Water comes out spout and shower head at same time

first id check shower head most has a screen in them and do get clogged from time to time since the water flows good from faucets

Shower heads have flow limiters installed in them. This means the water heater cannot sense the water flowing and therefore it wont come on. Take the shower head off and look in the end for a piece of plastic that is in the swivel. Remove it. Then put the shower head back on.

Bad PC board.
Bad sensor.
Don't buy a tankless because the problems can be endless.

Before blaming the tankless however.
Perform the yearly required maintenance on burner
Delime the unit.
http://waterheatertimer.org/pdf/Bosch_Descaling-heat-exchanger.pdf

Remove and clean aerators, replace low-flow shower heads with large water-wasting shower heads.

IMPORTANT check all your single-handle faucets for crossover.
Water getting cool after 15 minutes could be bad cartridge on another faucet.
http://waterheatertimer.org/Crossover.html
http://waterheatertimer.org/pdf/No-Error-Code.pdf

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

To clarify your question, are saying that you just had a new water heater installed in your house and the shower head in the bathroom can't get water through it? If that's what you meant, the water restrictor in the shower head is plugged with sediment. This often happens during water main shut offs and then turned back on. It shocks the sediment on the inside pipe walls. The first places it affects are shower heads and faucet aerators. Remove the shower head and clear the clog or replace with a new shower head. If this is not what you meant to say, then rewrite your problem.

if it has been constant from instalation it might be that ur shower head is rated too low to get enogh gpm ( gallons per minute) it will tell u most of the time around the shower head where it threads onto the spot of the wall what the gpm is. alot of the energy saver heads are restricting tehgpm to 1.5 or .5 gpms to conserve water well the water heater will only operate correctally around .5 gpm depending on model so if u are not pulling enough water u will not turn the water heater on to get ur hot water all the time so go to lowes ( or locally trusted hardware) and get a new shower head make shure u look for the gpm on teh head and check in ur owners manual to see what gpm ur water heater kicks on

P.S. if ur shower head is rated at higher gpm say 1.5 and it is corroded or pluged up say with lime it is going to restrict it causing a lower gpm

A 40 gal. electric water heater will give you aprox. 32 gal. of hot water in one setting, since they do not reheat as quickly as gas. So if you are using a newer 1.5 or 2.0 gpm shower head. You could take a 16 minute shower with straight hot water. If you are running 2 new shower heads at once you could get 8 minutes. If you using the tub spout the times decrease. Some of my customers like to turn up the temerature on the thermostat a little so they can mix more cold and get it to last a little longer. Be very careful doing this.
If your water heater has a lot of calcium built up in it the times will decrease.
With out seeing the heater, you mention it is a mobile home heater so I am guessing that it is bottom fed.
I hope this has helped.

There is a chance there is mineral deposits in the showerhead openings. You can try products sucha as limeaway by unscrewing the showerhead , soaking in the limeaway for a few minutes and rinsing with water. You may have to repeat this a few times to get it clean.

Take the aerator off the sink spout and see if your pressure is good without it on there. Sometimes they get plugged causing that problem, usually after you have had the water disconnected and turned back on again. If that works, do the same with you shower head. If the tub spouts runs good and not the shower head, then that is also a good indicator of that being the problem.

This is what you should have... Grohe Grohmix Thermostat Valve & Trim - 34436

Paraffin cartridge

Lever volume control

Grip ring temperature control

1/2" NPT female connections

Service stops

Includes rough-in valve and trim

Integrated temperature and volume control

Requires Twin Ell (03 671) for shower/tub combinations with diverter spout Note:

Requires Twin Ell (03 671) for shower/tub combinations with diverter spout

Grohe Grohmix Thermostat Valve & Trim - 34436

take the shower head off and make sureits not clogged,, if it's not then valve has dirt in it you need to take the cartdrige out or the stem depending on the diverter and clean it out, but it does appear to be dirt, maybe solder got into the water line and came up there