Plate Exchanger Sizes

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.

Which is the reason I asked the question.

Right now I have an outdoor wood boiler. I also have an EFM 520 in the basement not hooked up. My concern is will I get enough heat with this coal boiler. My OWB struggles when temps go below 10 degrees. I have to load it 4 times a day on those coldest days. After a 4 years of this I'm beginning to think a big issue with my current setup is the 30 plate exchanger.


So look at it this way. My house zones require 198,000 btu at full load. My OWB can put out 250. But my plate exchanger can only handle 180. So thats why I'm struggling to keep the house warm. I also have two propane space heaters for those coldest days.

Yes, it sounds like your plate exchanger is under sized.. But,, does your OWB have to be fired that hard to keep the water temp up to 170-180*?? if so, the BTU rating of the OWB is bogus.. What I'm saying is that if the 150,000btu plate exchanger can draw down a OWB to below target temps, then the OWB is not able to make 150,000BTU, much less 250,000.

So if your OWB is unable to keep up with the heat extraction by the 30-plate, 150Kbtu exchanger,, then the OWB is the problem..

IF the house water temp could not get enough heat through the exchanger, but the OWB water is hot, and the exchanger is hot inlet and outlet,, then the problem is the exchanger is too small.. and it can't transfer enough heat to the baseboard's water.

Greg L

Of course this is all hard to verify. The only thermometer in my system is on the boiler itself.
Doesn't matter I guess. The EFM should be big enough.

I also plan on putting something in the front living room. Hand fired or stoker. Maybe a pellet stove?


That room is always cold even when the rest of the house is warm.

More radiators wouldn't help? A new zone maybe? Scott

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.

When I installed my first hotwater baseboard system with my Dad at the ripe age of 15, we had available 'long-sweep' 90* ells for the 3/4" copper. They were intended to reduce the resistance to flow, increasing the efficiency of each zone.. Back then the only readily available circulator was the B&G -100 'Red Pump'. Now, we have all kinds of pumps with various flow and head ratings available.. and the new pumps are maintenance free, silent and burn less electricity.

I have not found 'long-sweep' 3/4" copper ells for purchase,,, only the short, tight ells... I'm sure the 'long-sweep' ells went away when a greater selection of pumps became available.

Greg L

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