Thermostat is off; So why is the heat still on?

May 15, 2000
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“You’ve got to do something; the heating is all screwy. Some of the rooms are roasting and the thermostat’s off.”

The call was urgent and the occupants of the affected rooms were not ordinary people. This was a residential care facility and those people were old and frail. They deserved prompt attention, so I was on my way immediately.

The only question: What to do when I got there. I hadn’t a clue. I’d gotten the same call before but found nothing wrong when I arrived.

The staff claimed the problem only happened “sometimes.” Great — ghosties and goblins!

Our contract was for design-build of the radiant heating system. I wanted to install individual room control, but the owner-builder thought that was too much. He thought everything would be fine with just four thermostats, one for each quadrant of the two-story building.

In the end, we compromised at 12 zones. We chose electronic thermostats with remote sensors so we could place the sensor in a representative room, but leave the control in the hall behind a tamperproof box, accessible only to the staff.

Efficiency was important, so we opted for two 199,000-Btuh condensing water heaters. I chose two Taco 0011s for circulation because their curves indicated they were capable of sufficient flow with the system going full tilt.

Our old, standby heat-motor zone valve was not compatible with the thermostats, so we chose a synchronous motor-actuated valve. Reliability was very important, so we picked a brand with an excellent reputation even though it cost a few dollars more; in this case it was Erie.

Hot lobby woes

With all this care, we shouldn’t be having problems. Yet, there it was.

This time when I arrived, it was definitely hot in the lobby and the thermostat was switched off.

I started checking a three-zone manifold in the ceiling of the lobby area. Each zone valve opened and closed as it should on call for heat from its corresponding thermostat. All of the wiring was correct.

Listening very closely, I opened one zone at a time and simultaneously operated the ball valves we installed to facilitate zone valve repair. There was a slight hissing as the middle valve approached full closed — telltale sign of a problem, as that zone was not energized.

Obviously, the zone valve wasn’t closing tight. I went to other overheated areas and checked. I found the same thing. A bad batch of zone valves, I concluded while cursing the manufacturer.

Obviously, I was going to get stuck replacing the whole lot and I wasn’t a bit happy.

Going back to my office, I tried the installer’s last resort: I read all of the instructions. Then I thought about it . . . it wasn’t the zone valve’s fault. It was mine.

What was my mistake?

All of my equipment selections had been good ones. They just didn’t work well together.

Taco 0011s have a shutoff head of 30 ft. The Erie zone valves could close against 23 ft. We had a 4-gpm bypass installed in the distribution piping, but on mild days with only a few zones calling for heat, the circulators still operated high on the left end of their curve and were forcing up to maybe 15 gpm through zones that supposedly were shut off.

On cold days, with many thermostats calling, the operating point shifted to the curve’s midsection where the zone valves could happily deal with about 15 ft of pressure differential.

The solution was not to replace the zone valves, it was to change out the circulators. I installed Taco 0014s. With the 4-gpm bypass and just one zone calling for heat, these circulators will never see 20 ft of head and the zone valves will stay closed.

Nonetheless, the system won’t starve for volume because out at the midsection of their curve, they can exceed the pressure and volume of the original 0011s.

The lesson for me was to pay attention to the interaction between zone valve and circulator. (In other words, pay attention to the system, not just the components.)

It turns out Erie has a selection of valves designed for varying pressure differentials. The one I used could only seal against 10 psi max.

Erie makes another model that can handle 100-psi differential and so do other manufacturers.

Editor’s note: Brand name products mentioned in this article illustrate the importance of choosing components that are compatible with each other. The difficulties experienced in this situation could be encountered with any mismatch of components. This article is reprinted with permission from the November 1999 Radiant Panel Report, published monthly by the Radiant Panel Association. Clinton is with Bay Hydronic.

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