Grohe Thermostat Mixer Problem

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DCpete816

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For the last year or so our shower will not get hot unless we also run hot water from the sink in this bathroom in advance of taking a shower. We run off of a tankless water heater and have a Thermostat controlled mixer. It's probably about 10 years old now. The part number on the mixer appears to be 47 379 which is a 3/4" version. When the handle is all the way to the left (cold), the water flow is great. when the handle is all the way to the right (hot), the water flow is greatly reduced and I presume doesn't draw enough to engage the Tankless water heater without some help (ie. turning on hot water in the same room from the sink). We also have a hand shower connected to the same manifold with it's own flow handle and it exhibits the same problem.

The sink in this bathroom has great water flow on both hot and cold water. And no other faucets in the house has any issues with hot water flow. So I am presuming it has something top do with the mixer in the shower. I have all the manuals which have part numbers, etc. Before disassembling, I'm just wondering what the "best-guess" is on my problem from the experts. That part is surprisingly expensive and don't want to just replace it, if it's something else.
 

Reach4

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How about working around this by modifying or replacing the showerhead to give more flow?

Another workaround could be to turn the temperature on the WH down so that more hot is used.
 

DCpete816

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How about working around this by modifying or replacing the showerhead to give more flow?

Another workaround could be to turn the temperature on the WH down so that more hot is used.

Thank you very much for your suggestions. On your first comment about modifying the flow, I don't see how. When the temperature is on cold, I get great water flow on both shower heads attached to the same manifold. So the flow through the shower heads are fine.

I'm not really following you on the second comment. How would turning the temperature down on my water heater, which is tankless, help increase the hot flow on my temperature controlled mixer?

Regardless, I'm more interested in fixing the problem rather than working around the problem.

Thanks again, Chris
 
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1. If the shower valve will not allow enough hot water to flow (above .5 - .75 gpm), the tankless water heater will not fire up and it will allow cold water to go through the unit. Given the static flow rate of the shower, it will take a lower flow rate (gpm) of hot water at 130 degrees (less hot water flow and more cold water flow to get to 105 degrees at the shower head than at 110 degree hot water (more hot water flow because it doesn't need as much cold water flow to drop the shower head to 105 degrees). Different ratios of hot and cold depending on hot temperature and even cold water temperature.
2. The "47 379" says its a Grohe brand reverse cartridge. Meaning someone piped hot water to the cold inlet and cold water to the hot inlet. The reverse cartridge corrects the mistake.
3. The shower mixer could have a clogged inlet screen at the hot side screwdriver stop or inside the cartridge itself. Or the cartridge could be bad. That could restrict the hot water flow at the shower mixer to a point the tankless water heater isn't kicking on because the water flow isn't enough to kick it on. By turning on a faucet, the combined flow rates kick on the tankless and hot water is now fed to the shower valve. Increasing the hot water flow rate through the mixer may be the problem. Again, clogged inlet or a bad cartridge. Swallow the $200.
 
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