New to the forums, I'm not a plumber but I have a master electricians license for whatever that is worth.
So I helped out my best friend yesterday and installed his new On Demand HW heater for him. It's a Stiebel Eltron 24 Plus Electric unit. It's a 24KW unit that has an integrated control that reduces the flow rate in times of increased demand in order to keep the outlet temperature consistent. So his house has 2 full baths, laundry, kitchen with DW, couple hose bibs. The Stiebel Eltron 24 Plus is rated to handle a 2 bath house and is actually the biggest single phase unit the manufacturer makes so a unit any larger requires a 3 phase supply and would probably be installed in an office or something.
The plumbing for the 2 Baths is close to the heater. The Unit is installed in a mechanical closet with laundry and the incoming water main, the Mechanical closet is back to back with the basement bathroom, and directly above the basement bathroom is the upstairs bathroom and adjacent kitchen. Basically one main plumbing wall.
So I ran (2) 60A 240V circuits per manufacturers specs to the unit, mounted it onto the wall, hooked up a couple stainless steel braided hoses to the cold inlet and the hot outlet, turned her on, set the temp at 109, yada yada.
Downstairs bath sink...hot water.
Downstairs bathtub...hot water.
Upstairs kitchen sink...hot water.
Upstairs bath sink...hot water.
Upstairs tub...warm...cold...luke warm..cold...luke cold..luke cold.
So me and my buddy spent a few hours screwing around trying to get it to work. Set temp up to 120, reset new heater, opened hose bib outside while running the tub, tried running it super slow to reduce the gpm demand on the heater, adjusted the mixing valve, etc.
So I shut all the fixtures off, shut the supply to the On Demand heater off, went to the downstairs bath sink and started running just the hot water.
Losing pressure...lukewarm...losing pressure...all of a sudden it regains pressure and cold water is coming out. Repeated this experiment a few times and also on the upstairs bath sink.
Turned the HW supply valve back on, went to the upstairs tub, took the mixing valve trim off and turned the knob on all the way to HOT. Put one hand on the incoming hot line to the valve and one in the water coming out of the faucet. Piping to the valve was getting HOT, water coming out remained luke warm/cold...then suddenly the incoming hot line would get cold.
So I think cold water is going through the mixing valve into the hot water line, due to greater flow from the water main than what is coming out of the new heater. The reason this wasn't happening when they had a tank was because the flow rate was higher out of the tank. Solution would be a new thermostatically controlled mixing valve.
So my question...based on my story can anybody tell me what the hell is going on and how to fix it?
All modern shower valves are required to have anti-scald technology. On most of them, they accomplish this with a spool valve that relies on there being equal supply pressure on both the hot and cold. I think what's happening is that because the tankless is restricting the flow, the valve's spool valve is responding and limiting the overall flow. To work well, that type of valve needs equal pressure on both sides, and the tankless system is restricting it.
I ran some quick calculations, and that unit should be able to raise the incoming water temp about 70-degrees at a flow rate of 2.5gpm, the volume for one typical showerhead. Depending on where you live, that may not be enough temperature rise - in the winter, my incoming water is barely above freezing, and to achieve 120-degree output, it would have to throttle back the volume quite a bit - those extra 18-20 degrees of temperature rise just wouldn't cut it and be unachievable at the desired volume, let alone trying to run two at once, or fill the tub or washing machine in a timely manner.
In the wintertime (and maybe all year if you have a deep well and the supply is cold), a tankless of that sort is optimally good for one fixture at a time. Anything more, and it would likely be unsatisfactory. You can run the numbers yourself: 1Kw/hr = 3415BTU/hr and it takes 1BTU to raise one pound of water (a gallon is about 7.6#) one degree.
You might try lowering the set temp to just above the desired shower temperature, this would put less load on the thing and allow it to flow more, keeping the hot/cold supply pressures more equal.
Throw in the minimum flow rate to trigger the thing on, and the pressure variances, and potential variations in incoming cold water, I've not had a great comfort level with the things.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013