3 bed two bath home with a tankless water heater. Front bath room is getting hot water but the bathroom in the back, shower and two sinks, not getting hot water. We had the system descaled with no luck.

What else could cause this issue?

You likely have cold water crossover happening at one of your faucets or shower valves. If you have any single handle kitchen or lav faucets, check the cartridges. I would also recommend checking cartridge at each shower or tub/shower valve.

Meaning it was all working fine and then one day it wasn't?

Dr. Venkman said:

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Meaning it was all working fine and then one day it wasn't?

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So I tested it again yesterday. The back bathroom only gets hot water of the front shower is on hot. It's like the back bathroom does not trigger the heater to kick on but the front does.

Tankless water heaters need a minimum flow in order to operate. Are your pipes galvanized?

Dr. Venkman said:

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Tankless water heaters need a minimum flow in order to operate. Are your pipes galvanized?

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No idea. We just bought the home a couple months ago.

If you turn on the back bedroom faucet/shower, can you hear the water heater turn on? I bet it's a minimum flow issue. If your pipes are galvanized, they start corroding impeding flow.

1. Replace tankless with tank.
2. Replace pipes with pex.
3. Install recirculating pump somewhere to trigger the tankless water heater to turn on when needed.

Dr. Venkman said:

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If you turn on the back bedroom faucet/shower, can you hear the water heater turn on? I bet it's a minimum flow issue. If your pipes are galvanized, they start corroding impeding flow.

1. Replace tankless with tank.
2. Replace pipes with pex.
3. Install recirculating pump somewhere to trigger the tankless water heater to turn on when needed.

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4. Install whole house water softener. This will extend the life of your heater.


FIFY

Remove the showerhead and see if that makes a difference. For faucets, remove the aerator from the nozzle these items can be what is preventing the flow rate from reaching at least a half a gallon per minute that's usually the minimum flow rate for the tankless heater to come on. If that solves your problem then you know you've got to get a showerhead or aerator that allows for higher flow