In the field... Robert O'Brien Technical Heating Co. Mt. Sinai , New York Indirects & Zone Valves? Zone valves and indirect water heaters? I’m guessing most guys would rather drink warm beer than install an indirect with a zone valve. Why? The answers given are usually the standard ones given about zone valves in general—slow recovery, not enough flow, bad practice, no domestic hot water (DHW) if circulator pump fails—but in reality the real answer is the same as it most often is in this industry: “We always do it that way.” I also felt that way, but over the course of years I ran across many jobs with a zone valve on the indirect, and by and large the customers weren’t complaining. I assumed they thought the reduced performance was normal, so I asked them and they proceeded to tell me how long their teenagers were in the shower and how happy they were with their decision to buy an indirect. How can this be? After doing a little research, I decided to switch sides! We install indirects with zone valves quite frequently and never have an issue. It’s a bit of a long explanation involving a fair bit of math but bear with me. Let’s assume we are installing a new boiler with an indirect, utilizing a zone valve. How do you size a pump for an indirect currently? To do it correctly, you need to know the head loss of the piping, fittings and heat exchanger as well as the desired flow rate and then choose a pump that will meet the gallons per minute (GPM) requirements at the head loss of the system. How many people do that? Usually not too many! Most people will choose a 007 or to be on the “safe side,” a three speed pump so they can turn it up. For an example, let’s use a popular 32-gallon indirect with a head loss of 1.9ft and a recommended flow rate of eight GPM to achieve its rated first hour output of 165 gallons per hour (GPH) at a temp rise of 77°F, assuming a boiler temperature of 180°F. What’s the head loss of the piping? The short formula, which works pretty well, is as follows: Total Linear Length x .06 = ft of head There is approximately four feet of head per 100 feet of pipe run, 50% is added to account for fittings which gives us .06. However, what about higher resistance devices installed in the piping, like flow valves and zone valves? These have to be accounted for separately and are rated by Cv. What the heck is Cv? The Cv rating represents that amount of flow needed in order to create a pressure drop of one PSI, or 2.31 feet of head, across a component like a valve. How do you convert Cv to feet of head? More math, unfortunately! (Flow in GPM divided by Cv)2 x 2.31 = head loss Let’s use a zone valve with a Cv of eight in our hypothetical installation; the higher the Cv number, the less resistance to flow. (8 ÷ 8) squared = 1 x 2.31 = 2.31 ft/head loss for zone valve What about the piping? Let’s assume we have 25 feet of piping from the boiler to indirect and back. Just a little more math: 25 x .06 = 1.2 ft/head Lets add it all up! Indirect: 1.9ft Piping: 1.2 ft Zone valve: 2.3 ft Total: 5.34 ft/head at 8 GPM What size pump do we use? Taking a look at a pump curve chart for a circulator that will provide 8GPM at 5.5 ft/head, an 005 is pretty close but an 007 is a better choice. What if we used a circulator and weighted flow valve on this job? Everything else would stay the same; all we would have to do is swap out the head loss of the zone valve for the flow valve. What is the Cv of a weighted flow valve? A 1" NPT flow valve has a Cv of 6.7, the same valve in sweat is 7.7 Cv. Remember, the higher the Cv, the less resistance. The installation with a flow valve actually has a greater head loss than the zone valve job? Yes, it does! This is how we can use zone valves on indirect water heaters with no homeowner complaints. The caveat when using a zone valve on an indirect is domestic priority. Without priority, you need to size the circulator for the zone with the largest head loss, which may not be the indirect zone and the flow rate for the all the zones combined. ICM 12 ICM/July/August 2016
ICM July-August 2016
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