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why can't i and thanks for help to date

hello. thanks to all for help over the years. please skip to last para if you want to bypass my bs.

i inherited a homemade primary/secondary system using takagi tk-jr tankless as heat source upon purchase of house, prior homeowner also kept oil boiler attached in p/s fashion (with return water protection) in case price of oil every drops lower than gas. house inspector poo-poo'd but previous owner showed gas records that proved he heated house for less than 1500.00/yr including dhw (indirect in p/s w/ priority) and that he previously used 1800-2000 gals of oil per year. sure enough 3 yrs later, the heating cost has been spot on + or minus for changes in gas prices and tk appears to be holding up. 

previous owner gave me "lesson" on what is what and even printed directions for system. unfortuantely i misplaced his new contact info or i would just call and ask him. the system has the heater plumbed into primary loop w/ flow controlled by balancing valve. off that is secondary loop to indirect for dhw with priority. another secondary loop feeds a 20 gallon re-purposed water tank (electric w/ 2 elements taken out and swapped with fittings to create "buffer" tank) which is controled by outdoor reset for the temp of water in the tank. the house heating is zoned off the tank with zone values. the secondary loop feeding the "buffer" has a taco 007 and a b&g balnacing valve to limit flow into the tank to match output of heater so supply water feeding tank does'nt mix with return water back to tank across the closely spaced tee's. that pumped died and i have replaced and everythings dandy.  when replacing the pump i saw the b&g set to what equates to 4 gpm. from the 007 pump curve i saw the pump can supply almost 30 gpm based on the head loss of the secondary loop, hence, i assume, the balancing valve to keep the supply temp to tank up.

so all that for my question. it seems valve must be restricting flow alot to keep the system balanced the way it was designed. the size of the pump greatly exceeds what is needed and its power comsumption is drawing alot more amps  than needed. i thought of replacing pump with differant one but 007 was cheap, easily available, and i didnt want to screw with his design as it has worked out so good to date.

can a 5amp ceiling fan type "dimmer" switch be used to dial down the speed of the pump, and by doing so, the power consumption? amp draw of pump is under 1 amp at full speed. switch is made to handle a motors load. i would have to adjust the balancing valve but could save alot on electric power use over life of pump and switch, box, wire cost less than $15.    if not, anyone know why not?  

thanks and sorry if boring or dumb
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