses. Oftentimes we leave the hot and cold wash faucets turned on all day long so that we can simply wash another horse without having to go to the trouble of turning the faucets on and off. When this happens, the barn apartment as well as all other fixtures in the barn do not have hot water. The instant you turn the wash stall faucets to off or open the hose sprayer to on, all is good with the other fixtures and they have hot water. Our plumber told us that there is nothing to be done for this problem. Anyone have any ideas?

Answer 1

When other faucets call for hot water, cold water is flowing through the hose connections (or mixer) into the hot supply instead of flowing through the heater.

Fit a check valve before the hot supply connection to the faucet to prevent this backflow.

Answer 2

You are going to have to shut off the water at the mixing faucet in the barn when not washing a horse--you can't just leave the hose nozzle shut off and the faucet valve(s) open. You will be able to get away with shutting off just the nozzle for short interruptions while washing one horse, but even then if someone draws hot water in the apartment while the nozzle is off, they would get no hot water.

A mixing valve (or pair of valves that go into a common mixing chamber) that is open to deliver a mixture of hot and cold into a hose that is then closed at the end will allow cold water to flow backwards into the hot water pipe if another hot valve is opened. This will occur with a tankless WH because the output side of a tankless WH has a lower pressure UNDER FLOW than the cold side. This is because the tankless WH has a restriction in it to achieve a set output temperature.

Having one mixing valve open to hot and cold but the output plugged would pressurize the output side of the tankless WH and would likely prevent the WH from firing up if another hot tap was opened. The tankless WH only fires up when the flow of water through it exceeds a certain amount (amount varies with mfgr and with settings on a particular WH). You cannot get a very low flow of hot water from a tankless WH like you can from a tank.

If this barn was serviced by a tank WH, then it would supply some hot water to the apartment even if the barn faucet were open to hot and cold and the hose shut off because the tank has no flow restriction like a tankless and it will deliver hot water in a low flow. This practice of leaving the mixing faucet open and the nozzle off would have worked to some extent in barns with a tank water heater. But I think that even with a tank WH a check valve in the hot supply to the barn mixing faucet would be necessary for acceptable hot water supply to a barn apartment if the mixing faucet in the barn is oging tobe left open.