Hot water temps differ from faucet to faucet

[Tom Wurtenberger]

I hope someone can help me figure this out. I've been noticing that the hot water temperature has been getting cooler and cooler at the kitchen faucet. So I have read a couple of posts on here and done the testing they suggested. Let me describe my system first.
It's a older house with galvanized pipes that I'm sure have lost flow rates. 50 PSI 5.27 GPM measured at hose bib. There is a whole house tankless water heater Bosch Aquastar 125 that is de-scaled annually. Pfister single lever faucet at tub/shower. Delta Touch 20 single handle kitchen faucet. Unknown possible Moen double handle lavatory faucet.
Here are the temps that I recorded.
Bath 120*
Kitchen 108*
Lavatory 108*
I shut off the cold water supply at Kitchen and Lav. (no shut off at tub/shower) and saw no change in temps. but if I turn Tub/shower to full hot the Lav and Kitchen temps will also rise to tub/shower temp. I then changed the tub/shower cartridge and saw a slight increase in temps across the board.
Bath 126*
Kitchen 113*
Lavatory 118*
As a side note flow rate at Bath/shower is off 3.44 GPM cold 2.05 GMP hot and this did not improve after cartridge change.
So I'm asking what else I should be trying?

 

[WorthFlorida]

I think you did good by changing the tub/shower cartridge and assume that there is no recirculation system.

The big difference is the flow rate of each type of faucet. The tub is essentially wide open, the kitchen faucet maybe 1.8-2.2 gallons a minute, the lavs are 1.25-1.5 gallons a minute, that is with new EPA compliant faucets with 60 PSI. Some states require 1.25 gpm for lav faucets so most are rated at that instead of EPA max of 1.5 GPM.

If you have pipes that run from an uninsulated basement, or through floor joists between floors or even near an outside wall where the ambient temp of those areas are cooler than the room temps, it's an area to look at that may be sucking some heat from the pipes. Outside temperatures may impact your readings. If the temps drop on very cold days its a little proof that your losing heat from the pipes.

Depending on the location from the water heater, the longer the pipe run to the faucet there is more time to lose heat. My home has all CPVC pipe where most of it is under the slab except for the second floor bath. I have all new faucets after a kitchen and both baths were remodel. I've noticed that the bath lav faucets never seem to get as hot as the shower hot water but it's hot enough. I've never taken the temps. However, the hottest temperature is no more than 120 degrees. 126 at the tub is much too hot especially if there are small children living in the household.

Here is an 11 year old article and it does a good job of explaining safe water temps. If you look up installations instructions on a new water heater, there is a chart on scalding vs time and temperature.
https://www.phcppros.com/articles/1898-what-are-safe-hot-water-temperatures

 

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[WorthFlorida]

Another cause of temperature difference are the aerators. The water is mixed with air and it will drop the temperature.

 

[Dana]

With insulated pipes after the first draw of the day the idling pipe temps will be higher, sapping less heat on subsequent draws.

Current IRC code-min for domestic hot water distribution pipe insulation (half-inch or bigger) is R3, and that generally presumes the pipes are fully indoors. If they are in a vented crawlspace or vented attic R5 pipe insulation wouldn't be insane.