Flow Control Valves

Mike Kusiak

I have been looking at a couple of 50 year old hot water heating systems recently, and am trying to understand the reason for the flow control valves. These are both on-off, single zone, diverter tee (Taco venturi) systems with B&G 100 circulators on the return to the boiler. The flow control valves are in the supply from the boiler, and the plain steel expansion tank is connected to a fitting on the valve. One of the valves is a Thrush, the other a Taco. They both seem to serve the same purpose. I could understand the need for the flow control valves to prevent gravity flow if the boilers had a tankless coil for DHW, or there was more than one zone, but why would you want to prevent gravity circulation in a single zone system? I would think that it would be more efficient to allow the heated water in the boiler to gravity circulate after the burner goes off. Otherwise the heat in the boiler casting and water is lost up the flue, since there is no flue damper on these boilers. There is not enough heat stored in the boiler to make the house temperature overshoot the thermostat setting, so I dont think this was the reason for the valves. Do any of you old timers know the reason for including the valves, or was it just standard procedure to use them, whether they were needed or not?

Tom M.

possibly,

> I have been looking at a couple of 50 year old

> hot water heating systems recently, and am trying

> to understand the reason for the flow control

> valves. These are both on-off, single zone,

> diverter tee (Taco venturi) systems with B&G 100

> circulators on the return to the boiler. The flow

> control valves are in the supply from the boiler,

> and the plain steel expansion tank is connected

> to a fitting on the valve. One of the valves is a

> Thrush, the other a Taco. They both seem to serve

> the same purpose. I could understand the need for

> the flow control valves to prevent gravity flow

> if the boilers had a tankless coil for DHW, or

> there was more than one zone, but why would you

> want to prevent gravity circulation in a single

> zone system? I would think that it would be more

> efficient to allow the heated water in the boiler

> to gravity circulate after the burner goes off.

> Otherwise the heat in the boiler casting and

> water is lost up the flue, since there is no flue

> damper on these boilers. There is not enough heat

> stored in the boiler to make the house

> temperature overshoot the thermostat setting, so

> I dont think this was the reason for the valves.

> Do any of you old timers know the reason for

> including the valves, or was it just standard

> procedure to use them, whether they were needed

> or not?

Tom M.

It's possible

that there used to be tankless water heaters on the systems or low limits on the boilers. If there are no openings, there may have been sidearm external tanklesses. Also, if there is a low limit or if there was originally one, overheating could occur in milder weather. Fifty years ago, boilers had a lot of water in them and most had low limits. You could have condensation problems in the spring and fall when the boiler runs once a day and starts up cold especially if there's no bypass to preheat the return water. Bottom line, do you see two plugs where a tankless may have been piped or does some of the wiring or controls look newer than the rest of the system? That would be my guess.

Mike Kusiak

System modifications

You bring up a good point on one of the systems (Taco valve), which originally had an Arcoliner oil fired boiler. This boiler had both a high limit and a low limit aquastat. The circulator would not run until the water reached the low limit temp, in which case the flow control valve made sense, as it allowed the boiler to come up to temp without gravity circulation. About ten years ago, the Arcoliner was replaced by a Burnham series 2, but the flow control valve was never removed, nor was a boiler bypass installed. I am concerned about the low return temps with this system, as I have noticed some rusting of the heat exchanger.
On the other system (Thrush valve) the gas boiler and piping is completely original. I know this to be true, as the system is in my father's house, which he built in 1954. The flow control valve makes less sense in this system, which only has a high limit aquastat, and no provision for tankless coil. This system, by the way, has worked perfectly without breakdown for 50 years, with only normal cleaning and maintenance, and replacement of the B&G 100 coupler in 1978!