Water hot enough heat - Bosch PowerStar AE115 Tankless Water Heater...

Hear is a list of possible causes for running out of hot water.

• Water temperature is set too low. Increase the temperature to approximately 125-130 F.
• Reduced the water flow when the water filter or fixture aerator are clogged.
• If there is a plumbing crossover, cold water is mixing with the hot water. One of the reasons for the plumbing crossover is if the single-lever mixing valve was installed, and it failed.
• Gas valve does not provide enough pressure; either due to the low gas pressure or if it is not open fully. Is the gas line sized correctly?
• Sediment and limescale build-up inside the heat exchanger. You might want to use the descaling procedure and flush all the deposits. The scale deposits act as an insulator preventing the heat exchanger to transfer the heat to water.
• Is venting system clean of the debris and does it provide enough fresh air for proper combustion?

When there is no hot water the problem can stem from lack of power or a faulty electric thermostat or a faulty upper electric heating element. When the water is not hot enough, the problem can be an undersized water heater, or crossed hot and cold connections, or a faulty heating element or thermostat.

If this is in a RV the propane monitoring valve may require more gas flow to turn on than the water heater requires. A central heating system requires substantial gas flow so when the monitoring valve opens to fire the central heating, there is enough gas flow to also fire the water heater.

I do not know how old this unit is or what model but many water heaters are not sophisticated enough to measure the temperature, but heat based on water flow and temperature rise. To start, tankless water heaters turn off every time you turn the hot water off. Lets use an example. You use the facilities, wash your hands then turned off the water and take a shower. You will bring the already heated water (from washing your hands) from the pipe up to the shower. Then the cold water that comes into the pipe from the reactivation of the water heater comes up the hot water pipe with warm then hot water just behind.

If this is not your problem, keep reading. Every unit I have been called to see were installed wrong. Check your installation manual to see if the problem with the system started with the contractor.

First, check the temperature of the water from the heater. If it is too hot and you are mixing too much cold water in, the unit will turn off because there is not enough water flow to keep it running.

Second, turn off the valve at the water heater and turn on the hot water at a faucet. If the water does not stop running, there is a "plumbing cross over" usually caused by a failing pressure balance valve in a shower.

Third, if the unit has not been serviced, go to bosch-climate.us to find a tankless water heating professional in your area. Totally Tankless LLC

http://www.hearth.com/talk/categories/main-hearth-forums.4/
You burn wood, and smoke goes up flueway.
You're not running chimney smoke through ductwork so you have a heat exchanger.
What kind of heat exchanger?
probably air since you're running through ductwork?
How much hot air or hot water is coming off heat exchanger?
How big of a space can be heated with that amount of heat?
Is the exchanger located in optimal location?

Considerations.
One room log cabin with fireplace will not stay very warm.
One room house with big wood burning stove in center, with hot flue pipe running across the room and exiting on far wall, will get warm-hot, but will cool off fast.
Why?
Because the BTU output of firewood is much less than electric, coal, oil, or gas.
Otherwise they would have made wood-burning steam locomotives. But the locomotive boiler cannot get hot enough with wood ... the boiler needs coal to produce enough BTUs to boil water fast enough to rotate the turbine and turn the wheels.

Maybe your wood stove output should be measured.
Don't forget a huge percentage of fire heat goes straight up flueway.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

This time of year the water comming in the house is much colder and heater has to work a lot harder to heat 40deg. water than ot does to heat 70deg water in the summer time. its normal.

Check cylinder stat , if fitted check 3 way valve is fully open on hw side . Check water level in header tank

You probably have a bad heating element. Probably the upper element.

How big is your bathtub? Is your water heater set on the hottest setting? What is the temperature of the hot water at the tap?

Keep in mind that hot water can cause severe burns. Children and those with diminished mental capacity can be seriously harmed by hot water.

The "40 Gallon" rating on the water heater is usually the amount of hot water the unit can produce in one hour. So the tank, and the amount of immediately available hot water, might be smaller than 40 gallons.

My bathtub is 2 feet wide by 1 foot deep and 4 feet long. This is 8 cubic feet or almost 60 gallons. So a 40 gallons of water at 120 degrees, mixed with 20 gallons of cold water (60 degrees), fills the tub with 100 degree water. My January cold water is 42 degrees, so it will take 40 gallons at 129 degrees to fill the tub. If the tub is cold to start it will take even hotter water.

So the water heater might be just fine.

Possible solutions:
1. turn the temperature up on your water heater- or the temperature control could be malfunctioning and not getting the water hot enough.
2. insulate your hot water pipes so that the water loses less heat on the way to the tub.
3. bigger water heater.
4. smaller bathtub, or less full bathtub.

most likely the diverter valve.. when hot tap turned on the movement of the water through the valve operates a diaphragm which pushes a pin/button//switch to tell the boiler to fire up