I've finally gotten around to adding the buffer tank to this system. We've had the system shut down and winterized since Sept and were only able to return to Maine to install the Buffer Tank the last week of April. Results are 'mixed'...
I've attached photos of the installation, but, basically, I've disconnected the boiler supply and return from the radiator loop manifolds, created a primary loop with the boiler feeding a 28 gallon hot water heater (used as a storage tank) and returning directly to the boiler. I've installed a closely spaced tee manifold to connect the radiators as a secondary loop into the primary loop supply line feeding the bottom drain port on the tank. The top 'Hot' port of the tank was then connected back to the boiler return, included in that line is a tee to an additional 2.1 gal expansion tank (the boiler has a 2.2Gal built-in, but calculations showed that I needed a bit more with this additional water in the system).
The secondary loop has its own circulator (Taco Bumblebee HEC-2 running in deltaT mode). This circulator pumps into the secondary supply manifold that feeds the heating zones through zone valves. The returns from the heating zones are gathered into a return manifold that feeds into the lower side of the hydronic separator.
The circulator deltaT sensors are placed on the shut off valve on the input side of the circulator and on the return manifold near the hydronic separator return port.
As I said, the results are mixed. When first starting up, the boiler runs for a good long time to bring the tank up to temperature. The time depends on outside temperature conditions due to the outdoor reset and the boiler modulation, but it's run between 16 to 25 minutes to get to a quiescent state. Once in this state, however, the boiler still seems to burn for a very short amount of time. Again, exact time depends on outdoor conditions, but it's almost always less than a minute.
I measured the return from the tank into the boiler and it seems to stabilize at an almost constant temperature that is just above the boiler's low trip point. The boiler has about 7-8 degrees C hysteresis, and seems independent of the outdoor temp (the trip points move with outdoor temp, but the difference between high and low remains the same).
The boiler, when it's burning, is definitely pumping hotter water into the supply side, you can see it rise on the boiler's output temp gauge, I've measured it at the zone supply manifold, and the circulator adapts its speed as the temperature rises. However, the boiler's output temperature seems to reach the high point very fast AND does not seem to be solely driven by the return temperature (which is always about 6-7 degrees C lower).
One thing I did discover, after tearing my hair out and replumbing the tank though practically every permutation (the tank has four ports which I tried to use for both primary and secondary ports), was that the boiler has an internal "automatic" bypass between the supply and return, just above the respective shutoff valves. Thinking that too much heat was circulating in the bypass loop and that this might be adjustable, I looked for an adjustment control. There is none. The only thing that appears 'automatic' about the bypass is that it is ALWAYs bypassing, the only 'control' is that the diameter of the bypass tubing appears about 3/8" vs the 3/4" piping I'm using for the hydronic loops.
Other than providing a safety in case the supply and return shut off valves are closed, does anyone know the purpose of this bypass loop? And, is there any way I can limit it so that more of the return heat is used?
The main reason I'm getting any reasonable use from this setup is that I've modified the boiler burner 'off' time limit from 3 minutes to 6 minutes thereby limiting the number of cycles it goes through in an hour. It sure would be nice, though, if the buffer tank actually served as a thermal mass for the boiler. Now it's nothing more than a storage tank, proving heat to the secondary loop when the boiler is not able to burn...
Tom