Bosch Combi Tankless - Hot water cutting out

[Kalki]

Hi, I have been browsing these forums hoping to find someone else describing my current scenario but have not found anything. After talking with Bosch support and a few local plumbers I am still at a loss for if this is normal behavior for the system and was hoping someone here could help before I pay to have a technician come to the home (unfortunately the plumber who installed the system moved out of state and is not reachable)

I have a Bosch Greenstar Combi unit.

Briefly the situation is this: after using the hot water for a task, the next time I use it, either it takes considerably longer to reheat, or will start hot and then become ice cold for 2-4 minutes.

As an example, say my wife takes a shower, and then some time later, even right away, I take a shower. The water will either take longer to heat up, or the water will heat up normally, and then turn ice cold for 4 minutes before heating back up again. I did try not turning the water off inbetween the showers to see if it was some sort of time dependent event, but the water stayed hot throughout both showers if the water is never turned off.

Is this normal behavior of the heater? Some sort of safety mechanism? The same situation occurs no matter which use of hot water (either shower, sinks, dishwasher, etc).

I appreciate any help or thoughts offered! Thank you.

 

[Leon82]

The water is cooling off in the piping while the heater is off and/or when it starts up a gallon or so will run thru the coil before it is heated to the setpoint. if you have a low volume head it will take a few min for the hot water to reach the shower again.

This is the cold water sandwich, installing a buffer tank inline will get rid of it because the cold water will dilute with the warm water in the tank.

 

[Sponsor]

 

[Dana]

A 4 SECOND cold water sandwich might be normal. A 4 MINUTE cold water sandwich is NOT.

Most tankless HW heaters & combis only need 3-5 seconds for the flue purge prior to re-lighting when potable water flow is sensed, with a correspondingly short cold water sandwich.

Care to share which model Bosch Greenstar Combi is installed in your house?

The -151 has a min-fire input of 36,000 BTU/hr, which is pretty high for a low-flow showerhead during the warm-water months, and could be a contributing factor. Sometimes anti-scald valves and shower mixers are interactive, and may initially limit the hot water side flow to something lower than that required for ignition at the heater when the shower plumbing is up to temp, even though once-lit it can still regulate the temperature. The -100 is only slightly better off, with a min-fire input of 34,600 BTU/hr.

If it only comes out cold even on a high flow faucet after a shower, the problem is probably entirely within the unit, but in no way should that be considered "normal". Many tankless units have an "overtemp" sensor that has to cool off if some threshold hot water temperature is reached on the potable side, and units that have minimum fire inputs as high as the combi units are more prone to that sort of symptom.

The lower you set the hot water temperature, the less likely it will have either flow sensing or overtemp problems. Most showers need a temp of at least 105F, most tubs at least 110F, but most people set their tankless HW heaters higher than that, some even set it to 140F, as if it were a tank that needed a higher storage temp to prevent legionella or something. Tankless units don't have to be set that high for safety, since they purge 100% of the water in the unit with nearly every hot water draw, and they don't stagnate for hours at legionella growth temps. It's really no different than the hot water distribution plumbing, that heats up when the water flows, after which the temperature decays toward room temp. To actually kill legionella as it passes through a tankless water heater would require water temps of 180F+.

So, as an experiment, try turning down the domestic hot water to something like 110F (or as cool as you can set it and still get a decent temp at the shower head), and see if the symptoms go away.

 

[Kalki]

Thank you for the response. The unit is a 151. I will try lowering the temperatures and update.

 

[Kalki]

A 4 SECOND cold water sandwich might be normal. A 4 MINUTE cold water sandwich is NOT.

Most tankless HW heaters & combis only need 3-5 seconds for the flue purge prior to re-lighting when potable water flow is sensed, with a correspondingly short cold water sandwich.

Care to share which model Bosch Greenstar Combi is installed in your house?

The -151 has a min-fire input of 36,000 BTU/hr, which is pretty high for a low-flow showerhead during the warm-water months, and could be a contributing factor. Sometimes anti-scald valves and shower mixers are interactive, and may initially limit the hot water side flow to something lower than that required for ignition at the heater when the shower plumbing is up to temp, even though once-lit it can still regulate the temperature. The -100 is only slightly better off, with a min-fire input of 34,600 BTU/hr.

If it only comes out cold even on a high flow faucet after a shower, the problem is probably entirely within the unit, but in no way should that be considered "normal". Many tankless units have an "overtemp" sensor that has to cool off if some threshold hot water temperature is reached on the potable side, and units that have minimum fire inputs as high as the combi units are more prone to that sort of symptom.

The lower you set the hot water temperature, the less likely it will have either flow sensing or overtemp problems. Most showers need a temp of at least 105F, most tubs at least 110F, but most people set their tankless HW heaters higher than that, some even set it to 140F, as if it were a tank that needed a higher storage temp to prevent legionella or something. Tankless units don't have to be set that high for safety, since they purge 100% of the water in the unit with nearly every hot water draw, and they don't stagnate for hours at legionella growth temps. It's really no different than the hot water distribution plumbing, that heats up when the water flows, after which the temperature decays toward room temp. To actually kill legionella as it passes through a tankless water heater would require water temps of 180F+.

So, as an experiment, try turning down the domestic hot water to something like 110F (or as cool as you can set it and still get a decent temp at the shower head), and see if the symptoms go away.

This completely solved the issue. The temperature was set at 140! I am wondering if the original plumber meant to turn it down when he was supposed to walk me through the unit after he completed the install. This walkthrough never happened when he had to suddenly leave the state.

Thank you so much for the help!

 

[Dana]

The default domestic hot water settings on tankless units & combis is usually 115-120F. If it was set to 140F it had to have been set there by th installer- probably someone accustomed to setting tank type water heaters to 140F to meet a code requirement, and unaccustomed to tankless water heaters & combis.

 

[Flapper]

Briefly the situation is this: after using the hot water for a task, the next time I use it, either it takes considerably longer to reheat, or will start hot and then become ice cold for 2-4 minutes.

That's because when you shut off the hot water after using it, the heat sort-of 'builds up' and trips the thermal cut-off, so when you turn it on, you get a blast of hot water followed by freezing cold water for a little while until the unit cools down enough to reset the cutoff and heat up again. I had/have the same issue. It is typical with cheap water heaters.