I had the same problem. The reason the water gets cold is that it runs through the water heating coil too fast, so it doesn't get hot. The mixing valve will add cold if your output is too hot, but if it's too cold the mixer can't help.
The cheap/easy solution, and this worked quite well for me, is to install a water-saving shower head. This will slow the flow through the heating coil in your boiler, and you should have good hot showers. If you want, try running the hot water very slowly while you shower. You should get hot water almost forever. If that works, get a new shower head so you can have a nice spray.
: My shower/bath water is hot for only a few seconds, then it get luke warm
: > to cool (at best). I adjusted the water temp on the furnace (I believe I
: > have Domestic Hot Water) since I have no hot water tank. It was at 140-160
: > and I have turned it way up to 180-220. I basically had no change in water
: > temp after 12 hours. The water from the tap is red hot. It would seem that
: > I never can get a good flow of hot water. I also notice a small tank
: > (about 5 GAL) which could be a mixing tank, but, I am unsure. Any way that
: > I can get my shower back?
: Regarding the furnace, I have just purchased the house (move in on 4/24) and
: yes this has always happened.
: Since my last message. I have taken another look and found a plastic dial
: with numbers on it and on the cast iron part of the valve(?) is embossed
: hot\cold. I assume that this is a mixing valve and have tried to adjust the
: valve all the way in one direction and then all the way in the other,
: letting the shower run for a 5-10 minutes each time. Both ways I
: experienced the same results... cold shower.
: I also noticed that the valve is tied into the system with three threaded
: fittings and looks VERY replaceable. I'm sure Home Depot would have the
: part and as far as installing it, the shut off for the furnace is at the top
: of the stairs, the hot & cold water pipes have spigots to turn them off,
: but, I do not see a spigot for the third pipe (I think it goes into the
: Is this normally a difficult task? I have replaced faucets and electrical
: fixtures and can perform small repairs to my car, but should I venture to
: expand my list of trades? Binx