The Case of the Rebellious Radiators

It was May, and my friend Benny the super was catching heat from the residents of his building. Despite the mild weather, the radiators in the apartments were “rebellious”: they were frying everyone to a crisp. What the heck was going on?

Benny ventured down to the sweltering boiler room and soon found the cause: the heating hot water circulator was pumping full blast even though the heating season was virtually over. Problem solved – shut off the pump. If only they were all this easy!

Then, complications. Initially, all seemed well, but within a day, the complaints began anew. Benny couldn’t believe it, so he visited a few apartments to see if maybe the weather was overheating everyone’s imaginations. No such luck; the radiators were still warm, although it seemed to Benny not as warm as before, and the residents were not happy.

Who to call? Benny figured the heating contractor might know what to do. The contractor came by, looked around the boiler room for a few minutes, and came back with a recommendation. “You need a couple of check valves,” he said, and it would cost Benny’s building $2,000.

Benny winced. Why hadn’t the contractor recommended the check valves when he installed the new domestic hot water tank several months ago? And why hadn’t the problem shown up before now? Benny decided he needed a second opinion, so he gave me a call. I asked him to send me the contractor’s proposal for evaluation.

At first I was confused. The proposal was vaguely worded – a common problem – and it took a while before I understood what the contractor wanted to do. Then I recalled that I had spoken to the contractor about this very job months earlier, when I had recommended the modification to the domestic hot water system. In the wake of an untimely failure of a tank-type water heater, I had told the building to replace it with an insulated tank that would be heated by boiler water. This would improve efficiency and reduce downtime.

The building had adopted my suggestion, but to save a few bucks had decided to let the contractor design the new system. Contractors have a lot of practical experience with equipment and its installation, but they generally lack the detailed insight required to design a proper system. Such was certainly the case in this instance.

The Hidden Flaw

The contractor had installed the new hardware in the middle of the heating season, and everything appeared to be fine, plenty of hot water and no breakdowns. But once the heating season began to wane, the flaw in the contractor’s design soon became apparent.

With the new system, a small pump was needed to send boiler water through a heat exchanger inside the hot water storage tank. Although the pump was small, when combined with convection in the heating hot water pipes, it was strong enough to send some hot water up to the apartment radiators. During the heating season, the effect of this unwanted flow was negligible; when the weather warmed up, however, it became a major nuisance.

Where had the contractor gone wrong? He had placed the small pump in the wrong place. He didn’t realize that the location of the pump in the heat exchange loop had a profound effect on the flow in the rest of the system.

This happens all the time. Buildings are always eager to keep costs down, and design engineers are viewed as optional at best and a waste of money at worst. Contractors are only too happy to take on the design function, and they’re usually willing to help out further by taking on the construction oversight function as well. Why waste money paying a third party to check on the work when the contractor will check its own work for free?

That’s good for them, but not always so good for you. Larger contractors sometimes have design engineers on staff, but smaller firms typically do not, and the counterintuitive aspects of heating/air-conditioning design frequently elude them.

Back on Track

I drew a sketch showing where the pump should be located and sent it to the contractor. He objected at first, but eventually I was able to persuade him to make the change – at no cost to the client. He was about to install a similar hot water storage system at a nearby building and agreed to modify the original “design” in similar fashion.

With the pump in its rightful place, the unwanted heat disappeared and the complaints stopped. The residents were happy and Benny was, too. One detail remained to be addressed, however. If you recall, Benny originally thought the problem was caused by the unnecessary operation of the heating hot water pump. That turned out to be a red herring, but was still a cause for concern. Why was the pump on when it should not have been?

That’s a story for another time.