System zoning-how to and how not to do

Zoning has become exceedingly popular over the last couple of decades. The university of Illinois did extensive testing in the 1960s end determined there was fuel savings when buildings were zoned. The university used only cast-iron boilers for their testing. They found out if you took a single zone home and applied two zones the fuel costs decreased. They also determined boiler operating efficiency went down slightly.

While zoning is extremely popular today some contractors have gotten overzealous with zoning. Some zoning is good but too many zones could end up costing the consumer more fuel usage unless it is done improperly and increased installation costs.

system zoning can be controlled using individual circulators or zone valves. The popularity of each is determined by geography. Most zoning with circulators is done in the east and most zoning with zone valve is done in the Midwest and the West. I have gotten asked many times which is better, zoning with circulators or zoning with zone valves. It is more of a case of personal preference of the installing contractor.

The argument of which is best is as old as Ford versus Chevy, Coke versus Pepsi, create other common on-going arguments. I personally and more concerned with the piping application than I am how it is zoned. Below are two examples one zoning with circulators, and 1 zoning with zone valves.

Manifold w-circulators
Zoned with circulators

Manifold w-Zone Valves

Zoned with Zone Valves

Today's boiler installations have moved from cast iron to more of the high efficiency modulating condensing boilers. while these newer boilers with their 5:1 to 15:1 turn down ratios makes zoning properly easier. these boilers will modulate the fuel input according to the load during the heating demand cycle. Cast iron borders are still very popular as a retrofit boiler but have one input, which means it'll always input 100% of the rated btu's.

The only hard and fast rule that I have for zoning high efficiency boilers is to keep the minimum zone BTU output equal to or greater than the minimum input of the boiler. If that is not doable there are other options, we will address it a little later.

Let us discuss zoning cast iron boilers. Since this boiler fires 100% on or off we must be aware of how much it will short cycle by the number of zones. We are looking for a 10 to 15 minute runtimes so having too many zones could cause severe short cycling. Even though the zone will heat properly with the boiler short cycling it does affect the boiler efficiency and the amount of service and maintenance required on the boiler over time. I would normally try to limit the number of heating zones or a cast iron boiler to 2 or 3. If you need many zones, I would consider installing a modulating condensing boiler instead of a cast iron boiler due to modulation to reducer boiler short cycling. If you still want to use a cast iron boiler, we can address piping with more zones a little later.

Cast iron boilers should be operated between a 20 degree and 40-degree delta T. With special piping applications we can work outside those parameters. To determine the flow rate through a boiler you would use the DOE or gross output rating on the tag and divide it by 10,000. For instance, if the boiler is 100,000 btus Doe output divided by 10,000 the maximum flow rate through the boiler would be 10 gallons per minute at a 20-degree delta T. divide that by two that would be your flow rate at a 40-degree delta T. Sorry 100,000 BTU boiler should have a flow rate between 5 and 10 gallons per minute. A Flow rate higher than 10 gallons per minute in this example would cause more boiler flue gas condensation and reduce the life of the boiler and increase fuel consumption overtime due to corrosion of the boiler heat transferring surfaces.

Zoning with Zone Valves

Let’s circle back to the argument of circulators or zone valves. If I were using zone valves I would have multiple zone valves and a single circulator. The best circulator to use with that application would be a variable speed circulator. These circulators use ECM technology and will reduce the watts, so it is easier on the electric bill. Variable speed circulators will speed up and slow down as zones open and close. Do not confuse variable speed circulators with multi speed circulators. The zone valve end switches can be daisy chained together and wired back to the boiler to start or stop the boiler and circulator. There are also multizone panels available for zone valves and offers a priority zone if you have an indirect water heater.

Zoning With Circulators

When zoning with circulator's I get very concerned about the flow rate through the boiler. Most zones require less flow than the size of the pipe his capable of flowing. But most of the time, the circulator's installed per zone will move more water than there's zone needs. The reason for the concern would be excessive flow rate through the boiler causing more condensation and corrosion.

For instance, a ¾” pipe which is most common for zones is capable of creating a flow of 4+ gallons per minute. using the calculation above if the boiler Has four zones it would be capable of moving 16 gallons of water per minute if all four zones are calling at the same time. This would shorten the life of the cast iron boiler. You can see by the following diagram how this could be a problem.

Moose Antler.jpg

I feel if zoning with circulator's it would be best to pipe the cast iron boiler utilizing primary secondary. When primary secondary piping is used the system flow is separated from the boiler flow. When piped in this manner you can control the amount of flow through the boiler with a flow regulator. The flow raked through the boiler can be different than the flow rate through the system with this style of piping.

For more information on cast iron boiler protection, you can use the search box in the upper right-hand corner and type boiler protection, buffer tanks, primary secondary piping, bypass piping, and three- or four-way mixing valves. You will see there are many ways of protecting cast iron boilers when needed.

Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, installations should be reviewed on a per job situation. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or its personnel accepts no responsibility for improper information, application, damage to property or bodily injury from applied information found on this website as it should be reviewed by a professional.