IN LAST MONTH’S article (pg. 40), we began discussing the many choices for the heating plant for the snow-melt system. This month we’ll get into the “guts” of the system that we actually installed in that big retail store, the part that most hydronic people want to see, the boilers.
In our case, the retail company’s design engineer had already selected the type of boilers we would be installing in this system. His choice was a compact design. It was a vertical-profile, low-mass, medium high-efficiency (85%) copper fin-tube boiler. We installed two of them.
The engineer’s original piping design had the piping for the boilers being performed as a secondary to the system distribution primary. The boilers were piped with “bull head” tees, which are common among engineering circles. I substituted his design with a parallel reverse return design for the near boiler piping. I would have preferred to do something differently, but as I have said, this was a pre-engineered system and the engineer didn’t want me messing too much with his design.
In many cases, the design is up to you as the “design/builder.” In this case, as it turned out, this system works well. It is set up as follows.
There is a main distribution loop consisting of 4-in. pipe with a 712-hp pump moving the glycol/water mix through the primary loop. The two boilers are piped parallel reverse return and are connected to the primary through a four-way mixing valve. A single 2-hp circulator provides fluid flow for the two boilers.
The function of the four-way valve is to protect the boilers from low entering water temperatures and the long-term condensate production potentials associated with that condition. The four-way valve provides temperature control of the fluid going to the snow-melt system, thereby avoiding thermal shock to both the snow-melt system and to the snow-melt boilers.
Although it works adequately, the four-way valve has a tendency to “hunt” quite a bit in its efforts to maintain all the different parameters it’s trying to control.
I would have preferred to do the job with a small secondary loop around the boilers, with the boilers piped as input secondaries, and a variable-speed injection pump taking care of the Btu bridge between the heat-source loop and the snow-melt loop. It has been my experience that this has a tendency to cut down on control hunting and on/off short cycling of the boilers as they hit their high limits waiting for the four-way valve to get caught up with the program.
As to the boilers, I would have preferred to use high-efficiency condensing boilers. Even though their initial cost would have been significantly higher, their operating efficiencies can approach 98% during startup and can maintain 94% all day long if need be.
Avoiding the cost of running flue pipe through a large building can also be a significant savings. Most high-efficiency boiler manufacturers have a plastic flue pipe available that is significantly less expensive than the pressure-rated stainless steel vents that were required for this job.
If your choice is to specify and use high-efficiency boilers, make sure that the owner is aware of the annual maintenance requirements that are associated with their operation. Remind the owner on an annual basis by either sending him a notice in the mail, or by showing up at the facility to perform the required preventive maintenance.
Another recommendation if you are going to be using high-efficiency condensing appliances is to make sure that you have included the cost of a condensate pre-treatment system. This is a large plastic container that has crushed marble rock inside it. This neutralizes the acidic condensate before it is discharged into the sanitary sewer.
If the condensate is not neutralized before it is sent to the sanitary sewer, and the drain lines are metallic, i.e., cast iron, the acidic nature of the condensate with a pH of about 5.5 will cause the cast-iron drainage system to fail. It will eat the bottom right out of a cast-iron floor drain’s P trap, and it will also take out the flow line of the drain until it reaches another branch of the sewer where it will become diluted.
Condensate pre-treatment is not required by code at present, but I anticipate it becoming a code requirement in the near future.
After seven months, this completes my series on hydronic snow melting. Tune in next month as we begin looking at apartment complex retrofits.
Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at [email protected] or by phone at 303/778-7772.