When Mind-Reading Fails to Work in a Retrofit

ONE OF THE interesting aspects of designing a retrofit mechanical system for the older apartment complex that I described last month (November, pg. 30) is trying to look into the minds of the people who designed the system in the first place. Sometimes you think you have a good handle on what they had in mind, and then sometimes all you get is a blank screen with a test pattern. On this job, I got

ONE OF THE interesting aspects of designing a retrofit mechanical system for the older apartment complex that I described last month (November, pg. 30) is trying to look into the minds of the people who designed the system in the first place. Sometimes you think you have a good handle on what they had in mind, and then sometimes all you get is a blank screen with a test pattern. On this job, I got the latter.

It was evident that at some recent point in time, the primary circulator had been replaced. It was not evident, however, if this pump was larger than originally designed, or if it just had not had its impeller trimmed to meet performance requirements.

In any case, based on interviews with maintenance personnel and management, the pump was grossly oversized in its feet of head capacity. They explained to me that the residents kept their thermostats turned all the way down, and the apartments still overheated to the point that residents opened their windows to counter the sweltering heat coming from the perimeter baseboard convectors.

In reviewing the existing physical plant, I discovered a bypass around the pump that was wide open. It was much smaller than it should have been, which is a part of the reason that the heating pump was capable of causing the individual zone valves in the apartments to bypass.

I don’t believe the typical fix of installing a pressure-activated bypass valve would have done much good in this scenario. Maybe six of them in parallel and staged, but not one by itself. So, as a part of Phase 2, this oversized circulator has to be addressed. The options are as follows:

1. Leave the pump as is.

2. Replace the pump with a smaller pump.

3. Install numerous pressure-activated bypasses around the pump.

4. Install a variable-speed motor controller based on temperature differential.

5. Install a variable-speed motor controller based on pressure differential.

Let’s take a quick look at each option and discuss the benefits and problems with each.

Option No. 1, leave the pump as is, is obviously not a wise decision. The use of windows to control comfort conditions is an expensive way to go in terms of cost of operation. It also does not sit well with the occupants of the building. It had been proven that it was not a good move, which was the reason our company got involved in the first place.

Option No. 2, replace the pump with a smaller pump, sounded like a good idea, but the complex was a long city block in length, and a short city block wide. There were no records of what the piping distribution sizes were, and the mains were all underground. It would be virtually impossible to determine the pipe sizes underground short of digging up the lines and exposing them.

Option No. 2, although sound in principle, would have been nearly impossible to perform correctly without by-gollying and wild-guessing a new pump size.

Option No. 3, install numerous pressure-activated bypasses, was physically possible, but it would have been a tough cookie to figure the proper settings of the paralleled pressure-activated bypasses. I suppose a person could have spent a lot of time on site adjusting and tweaking the settings for the bypass valves, but as conditions changed, so would the dynamics of the system.

Option No. 4, install variable-speed control based on temperature differential, holds an excellent potential, but only if the piping distribution system was piped in parallel reverse return. Any other type of piping configuration (parallel direct return) would cause severe short-cycling of the water flow to those units nearest the pump and would starve those units farthest from the pump.

In short, because nobody knows what the piping distribution system looks like, it was an unworkable solution.

Option No. 5, install a variable-speed control based on system pressure differential across the pump, appears to be the best workable option on this project. With so many unknowns, it would be best to design the system to maintain a pressure differential across the pump just below that of the zone valves’ close-off capacity. Although it still may be in excess of what’s required, it will eliminate the zone valve bypassing that had been occurring.

That, in and of itself, should result in a substantial reduction in thermal energy consumption, to say nothing of the kilowatt-hour reductions associated with running the 5-hp circulator at lower speeds.

There are many manufacturers of variable-speed motor controllers. I would suggest that you hook up with one that has experience in the hydronic heating business. There are numerous hydronic heating pump manufacturers that make variable-speed motor controllers to work with their pumps. Consult them with your application and get them to provide you with a quotation based on your project.

Most of them have the ability to show the economics on the electrical side in reducing KWH consumption, wear and tear of the circulation components and energy savings provided by residents not having to use “double-hung” thermostats to maintain reasonable living conditions within their dwellings.

Do yourself, your company and your customers a favor and do a Google search on “variable speed pump controls.” There’s a plethora of information out there just waiting to be soaked up.

As Phase 2 of this project gets underway, I will keep you posted as to the changes and results.

Until then, Happy Holiday Hydronicing!!

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.

TAGS: Hydronics