Joe Fiedrich
The new mechanical room. Left to right are a 42-gal. Viessmann Vito DHW tank, a Viessmann Vito Rond cast iron boiler, a Grundfos circulator and (on the wall) a Zilmet Model ZFT expansion tank. Above, on the wall, is a Viega Model 12152 Enhanced Mixing Station (with another Grundfos circulator on the left-hand side).

Updating a 40-Year-Old Hydronic Heating System

June 24, 2022
Improvements in technology and changes in system design tell a fascinating story.

Hover over main image for caption information.

A typical oil fired forced hot water heating system, installed in 1980 was ready for replacement. The original boiler room equipment was removed, which consisted of a boiler with tankless DHW coil and oil burner, the oil tank, three zone valves for copper fin tube convectors, a circulator, one zone valve for a basement fan coil unit heater, zone controls and an expansion tank.

Nothing surprising, just a typical hydronic heating system built in the Northeast and Midwest in the '60s, '70s, and '80s in the United States, of which there are hundreds of thousands still in operation today.

Not only was the boiler room equipment replaced with new updated components but the entire control strategy has been rearranged from intermittent to continuous circulation. Rather than injecting high water temperature intermittently into the system, we are now modulating the water temperature based on room temperature and/or outdoor temperature set point while keeping the circulator running continuously.

The Control Strategy

A self-regulating circulator is adapting its properflow rate through the system regardless how many zone valves are calling. A non-electric room temperature sensor or an electronic control resets the water temperature to the infinite degree by sensing the room temperature of the highest heat loss area of the house.

The rest of the rooms still have the option to close zone valves occasionally based on their settings and occupation. As an alternative, flow meters at the manifold can be fine-tuned to eliminate zone T-Stats and power heads (however, that degree of fine control is not for the average homeowner).

The non-electric room temperature control is an inexpensive control strategy since no expensive electronic reset controls are required. All-in-all, a very cost effective control strategy that provides a simple and reliable building temperature control that will last for many years to come.

By resetting the water temperature at the diverting valve between the heat source and the distribution manifold to the Nth degree the precise BTU input is being offered to the building without over- or under-shooting room temperature. The rest of the temperature control is accomplished with electric zone valve power heads. 24V zone T-Stats will act as high limits. This temperature control method will assure a very high degree of improved heating comfort with a significant reduction in burned fuel.

The building is being offered the precise amount of BTU's at any given outdoor or indoor condition, regardless of external or internal heat gains due to solar gain or people.

Keep in mind the most ideal way of controlling a hydronic system is keeping the water moving continuously, fine tuning the water temperature to the building under any condition to supply just the right amount of BTU's to satisfy the instantaneous heat loss to the building.

High Efficiency

An indirect-fired 42 gallon water heater adds to the efficient boiler room package with endless supply of  hot water. The boiler, DHW tank and piping are all well insulated, which will ensure a cool boiler room while putting the BTU's where they are needed, adding to the fuel savings. The key link between the heat source and the distribution manifold must be a three way mixing or diverting valve. The advantage of a diverting valve is the option of using non-electric low cost controls. However, if electric controls are desired, you still have the option to do so with a divertingvalve. Room temperature feedback from the highest heat loss area of the building is mandatory in
either case.

The final component replacement for our system was a double wall polyethylene/sheet metal oil tank with a 275 gallon holding capacity. A safe, clean and space saving solution.

Proper Distribution

Due to a former kitchen remodeling project, a large section of the existing baseboard radiation was removed resulting in insufficient BTU output in the kitchen and the adjacent dining room.

For that reason radiant panels were added from below the sub floor between the basement joists to compensate for the output. Kick space heaters or fin tube radiation were not feasible. The panels were stapled with the aluminum side facing up. 5/16" oxygen-tight  PEX tubing was snapped into the panels. 8 inch fiberglass paper backed insulation was added between the joists with a 3 inch air space between radiant panels and insulation, craft paper facing down.

Dramatically improved heating comfort will be realized next winter with a significant fuel reduction. With fuel prices going through the roof, this revamp is a blessing in disguise.

Additional fuel savings will be realized with the use of temperature setback thermostats and master controls.

Constant low water temperature circulation alone adds 1% fuel savings for every 3 degrees of system water  temperature reduction versus high temperature 180 degree intermittent operation, which is causing overshooting of room temperatures, temperature swings, more convection heat than desirable radiant heat, as well as boiler room and piping transportation heat losses to get the water temperature to where it is needed at your radiators, convectors and radiant systems.

Compare and Contrast

A typical hydronic heating system from Europe, where forced-air systems are (virtually) unknown, uses constant circulation. Advantages include:
  • Significant fuel savings
  • Increased heating comfort
  • Increased electrical power reduction by eliminating multiple zone circulators and using a single, power-saving, low-wattage circulator with zone valve and flow meter adjustments
  • A "perfect marriage" in combination with heat pumps, electric boilers, solar systems and wood-fired boilers
  • Small, constant BTU draw at the heat source eleminates problems related to on-off high temperature water distribution systems
  • Low water temperature heat sources are now becoming super-efficient hydronic systems

System Components Used in This Installation


Model Vito Rond, 100 model VR 1-22,  80 MBH Output                                                     

With Riello oil burner F3, Series 40                                                             


Model Vito Cell  Stainless Steel 300 - V, 42 Gallon Capacity                                              


Tank Model DWT 1000L, 275 Gallon Capacity                                         


QuikTrak Aluminum/Plywood Panels, plus 5/16" Oxygen Barrier Pex Tubing                                               


Model # 12152 Enhanced Mixing Station for (10) Circuits                                   


Copper ProPress 1/2", 3/4", and 1" fittings                                                                                  


Model ZFT 18 R                                                                                                


Model 101 22 97 Wall Mounted T-Stat with 33ft Capillary and Valve Actuator        


(OPTIONAL) PRORADIANT BASIC HEATING                                                           


(OPTIONAL) VITOTRONIC 100 CONTROLLER #7834 238                                          


FAN COIL UNIT                                                                                                   


Part #15069                                                                                                         


Mixing Station Modulating Circulator # 99287244 ALPHA2                                         
Fan Coil Unit Circulator # UPS 15 - 58 three speed                                                        
DHW Tank Circulator Model # UPS 15 - 58 three speed                                               

AMTROL DHW TANK EXPANSION TANK MODEL # ST5                                                 

MILWAUKEE FITTING PRESS TOOL MODEL #2773-20                                                    

CALEFFI BOILER FILL AND BACKFLOW PREVENTOR MODEL #573                                    


CALEFFI DHW TEMPERING VALVE MODEL #521                                                        

Joe Fiedrich is a former hydronic heating columnist for CONTRACTOR. He was the president of Stadler Corp., which is now part of Viega North America, and he is the inventor of the Viega Climate Panel and Uponor Quik Trak. He can be reached at [email protected]. Also, visit his company

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