Hot water pressure problem

We recently remodeled a bathroom with new shower plumbing with a Grohe thermostatic valve and body sprays. Additionally we replaced the tub rough and trim. The problem is that the pressure on the hot water side drops significantly after about 5 seconds when running either the tub or shower in hot only. When switching on the cold only the pressure is great. If the tub or shower is running hot only and we turn on the lav faucet the hot water only trickles out. This bathroom has a dedicated 40 gal electric water heater located in a closet 10 feet away. One thing we noticed is that the supply line going into the heater heats up as hot as the hot water pipe within 10 to 15 seconds. Not sure if this is part of the pressure problem, but I would think that the supply line to the heater would only get colder as water flows into it. You advice is greatly appreciated.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Is the dedicated water heater new or was it there before.
Other than that, I suspect the that it is the thermostatic valve that protects you from being scalded. It may have been defective. Check with whoever did the work . I believe Grohe is good stuff.
--
Joseph Meehan

Dia \'s Muire duit
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

The water heater was there before. It is the cold water supply to the heater getting so hot that has me concerned now. It seems that there is some kind of loop feeding hot water back to the heater after the hot water runs for 10 to 15 seconds. The cold water supply pipe gets as hot as the hot water pipe exiting the heater.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Where do you take the cold off for the tank and the tap if at the end of a line and tee'd you are sucking it outwhen you have the tap turned on. Or you have your connections crossed not the first to do that. send picture of set up
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Don't have pictures of the rough work but it does sound like some connections are messed up at a tee or valve and the hot water is getting back into the heater after a few seconds. Is that correct?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

The cold should always go to the house then the tank also this should be no closer than 3 feet from entrance to tank as the possibility for suck up acures when cold is on . either way without seeing it this is possible but picture would confirm get a plumber pay him for a opion and tell him you will fix yourself whats a few bucks.
This is what it sounds like to me have seen it before diy
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I think the cold might be going through the thermostatic shower control and then to the water heater possibly sending hot water back to the heater. Thanks for the input, I will have an experienced plumber take a look at it.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.