At this time of year I occasionally have more heat than I need since I now have 4- 4X8ft Sun Earth collectors (added 2 last year).  I have gotten various suggestions for solutions to this issue, including a thermostatic 3 way valve that dumps heat to an outside radiator, and boat covers for the extra collectors.  I have yet to implement these suggestions.

Excess temperature in my storage tank drives the pressure over 100psi, at which point the P/T valve at the top of the tank dumps hot water all over the place!  (I now have a bucket in place under the outflow!).

I figured that if I could just back the pressure off a bit somehow, that I could continue to store more hot water.  I was able to do this manually by turning up the thermostat for the radiant floor loop, the excess heat is then dumped to the floor and the pressure plummets in a few minutes.  When the tank measures 165F at the top( outflow), and 90psi, the water coming out the bottom is no more than 75-80F before it goes into the heat exchanger, this is due to the deliberate temperature stratification in the tank.  I figure I could squeeze more heat into the tank if I could just dump the pressure.

My solution is to replace the pressure gauge at the top (outflow) of my storage tank with a gauge that has an adjustable electrical set point.  Now, when the pressure reaches 90psi the set point closes.  I wired it in parallel with the radiant floor loop thermostat so that hot water is dumped into the slab.  The result is that in less than 5 minutes the pressure is down below 90psi in the tank.  The gauge will continue to cycle on for a few minutes throughout the afternoon for a minute or so on very sunny days.
                              
(the light in the gauge goes OFF when the set-point is reached - actuating the floor circulation pump)

Since the heat dumped to the floor is negligible (it takes 3 hours to see a temperature rise at the thermostat for the radiant floor) the solution works fine for the occasional spring and fall warm days.  I believe that this allows me to store extra heat in the tank.

I still plan to drain and flush the collectors when the weather turns hot, but this seems like a viable interim measure.

Above are images of the gauge installed at the top of my tank.  I got the gauge from C&H Surplus in Pasadena CA for about $20.00.  (Unfortunately C&H no longer sell the same gauge ore any other suitable gauge with set points.  I'm not sure where you would find a similar one at this point)

UPDATE: January 10, 2005
I read a brief article in Home Power magazine (#105 Feb/Mar 2005, page 100) about Expansion Tanks by Chuck Marken.  He describes the purpose of a tank this way: "An expansion tank gives a solar water heating system elbow room, preventing burst pipes when pressure rises as the water gets hotter."  Out of curiosity I asked him if he thought that my tank was correctly sized.  He pointed me to the Amtrol web site:
http://www.amtrol.com/extrol.htm
Download their product brochure, this document has sizing information for residential hydronic systems.

I determined that my tank (visible at upper left of the image above) is way undersized.  I have a #15 tank which has a 2 gallon volume, what they recommend based on my system specs is a #60 tank with 7.6 gallon capacity.  Presumably this would eliminate the pressure issues.  At a cost of about $90.00 I will be replacing my tank as soon as it is practical.

UPDATE: April 2, 2005
I installed  the expansion tank.  Click here to learn more.