I recently had the opportunity to look over a "gut-rehab" project involving a once stately three story, circa-1800s house only a few miles from where I live. The ambitious owner was ripping the whole place down to its wooden skeleton, hoping to one day restore it to its former grandeur. At the time of my visit, this aspiring Bob Vila had already spent months of quality time with his crowbar, sledgehammer and shovel, removing the old horsehair plaster and lathe walls and ceilings.
Given the sweat equity already invested - not to mention how he'd be spending his nights, weekends and vacation time over the next few years - said owner was adamant about a high-quality heating system. One worthy of the reincarnated mansion it would be serving. Needless to say, hydronic heating was the only option we discussed.
This Old HouseA walk through the gutted house revealed floors several inches out of level due to settling. These floors were framed with an assortment of timbers running in various directions. Thick wood planks lay on top of this framing with several layers of who-knows-what on top of them. It quickly became evident that a radiant floor system would be difficult and painstakingly slow to install.
After discussing many other hydronic options we settled on a combination of fin-tube baseboard and panel radiators. The combination provided a good match for the owner's aesthetic tastes, zoning requirements and budget.
But how should these heat emitters be piped? Was it practical to run numerous circuits of rigid copper tubing from the upper floors down to a new boiler area in the basement? How about a Monoflo loop or two-pipe reverse return on each floor level tied to a common riser system? How could we route the necessary 1-inch copper tube through the maze-like floor framing without creating wheelbarrows full of sawdust?
Then came the thought of running 1/2-inch PEX or PEX-AL-PEX tubing to and from each heat emitter - a technique that's come to be known as a "home run" system. The holes would be small, and the tubing could be pulled up into and through the framing much like electrical cable.
But it was quite a distance from the third floor down to the basement. A lot of tubing would end up as "leaders" between the rooms where the heat emitters were located and the manifold in the basement.
Fortunately, we spotted a vertical chase next to an old chimney near the center of the house. It was a perfect route for a pair of vertical copper risers that could run from the basement all the way to the top floor. Hey É Why not marry the concept of the home run system to a common vertical riser system? Relatively short home run circuits of 1/2-inch PEX or PEX-AL-PEX tubing could be routed out to each heat emitter from the centrally located manifolds on each floor. Zoning could be accomplished using low-voltage valve actuators on each manifold, or nonelectric thermostatic radiator valves at each heat emitter. The flexibility of hydronic heating comes through again.
Fixed Or MixedThere were three possible ways to control the water temperature to the riser system:
- 1. Supply water directly from the boiler while operating the latter on a fixed high-limit setting.
2. Supply water directly from the boiler, but reset the high limit based on outdoor temperature.
3. Make the riser system into a separate loop and meter heat into it using a variable-speed injection pump.
The first method was arguably the simplest and least expensive. However, supplying high-temperature (180 degrees F) water directly into cool piping and heat emitters sets the stage for expansion noises. Imagine that poor owner - a guy who now needs all the sleep he can get - waking up to those ticking sounds and thinking he'd just restored a haunted mansion! A fixed (high) water temperature also makes it harder for the thermostatic radiator valves to stabilize heat output during mild weather.
Method No. 2 was an improvement since it partially reduced water temperature during milder weather. However, since a conventional gas-fired boiler would be used, the amount of reset was limited by the need to keep the boiler return temperature high enough to prevent sustained flue gas condensation.
The third method allowed for a "deep" reset of the distribution loop, as well as boiler return temperature protection. The gradual changes in water temperature appropriated by the reset control would also minimize any expansion noises. Being one who loves the feel of a lukewarm radiator on a cool April evening, I recommended this method. A schematic showing the concept is given in Figure 1.
Vertical VersatilityThe stacked home run system is adaptable to almost any combination of panel radiators - fin-tube convectors, towel warmers, cast-iron radiators or radiant baseboard. Balancing valves on each manifold can be used to compensate for the flow resistances of different home run circuits serving different heat emitters.
Another option worth considering includes panel radiators with externally-mounted bypass valves that allow several panels to be strung together in what looks like a series circuit. However, as the thermostatic valve on a given radiator closes, flow coming into the bypass valve does a U-turn and heads right back out toward the next panel. It's like having diverter tees at each panel without the necessary piping under the floor. This technique could further reduce the amount of tubing routed through the floor framing (see Figure 2).
Still another possibility is a small zone of radiant floor heat tied to one of the higher temperature manifold stations. If the water temperature supplied to the home run manifolds is: (a) higher in temperature than that required by the floor circuit, and (b) reset based on outdoor temperature, then it's possible to achieve a proportional reset of the water temperature supplied to the floor circuits using a couple of manually-adjusted valves. (See the May 1999 Hydronics Workshop column, "Two For The Price Of One," for technical details on proportional reset.)
If the water temperature supplied to the home run manifolds is set at a fixed high temperature, then a thermostatic mixing valve should be used to interface the lower temperature radiant system. These options are shown in Figure 3.
If reset water temperature control is used, I hesitate to suggest installing an air handler, fan-coil or kick space heater. The lower water temperatures during mild weather could lead to uncomfortable air discharge temperatures. Some fan-coil units also have thermostatic switches that may not turn the blower on at water temperatures below 140 degrees F.
Remember The BasicsBecause of the extensive (valved) zoning, it's imperative to install a differential pressure bypass valve and use a distribution circulator with a relatively "flat" pump curve. These measures will prevent the whistling sounds that often occur when a high differential pressure is imposed across only one or two open radiator valves.
To ensure fast air venting, install good quality float-type air vents at the top of each riser. To make sure they work properly in what may be a relatively tall system, the setting of the feed water valve needs be adjusted to ensure a minimum of 5-psi static pressure at the top of the system.
Be sure the air pressure in the diaphragm expansion tank is also adjusted to match the static pressure it will feel when the system is filled. As always, adjust the pressure in the tank before adding water to the system.
If thermostats and valve actuators are used for zoning, wire the end switches to signal for heat and turn on the distribution circulator. If zoning with nonelectric thermostatic valves, use the warm weather shut down function on the reset control to turn on the distribution circulator and enable boiler firing. The differential pressure bypass valve prevents "dead heading" the circulator in the event all zones are closed at the same time.
The physical and emotional energy involved in a DIY restoration of such a large old house is enormous. Needless to say, the owner faces many difficult tasks before he can relax and enjoy the fruits of his labor. But thanks to the flexibility of modern hydronics technology, when that time finally comes he'll bask in comfort far superior to that ever experienced by the home's previous occupants.