why no hot water unless i leave the bathtub faucet slightly cracked running a small stream?

why no hot water unless i leave the bathtub faucet slightly cracked running a small stream? Water is not heating properly. sometimes its just barely barely warm.

Turn off the faucet and look at the heater after an hour or so to see if there is any leakage into the tundish. The expansion vessel may have failed.

EDIT:

Have any of you even seen a pressurised HW system? 

Edit to ADD: this is a strange thought, but do you have a standard water heater or do you possibly have what is called a "Hot Water On Demand" water heater?  By the action of constantly running the water to get HOT water, it sounds like an On Demand water heater that is slow to heat.  

Original Message: Most water heaters have two heating elements.  One of yours has gone out.  Check them with an ohm meter.  

https://www.google.com/search?q=how+to+check+water...

Edit: definitely sounds like you have a cross over somewhere. Either a direct crossover or a bad mixing valve. If you can shut off the cold water after the T for the hot water tank, shut it off, relieve pressure in the cold water line by opening a faucet and test your hot water. With your cold water still off, leave all your faucets closed for some time and try your hot water again. If you get hot water after running a faucet, you’ll know you have cold water entering your hot water line somewhere and you’ll have to diagnose that. 

It sounds like a bad mixing valve. It could be the mixing valve on your tub itself causing your problem or it could be elsewhere in your system. You can diagnose it by shutting off the cold water supply to your hot water tank and opening the hot water valve on your faucet. You should get water until pressure within the tank is gone. If water continues to run or continues to run at a lower pressure, you have a cross over somewhere in your system, it could be through a mixing valve or a direct cross over between hot and cold lines after your water heater. You’ll have to go around to each faucet and check the temperature of both the hot and cold water lines to see which mixing valve is causing the cross over. The problem could also be the mixing valve in your washing machine, so turn those valves off before checking each faucet. 

@boy boy -- what you describe shouldn't be happening.  After the tank is left on overnight [or all day], it should heat the entire 40 gallons to the target temperature.  Now, it is possible [just barely, imho] that the hot water line from the tank to the shower runs through an uninsulated space [crawl space or basement under the house?] that has become very cold [below freezing in much of North America this time of year] and that the hot water has to heat the entire length of water line before it gets to the shower.  in that case, what one does is feel the hot water line down there to see, just after turning on the hot water, if/how soon it heats up.  And if things are as I describe, the solution is to insulate the hot water line in the crawl space [or basement].  You might get the same effect if the hot water heater is in an unheated garage ...  -- grampa  {I've lived in the North and shoveled snow in winter ... have a few decades experience with the winter cold.}

If you just can't figure it out ... find a local handyman.  He'll be lots cheaper than a plumber.

im guessing your water is heated by  an electric heater ..not sure where you are in America ..but its winter ..and the water is VERY  cold ..probably 10-15 degrees colder than summer ...your water is heated by passing over a heated element ..so its by transfer ...so the water is passing over the element for a split second ...as its so cold there is not enough time in a split second to heat transfer successfully...in summer the water has a better chance of transfer heat  ..in the uk out electric showers work similar ..we  had same problem 30 years ago ..average element was 7.5 kw ...now they are 10.7 kw and work well at all water temperatures ...you could probably upgrade yours ..but you would need to upgrade the electrical supply cable and fuse as well