Steve.N wrote:Greg and Yanche,
I agree with both of you, in fact the plate exchangers are rateded by flow and not temp input. What I am talking about though, and this is from observation, is this, There is an existing oil boiler rated at 120 K BTU that heats a house fine. I install an outside boiler rated 300K BTU and interconnect with an exchanger rated 250K BTU and can't get enough heat to maintain the house during extreem weather. If I remove the exchanger all is fine. This hasn't been the rule only an exception on a few installations. One of the problems with the outdoor boilers is no pressure so you are limited to 180 or 190 deg water temp to avoid boiling. My feeling is that the radiation in the house is designed too close to the maximum limit and the loss across the exchanger and the lower water temperature will not allow enough heat to be radiated into the house.
What must be done to engineer any hydronic system is the determine the system "resistance". Resistance is just that, the resistance of water to flow. Pipe, elbows, valves, heat exchangers all have a resistance to flow. The component manufactures determine by testing or calculation the resistance and express in the equivalent of straight pipe resistance per foot. Once you know the system resistance you evaluate several points in a flow equation. The flow curve shows the total head resistance vs. flow of your piping and hydronic components. The shape of the curve is parabolic with an origin going through (0,0). You then superimpose this curve on a family of pump curves, to select your pump. See the attached pdf for an example of pump curves. For a given pump the intersection of the system resistance curve and the pump curve will be the operation point. It determines the flow rate. You can not change it unless you change some component. Given the flow rate you use the boiler supply temperature and the heat emitters (baseboard, radiators, piping losses) to determine your BTU delivery capability. Then you look at you building heat loss calculation and see if it works.
Like the electrical analogy in a series circuit, the highest resistance limits the performance. Rule of thumb design guides were developed to allow plumbers to design simple heating systems. They are not very useful in trade-off design approaches. Clearly in your case the heat exchanger is the limiting hydronic component. Solutions would include a larger heat exchanger, increasing the operating temperature on the supply side of the exchanger and increasing the flow. All have trade-offs in cost and performance. There are two approaches to a solution. One is an engineering design on paper or upgrading components until you find the right one. Kind of like the auto mechanic that fixes the car by changing parts until he finds the one that makes it run.