in condensing mode if it's the only one calling for heat. Sure, I could drastically oversize the radiator, which would allow for a lower supply water temperature. But this 6-foot by 10-foot room has a good portion of the walls taken up by doors and the fixed glass on the doors. My clients were excited about the prospect of a comfortable breezeway because of my "great" idea, yet I still wasn't sure how I would solve this dilemma while maintaining simplicity and efficiency. I let things bounce around in my head for a day or two, confident the solution would come to me. It almost always does at some point, and this time it popped up while watching a Cubs game on TV from our home in Vegas. I'm not sure how that works, but I remembered seeing a towel warmer being piped in parallel to an indirect tank in an old Viessmann guide - pulling all its heat from the tank and not the boiler. That would be the key. I want the heat to be drawn from the 50 gallons of 140-degree water stored inside the tank rather than 2.5 gallons of water stored in the boiler's small fire-tube heat exchanger (see Figure 2).
Simple, efficient piping and controls
It's easy enough to plop a chunk of cast iron onto a drawing. Now, it needs to be piped and controlled in such a way that we remain true to our purpose of keeping it simple and efficient. I didn't find the drawing in the newer Viessmann application guides, but the concept is the same. The breezeway radiator will need its own circulator, pump relay, thermostat, purge port and a couple of well-placed swing check valves. Take a look at what I drew up in Figure 3, and here's how it's going to play out: 1. On a call for heat from the breezeway thermostat, the pump relay bangs on the dedicated circulator for the radiator. That's it. The boiler pump and the indirect pump will remain off. The relay end switch isn't used because we don't want the room thermostat prompting the boiler burner to fire. 2. As water is drawn from the tank, the temperature of the water in the tank drops below its setpoint. The indirect water heater sensor senses it and triggers the boiler burner to fire and ramp up. Its purpose is to heat the tank to satisfy the tank sensor setting. It will continue to do that until the breezeway thermostat is satisfied, shutting down the dedicated pump until the cycle repeats itself. The beauty in this is that it's unlikely the burner will shut down and restart before the room thermostat is satisfied because of the stored thermal mass within the tank. 3. The two check valves in the supply and return pipe from the boiler to the indirect ensure we won't pull heat directly from the boiler when the breezeway zone calls. When sizing the radiator, it's important to meet the heat loss of the room on the coldest day of the year with 140-degree water because that's where you're capping the indirect tank's water temperature. You're probably scratching your head over the fact that the radiator supply is tapped into the indirect tank's return to the boiler, and the radiator's return is tapped into the tank's supply. The best way I can explain it is this: If you switch them to what looks like the logical way, the boiler would now be supplying the radiator directly and not the indirect water heater, which is the exact opposite of our goal. Believe me, I tried it on paper. The swing checks are there to help ensure this. You've got to love the simplicity of this. I know I do. German engineering is tough to beat, or so I hear. The indirect water heater serves a dual purpose, acting as a buffer between the small microload zone and the boiler while still providing potable hot water. The efficiency doesn't suffer, and you won't add thousands of burner cycles over the years because of a single radiator. And don't forget about this option for a towel warmer or two, such as Viessmann's Application Guide once illustrated. It's a shame that hydronic systems are not used on every home and project out there - there's so much flexibility with them. l Stephen Minnich has nearly 40 years of experience in the heating industry, primarily designing, servicing and installing hydronic systems. He's moved from the Chicago area to Las Vegas and has hung up the tools for good. His newest venture is Minnich Hydronic Consulting & Design. He can be reached at stephen.minnich@yahoo.com or 630-291-3028.
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phc June 2022 www.phcppros.com Figure 2. The radiator is ready to be piped. Figure 3. The final product.
You've got to love the simplicity of this. I know I do. German engineering is tough to beat, or so I hear.
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