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The check was used on an aqua-booster to direct the incoming cold water though the coil to help keep it clear. But it will drop the pressure ….

You have a bypass ……. Shut off the hot water and then open a hot facet , if water runs out , then you have a bypass …

Hope this helps

It does take a lot more piping to make two boilers work on steam if run at the same time. Two boilers with manual changeover is ok. But to work automatically controlling condensate and water level is a issue. With 2 boilers you usually size them for 80% of the load. can probably go less. With 2 boilers the idle boiler fills with condensate as it acts as a condenser so now you need overflow traps and a feed tank. fed water isolation valves and controls. It gets $$$$

I would stay away from 2 unless size is an issue its a lot more work.

That said 2 will be more economical to run but more install $$$ and more maintenance.

Its alot more $$ & piping to do two boilers....it's got distinct advantages, but..…Mad Dog

Using 2 boilers with Steam, you will most likely have both boilers start at the same time and produce enough steam to get the pressure up to about 3/4 of the operating pressure then have a boiler drop out.  The other boiler will keep operating until the call for heat is satisfied.  If the one boiler can not keep the pressure high enough because the load is greater (because the outdoor temperature is lower and the radiators can't keep up with the amount of heat needed in the rooms) then the second boiler will cut back in if the pressure drops.

Another way is to operate only one boiler when with an outdoor temperature sensor in series with the other limits.  

The best way is with a Heat Timer staging system with a little more logical inputs that take into consideration several inputs like outdoor temperature, boiler pressure, and cycle timing.  

Two boilers are usually more economical to operate and sometimes less costly to maintain since you are using lower price boilers and parts.  Sometimes there is an economy of scale with performing two maintenance procedures without moving your tool box and truck to another customer so the actual maintenance per boiler is less expensive.

With Hot Water the answer is easier.  It is definitely more efficient to use two boilers.