Zone Valves or Circulators?


By Bruce Marshall, Emerson-Swan


In my hydronics classes I always like to ask the attendees which they prefer, zone valves or circulators.

A few years ago most of the guys were in the circulator camp with a few hands raised for zone valves. I would always ask a couple of the circulator guys their reasoning. The answer was always this. “If there was only one circulator and it failed, the whole building was without heat.”

This was a valid argument 40 years ago, when three-piece circulators were the norm. Water lubricated circulators came into being in 1963 when Taco invented the Perfecta, but they didn’t really catch on until about 10 years later. We in the heating industry are not early adopters; we would rather wait until someone else uses it for a while and let them get the bugs out.


Using Less Electricity

Today’s circulators are much more reliable than the old three-piece jobs, and failures are rare, so that 40-year-old argument no longer holds water. The main focus today is energy usage, especially electricity. Just take a look at all the regulations we have seen in recent years: the demise of the incandescent light bulb and the new electric water heater standards that will kick in next year are but two examples of government mandates to reduce the amount of electricity usage. Four circulators running, each pulling 80 watts equals 320 watts. One circulator and four zone valves could use as little as 86 watts. If a variable speed circulator is used instead of a fixed speed unit, the wattage could run in the low 20s.

In order to heat a space you have to “deliver” X amount of BTUs into it. In order to cool a space you have to “remove” X amount of BTUs from it. Long before you answer the question of zone valves versus circulators you have to know how many BTUs you need in each zone.

If you use zone valves then you have to ask what “size” circulator or zone valve do I need to let enough water circulate in order to “deliver” that many BTUs to the zone. Water will always take the path of least resistance, so the circulator must be sized to accommodate the zone with the most resistance plus be able to deliver the total gallons per minute for all the zones. The Universal Hydronics Equation is used to determine the GPM requirement of the system.

GPM = BTUh ÷ (Delta T x 500) or simply GPM = BTUh ÷ 10,000

Once the GPM has been determined, you next need to calculate the highest amount of resistance, and it’s not necessarily the longest zone. The formula for this is:

Total Linear Length ÷ .06 = ft of head

The .06 number accounts for 4 feet of head per 100 feet of pipe plus 50 percent to account for the fittings. This is used for pipe runs or baseboard loops. When sizing for a higher resistance component such as a hydro-air unit, you would need to know the feet of head of the unit plus the length of the supply and return pipes – the hydro-air unit could be closer to the boiler but have more resistance than the baseboard loop. If it’s easier for the water to go through the baseboard then that’s where it will go and not to the air handler.


Toward Better Efficiency

I mentioned earlier in this article that using a variable speed circulator is more energy efficient from an electrical standpoint, but it can also make the entire system more efficient as well. North American residential system design is pegged to the temperature differential between the supply and return (Delta-T). It’s part of the Universal Hydronics Equation, it’s how we size boilers, select pipe size and determines the amount of radiation to put into a room.

We also zone our systems and turn them on and off with a thermostat – all in an effort to deliver comfort. If we use a Delta-T based smart circulator like the Taco Bumble Bee or VT2218, the pump will react to how many zones are open, which zones are open, and even how cold it is outside, automatically and without an outdoor sensor.

These infinitely variable speed Delta-T circulators deliver only the BTUs required to match the actual heating load required by each zone. The entire system runs at peak efficiency, no matter the time of year, accurately providing optimum comfort and real electric and fuel dollar savings.

As for electricity savings, combining an ECM circulator such as the Taco Bumble Bee or VT2218 with Zone Sentry zone valves in place of traditional PSC circulators, your customers could expect to save as much as $20 a year for each pump replaced. This might not seem like a lot but when you look at a multiple zone system with pumps, the savings add up year after year.

How does reduced delta-T affect a system? Consider its impact on a modulating condensing boiler. If the system is designed for a delta -T of 20°F, but achieves only 12°F to15°F, the amount of run time the boiler spends below the point of flue-gas condensation will be affected.

If a reset control is telling a boiler supplying heat to radiators to fire to a high limit of 142°F on a 20°F day, a delta-P circulator programmed based on estimated system head loss may wind up sending 130°F water to the boiler. That is the condensing point, which makes the boiler work at, say, 86 percent annual fuel-utilization efficiency (AFUE).

A pump programmed to deliver a 20°F delta-T, on the other hand, will send 122°F water back to the boiler, creating more condensate and allowing the boiler to operate closer to its design.

So for all you circulator guys out there, maybe it’s time to take another look at “but that’s the way we’ve always done it.” We’ve moved on, and so has the equipment. And your customers deserve the highest efficiency and comfort that you can give them.

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