In the last year, I’ve had the opportunity to look over some real winners and some real “lemons.” Here are a few of the deviant details I’ve come across. Some of these flukes appeared in manufacturers’ literature, others were assembled by contractors who didn’t take the time to carefully think about how the piping details affect overall system performance.
1. Design-As-You-Solder: Once in a while I’ll see a piping system that seemed to start off with one design concept in mind, (say two-pipe direct return piping), only to morph into another concept (like primary/secondary piping) along the way. The resulting hybrid seldom works as intended.
Figure 1 shows a piping schematic for a small commercial building I recently visited. The system supplied both radiant floor heating and several hydronic air handlers. The two boilers were piped into a common loop in what appeared to be the start of a primary/secondary system.
The problems began when the installer connected the various sub-circuits into the primary loop as if they were being connected to a two-pipe header system. Either he didn’t understand the closely spaced tee concept of primary/secondary systems or he “chickened out” at the last minute, thinking he should rely instead on the trusty two-pipe “header” approach.
Anyway, circulators (P1) and (P2) were both high head circulators. The ball valve shown in the boiler loop was almost closed. I was standing in the boiler room when one of the air handlers called for heat. Its 3/4–inch zone valve opened and began resonating a sound you could have heard from outside the boiler room.
The valve was experiencing flow driven by its zone’s own high head circulator (p2) plus a portion of the pressure differential created by the other high head circulator (p1) trying to push flow through the mostly closed ball valve. This layout didn’t allow the pressure differentials created by the two circulators to be isolated from each other as would a true primary/secondary system. The pressure differential created by circulator (P1) pushing flow through the mostly closed ball valve even has the potential to induce flow through the other zone circulator piping unless it was blocked by a positive shut-off device, such as a zone valve or manifold valve actuator.
As a side note, both boilers in this system fired simultaneously (unless one was turned off with a manual switch). This system beckons for an automatic multiple boiler controller and the higher seasonal efficiency it could provide.
Lest you be tempted to morph various hydronic piping details together without careful thought, here are some words for the wise to live by:
Choosing a hydronic piping system is like choosing a marriage partner: Study the “options” in great detail, make an informed selection and then stay committed to your choice.
2. All Choked Up: Another piping design that caught my attention was an array of seven hydronic zone circulators, each neatly piped with 3/4–inch copper tubing and a flow check, as shown in Figure 2. Each served a heating zone in a fairly large house. The problem was that all these circulators were piped into a common 3/4–inch copper header!
Think about what happens as more and more of the circulators operate simultaneously. As each additional circulator turns on, the 3/4–inch header quickly becomes a serious bottleneck in the system. Flow through individual zone circuits decreases as more circulators come on line because the common header represents an ever increasing head loss that is in line with all of the active zone circuits. Meanwhile the increasing flow velocity in the common header piping will create flow noise and perhaps even eventual erosion of the copper tubing. The basic detail overlooked was the use of a generously sized header to minimize flow noise, as well as flow interference between simultaneously operating circulators.
3. Tee’d Off: Still another recent find is depicted in Figure 3. Here we have a zone valve on each side of a common tee. The side port of the tee feeds into the system circulator.
There are a number of problems here. First, the zone valves should be on the supply side of the system rather than the return. This detail prevents hot water from migrating upward from the boiler into inactive zone circuits.
Second, the zone valves can create significant pressure drop and turbulence just upstream of the circulator inlet. Given the right combination of pressure and temperature, this system extends a cordial invitation to cavitation. The circulator inlet is also too close to the side port of the tee. As a rule, allow at least 10 pipe diameters of straight pipe upstream of any circulator to reduce turbulence at the inlet.
Finally, assuming both zones could operate simultaneously, a “bullhead tee” configuration is created in which two flow streams impinge directly into each other. This wastes head and creates flow noise. There’s actually one other piping detail shown that isn’t ideal. I’m sure some of you have already spotted it. Hint: Something blue is in the wrong location.
4. Bottle Rocket: Every once in a while a great comparison comes along to describe the behavior of a system. A wholesaler recently told me of a boiler connected to a snow-melt system that, in his words, “sounds like a bottle rocket” when a slug of ice-cold glycol solution flows through it. The system piping he described is shown in Figure 4.
Apparently the contractor decided a bypass circulator could always maintain the fluid entering the boiler at a temperature high enough to prevent condensation. However, the thermal mass of a large cold slab is an awesome opponent. Without proper detailing and temperature responsive controls, a large thermal mass quickly takes control of system fluid temperature, and rapidly pulls it down to within a few degrees of its own temperature. This boiler apparently gets a few seconds worth of warm up time at the start of a snow-melt cycle before an ice-cold slug of glycol enters it.
It’s only speculation on my part, but I’m guessing the high pitch screeching sound he described may be the result of copper tubing sliding through steel or cast-iron manifolds inside the boiler as the materials rapidly change temperature? Whatever the cause, this sound isn’t healthy.
This system definitely needs a way of preventing boiler return temperature from dropping so low. A variable speed injection system with a return water temperature sensor would do it, as would a motorized mixing valve with the same type of return temperature sensor. Another option would be a non-electric thermostatic valve that limits — or totally prevents — heat from leaving the boiler loop when the return temperature is too low.
Don’t Forget the Basics: These systems were all designed by good people with good intentions. The designers just overlooked some of the fundamental “building blocks” of good hydronic design. One way to avoid such glitches is to constantly remind yourself of the fundamentals:
- Hot water “wants” to go up, and given the opportunity, it will.
- Circulators don’t like low pressure or turbulence.
- Header piping for multiple zone systems should be generously sized.
- Zone valves don’t take high flow velocities quietly.
- Boiler bypass loops (alone) are no match for a large cold thermal mass.
After you’ve sketched out a piping schematic, run the “virtual system” through each of its operating modes in your mind. Think about combined flows in common portions of the system under different operating modes. Think about how the system will guide itself through cold start-up conditions, especially if it contains a large thermal mass (such as a concrete floor slab). Finally, do your creative thinking with a piece of paper (or a computer) before you start putting things together on the job.
Abstract sculptors create on the fly — and the results usually speak for themselves. If your completed piping system looks like it belongs in the Guggenheim Museum, maybe you’re in the wrong line of work!
I’ve yet to meet a hydronics practitioner who doesn’t have at least one story to tell about a “creative” piping layout they’ve encountered that suffers various anomalies while attempting to perform its intended functions. If you’ve come across a hydronic schematic that’s a real dud, and you’re willing to share it with others, please fax me a sketch and a description. Be assured names will be changed to protect the guilty. I’ll put the best of the worst together in a future column so other PM readers won’t repeat the same mistakes.
After you’ve read this column get out a piece of paper and sketch what you think would be an ideal correction for each of the piping systems shown. Be advised that in all cases there’s more than one way to fix the problems described. Next month I’ll show some of my ideas for making these systems work better.