Hello all. First time post and can use any help I can get...

8 months ago we had a Delta Linden 17 shower/tub unit installed in 2 bathrooms. About 2 months ago, the master shower would either never get warm, or it would not be able to maintain heat. It worked perfectly for the 6 months prior and the second bathroom shower is having no issues. No other faucet in the house is having an issue.

I am able to get Hot Water to the shower by running the hot water in the master bath sink. However, if I turn off the sink, the shower loses heat in 3-4 minutes. Just recently discovered that I can direct water to tub faucet (instead of shower) and water will get hot again and remain hot. When I switch the flow back to the shower head, it gets cold in 3-4 mins.

Hot water heater is a 5 year old tankless gas heater and we have had no issues. I did a cleaning/descaling just to be sure and issue remains. In addition to the HW heater cleaning, I have replaced the valve in the shower, hoping that would resolve it but issue remains. I have also adjusted the temperature limiter to as hot as possible... no luck.

I just can't wrap my head around why 1) issue would start 6 months after install and no where else and 2) why the tub faucet is able to maintain hot but the shower is not... why would HW heater be able to maintain hot to faucet that has much higher flow rate?

The shower head is reducing the water volume, to a point that the water heater isn't sensing the flow to turn on the burner. Try cleaning the shower head. You can also swap heads between the 2 showers and see if the problem goes to the other shower.

Ahhhh....that's when the water gets hot, when you run the sink or tub, the added volume turns on the demand water heater. Shower head alone won't flow enough volume to trigger the water heater. Great diagnosis!!!!

Dropping the showerhead in vinegar now. Thank you for the suggestion! My line of thinking was the opposite... I'm used to HW heaters not being able to keep up with water flow (from a tub faucet for example) so it would have made more sense to me if the faucet was cold and the shower hot. Thanks again and I'll update what I find!

Steve it appears that the issue is not the shower head. I cleaned it well and swapped it with the shower head in the secondary bathroom. Issue still remains. The MB shower head did work on the second bath shower when swapped. The second bath shower head produced cold water on the master bath.

I also noticed that the master bath shower flow is much less than the flow in the second bathroom. I checked the pipe extending from the wall for blockage and there was nothing restricting the flow. However, given that the tub faucet is pushing enough volume, it seems like something is causing the reduction in flow to the shower head. I have already replaced the cartridge on the unit.

May have to break down and get the installers back out...

The low flow from the MB shower arm confirms Steve's diagnosis, and swapping the shower heads rules that out as the cause. Is there any thing on the end of the shower arm that the head screws into that may contain a flow restrictor? You may try unscrewing the shower arm and seeing what you can see in the elbow in the wall. You have successfully confirmed that there is a restriction in that shower path, now just need to find out what it is.