I set up my first home office in 1983. It consisted of a 14 by 14 foot corner of our basement, and was heated through the combined efforts of a wood stove and table fan. Along one wall I built a 7-foot drafting table out of a door panel to support a huge drafting machine I bought in an attic sale. Let’s just say I kept capital expenses to a minimum in those days.
Since then lots of people and hundreds of projects have flowed through that office. Unfortunately most of that traffic also flowed through our house. And more often than not did so when other family members were practicing musical instruments, experimenting with the stereo or just trying to sleep. You get the picture.
Finally came the big clue it was time for a change. During a phone conversation with a business contact I was asked, “Is that a washing machine I hear running?” Embarrassingly, I admitted the caller had a good sense of hearing. It was time to face the fact that a washing machine shouldn’t be a normal background sound in a consulting engineer’s office. I finally conceded to what my wife had been subtly hinting at for a some time. It was time to move Appropriate Designs to new quarters.
In 18 years of working with energy-efficient construction, I had come across many details that I wanted to try first hand. But “experimenting” on someone else’s building doesn’t always make for a restful night’s sleep. A new building of my own could provide both the sorely needed office space and a test bed for some of these new methods and materials. Besides, if some of these “experiments” didn’t work out as planned, I didn’t expect the owner would sue me.
I didn’t have to look far. Sixty feet from our front door (and at least 90 feet from the washing machine) was prime real estate heretofore serving as our side lawn. Here we would build a modest 20 by 32 foot structure and pack it full of functioning hydronic hardware in anticipation of future show-and-tell sessions.
Better Than A Sandbox: Con-struction began in late April of last year. My friend and fellow hydronician Harvey Youker executed all these ideas in hardware. Harvey kept things moving considerably faster than my usual do-it-yourself pace allowed. Without his patience, craftsmanship and attention to detail, this project would have been a long and stressful experience.
The first new system we tried was a shallow frost-protected foundation. There’s a drawing of it in Figure 1. Rather than digging down 4 feet to place footings below the local frost line, we excavated only about 2 feet deep. A poured concrete footing and stem wall were constructed and then the key component, a horizontal band of 2-foot by 1-inch extruded polystyrene board, was laid horizontally over the outer portion of the footing. It acts as a “thermal break” preventing frost formation below the footing.
According to research reports from the National Association of Home Builders, this system has proven itself in northern states as well as hundreds of structures in northern Europe. My wife and I spent several evening’s worth of “quality time” together installing footing drainage, underslab plumbing, and getting things ready for installation of the slab heating system.
Next came installation of the tubing for the ground floor. As a prelude to an experiment, we divided the office area down the middle and installed 1/2-inch PEX-AL-PEX tubing at 9 inches o.c. spacing down one side. The other half of the floor was tubed at 6 inches o.c. over the outer 4 feet and 12 inches o.c. within interior area. Both areas used about the same amount of tubing, but one area has more tubing near the slab edge. Thus far it’s been hard to feel any difference. Eventually I hope to instrument the floor and document actual surface temperature profiles.
We framed the building with 2 by 6 foot studs spaced 24 inches o.c. and in line with roof framing. This yields more area for insulation in the walls, and optimizes load transfer from the roof framing down through the walls. The entire second floor was framed using attic trusses. They go up fast and create a very usable Cape Cod-style interior space for a second floor.
Walls were insulated with a 1-1/2-inch layer of urethane sprayed directly onto the back side of the plywood sheathing and a 4-inch fiberglass batt taking up the rest of the 2 by 6 foot cavity. A 6-millimeter poly vapor barrier completes the system. The result is an R-24 wall that, thanks to the sprayed urethane, is very well sealed against air leakage.
Saving The Best For Last: In a few weeks we completed the remaining carpentry and wiring and were ready to install the hydronics hardware.
The design heating load of the building is only about 18,000 Btu/hr. Although a water heater would surely have sufficed from a capacity standpoint, I wanted to try out a direct vent oil-fired boiler. The smallest available capacity was about 60,000 Btu/hr. The challenge was to marry the small heating load with the obviously oversized heat source in a way that prevented short cycling and its resulting decrease in efficiency. It was the perfect setup for a buffer tank.
The goal was to keep the burner’s on-cycle time reasonable (say 10 minutes) by using the thermal mass of the buffer tank to soak up the excess Btus when the burner was firing. When the boiler finally reaches limit, or is otherwise shut off by the operating controls, the buffer tank becomes the “alternate” heat source for the small loads that may still be calling for heat. The deeper the temperature cycling range of the buffer tank, the longer the burner cycle time. To allow either the boiler or buffer tank to serve as the heat source, we connected both as secondary circuits to a common primary loop routed overhead around the 6 by 9 foot mechanical room. This arrangement allows either source to be “online” at any given time depending on the control strategy used.
The boiler is coupled to the primary circuit using a variable speed injection system as shown in Figure 3. The control operating the injection pump (IP3) is set up to protect the boiler from low return temperature as well as provide either setpoint or reset control of the primary loop temperature. The boiler has its own bypass loop to which the injection risers are connected. When a load (other than domestic water) calls for heat the primary circulator (P4), buffer tank circulator (P7) and boiler injection control (IC3) are all turned on. The boiler injection control senses the primary loop temperature. If it’s warm en-ough, the boiler is not fired and no flow is routed through the boiler. The buffer tank is the heat source for that time. Assuming the load persists, the buffer tank and primary loop slowly drop in temperature.
At some point (determined by the settings and operating logic of the boiler injection control) the boiler is fired. The injection circulator starts transferring heat from the boiler loop into the primary loop. Heat is now flowing both to the active secondary loads and into the buffer tank. The boiler loop circulator (P2) is “latched” on for the duration of the call for heat. This allows the thermal mass of the boiler to remain online with the primary circuit even when the burner is not firing. During such times the boiler temperature is always above the primary loop temperature so heat can only move into the loop. Partially discharging the thermal mass of the cast-iron boiler further lengthens the firing cycle of the burner over what could be provided by the buffer tank alone. This past January, I observed firing cycle lengths in the range of 10-15 minutes. Considering the boiler is more than 300 percent oversized, I felt the operating strategy was working fairly well.
Four heating loads are piped off the primary loop:
- A variable speed injection system serving the slab floor circuits.
- A two-way thermostatic valve serving the mixing device for manifold supplying two panel radiators.
- A “swapable” variable-speed injection assembly supplying a manifold that serves two floor circuits, a heated wall, and a heated ceiling on the second floor.
- A “high temp” Monoflo® tee-propelled circuit for an area of suspended tube floor heating.
In addition we installed two extra pairs of closely spaced tees with threaded ball valves on their side ports to serve as future secondary circuit take-offs if needed. Primary loop flow can also be routed through a flow meter if desired. Piping that allows differential pressure measurements across the primary loop circulator was also installed. Harvey managed to install a 275-gallon fuel oil tank in a closet under the stairs with at least 1/2-inch of room to spare.
The second floor space provided by the attic trusses offered modest but adequate areas to demonstrate several types of hydronic radiant panel heating. There are currently five types of site-built panels installed. Each is served by a separate circuit and can be operated independently or in combination with others.
Three separate areas of the wood-framed upper floor are heated. One uses 6-inch aluminum heat dispersion plates supporting 1/2-inch PEX-AL-PEX tubing 8 inches o.c. below the 3/4-inch plywood subfloor with low pile carpet glued down on top. Another uses the same tube/plate system with 3/8-inch laminated hardwood flooring. The third uses tubing suspended on hangers 2 inches below the plywood and enclosed on the sides and bottom by an envelope of reflective insulation.
We tried out a new “sandwich” for the wall and ceiling panels. It combined foil-faced polyisocyanurate foam “furring strips” and contact adhesive with the same 6-inch aluminum heat transfer plates placed 8 inches o.c. A cross section is shown in Figure 4. I think there are some interesting performance and installation advantages to this approach, and will de-scribe them in detail in a future PM column.
Throwing The Switch: We moved Appropriate Designs into the new office this past November. As always it’s a thrill when a new floor warms for the first time, especially after realizing those Btus made it through the more experimental portions of the new system. After spending the winter in the new office I can only describe the comfort as “indulgent.”
As time allows, I plan to monitor the new systems both qualitatively and quantitatively. I hardly ever miss a daily “feel, watch, and listen” session in the mechanical room. Minor adjustments to controls have already helped tweak system performance.
This project is, as the old saying goes, a dream come true for me. Although I’ve been fortunate enough to design and spend time in many hydronically heated buildings, this one is obviously special. It’s my chance to live with and learn more about the subtle characteristics of hydronic heat. To experiment with ideas that might someday be used in many other systems. To show others just a few of the many possibilities that hydronics has to offer. And what I learn I’ll pass on as the future allows.