Instant hot water issues

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GWolfe

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Hey everyone this is my first post, and I usually don’t resort to forums to ask for help but I just can not figure out my issue.

I have a Bosch AQ 125 - BNG tankless water heater. Which works great. Except for when we try to take showers. I have read about the “cold water sandwhich” but that doesn’t seem to be the case in my scenario.

the hot water works great when running the washing machine, dishwasher, even the sink. I can run them forever and the water never gets cold. Even if I just run the bathtub on hot water, the water never gets cold, it’s great.

however the issue is when we turn the water from the tub to the shower head.. within a minute the water gets freezing cold. It does not get warm again unless you switch the water back to the tub, shut off the hot and cold water and run it for a minute and then put it back to the shower head. I am so lost and frustrated. I love showers but, this entire issue has kind of ruined the experience for me.

I just had to pay to replace the sewer line so I can’t really afford to have someone come look at the plumbing. Any help would be greatly appreciated. Thank you for your time.
 

Geofd

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Hey everyone this is my first post, and I usually don’t resort to forums to ask for help but I just can not figure out my issue.

I have a Bosch AQ 125 - BNG tankless water heater. Which works great. Except for when we try to take showers. I have read about the “cold water sandwhich” but that doesn’t seem to be the case in my scenario.

the hot water works great when running the washing machine, dishwasher, even the sink. I can run them forever and the water never gets cold. Even if I just run the bathtub on hot water, the water never gets cold, it’s great.

however the issue is when we turn the water from the tub to the shower head.. within a minute the water gets freezing cold. It does not get warm again unless you switch the water back to the tub, shut off the hot and cold water and run it for a minute and then put it back to the shower head. I am so lost and frustrated. I love showers but, this entire issue has kind of ruined the experience for me.

I just had to pay to replace the sewer line so I can’t really afford to have someone come look at the plumbing. Any help would be greatly appreciated. Thank you for your time.
What brand is your shower valve some brands have a stem/cartridge for a hot water heater and another for a tankless application you may want to call the manufacturer k
 

GWolfe

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What brand is your shower valve some brands have a stem/cartridge for a hot water heater and another for a tankless application you may want to call the manufacturer k

The shower setup is pretty old, it is one of the older style 3 knob setups. I don’t think it would have 2 different attachments. Though I could be wrong, or I wouldn’t be here on this forum 😅.

How would I go about finding who made the shower stem?
 

Geofd

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The shower setup is pretty old, it is one of the older style 3 knob setups. I don’t think it would have 2 different attachments. Though I could be wrong, or I wouldn’t be here on this forum 😅.

How would I go about finding who made the shower stem?
That kills that theory
The shower setup is pretty old, it is one of the older style 3 knob setups. I don’t think it would have 2 different attachments. Though I could be wrong, or I wouldn’t be here on this forum 😅.

How would I go about finding who made the shower stem?
I re read your post how much water is your shower head putting out could it be it’s starting to fail and using more water that the tankless can make up??????
 

GWolfe

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That kills that theory
I re read your post how much water is your shower head putting out could it be it’s starting to fail and using more water that the tankless can make up??????
I doubt that the shower head is putting out more water than just the regular bathtub spout is, considering there is no restrictions straight to the tub spout. Does that sound right? I’m not exactly sure. But I would assume that the shower head uses less water. However when I look at the shower head it says that it’s rated for 2.5 GPM. Thanks for the reply
 

Geofd

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I doubt that the shower head is putting out more water than just the regular bathtub spout is, considering there is no restrictions straight to the tub spout. Does that sound right? I’m not exactly sure. But I would assume that the shower head uses less water. However when I look at the shower head it says that it’s rated for 2.5 GPM. Thanks for the reply
take the shower head off then try it like that this way it’s like using the spout just to see what happens
 
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Mitchell-DIY-Guy

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My guess is your shower valve is faulty; the diverter specifically. If you are lucky you can (if you shut off all the water supply) figure out the bad parts with the right tools, take it apart "in place" and replace the defective parts.

So, here is what MAY be happening; you get the hot, cold and mixed warm water out the tub spout yes? You get the same out of all your faucets, washing machine, yes? But when you twist that diverter now to move the water from the tub spout to the shower, THAT'S when you lose the hot.

So, the diverter is bad...at least that's my theory! Below is a parts list for Price Pfister, you can see all the parts. You have to figure out what brand you have and also figure out if you can still get parts for it.

 
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SHEPLMBR

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I doubt that the shower head is putting out more water than just the regular bathtub spout is, considering there is no restrictions straight to the tub spout. Does that sound right? I’m not exactly sure. But I would assume that the shower head uses less water. However when I look at the shower head it says that it’s rated for 2.5 GPM. Thanks for the reply
I think Geof is meaning that it may be restricting too much flow to where the heater is not kicking on.
 

Mitchell-DIY-Guy

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I believe the Bosch unit turns on with a .5 GPM flow rate. That would have to be one highly restrictive shower head, or a very low volume in order to not meet that flow...
 

SHEPLMBR

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I believe the Bosch unit turns on with a .5 GPM flow rate. That would have to be one highly restrictive shower head, or a very low volume in order to not meet that flow...
could be build up restricting, who knows? It is only when the shower head is on.
 

Geofd

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My guess is your shower valve is faulty; the diverter specifically. If you are lucky you can (if you shut off all the water supply) figure out the bad parts with the right tools, take it apart "in place" and replace the defective parts.

So, here is what MAY be happening; you get the hot, cold and mixed warm water out the tub spout yes? You get the same out of all your faucets, washing machine, yes? But when you twist that diverter now to move the water from the tub spout to the shower, THAT'S when you lose the hot.

So, the diverter is bad...at least that's my theory! Below is a parts list for Price Pfister, you can see all the parts. You have to figure out what brand you have and also figure out if you can still get parts for it.

I believe the Bosch unit turns on with a .5 GPM flow rate. That would have to be one highly restrictive shower head, or a very low volume in order to not meet that flow...
Did you try removing the shower head and diverting the water to the shower arm,I think that would mimic the tub spout
 
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