Published: August 13, 2015 - by Ray Wohlfarth

Categories: Commercial Heating

Ray Wohlfarth Pic

Star Trek has always been one of my favorite TV series and collection of movies. I like when Captain Kirk pleads with the chief engineer "Scotty" for more power.

While we may not have to jettison and explode our warp core to get more power,  there could be things we do that affect the performance of our heating systems.  I am reminded  of a call I received during a bitter cold snap. When I answered the telephone, I was told, "Your boilers aren't working. There is something wrong with them." The person on the other end of the telephone call  was the director of maintenance for a nursing home where we sold three boilers about four years before.  He informed me that he had even raised the boiler temperature setting from 180 to 200 degrees F but the perimeter rooms in the building were cold. He also told me that they were going to have to evacuate the facility if we cannot come up with something quickly. My service technician was close and I dispatched  him to the site.

The technician called and verified that the boilers were indeed at 200 degrees F and the outer perimeter of the building was cold and the seniors were freezing. The distribution system  used  a main air handling unit that sent  60 degree F air to the building. It had hydronic reheat coils to raise the temperature in the rooms, if needed. My technician said that the owner had opened all the balancing valves for the reheat coils in an attempt to get more heat without success. My technician was at a loss for what to do. Upon questioning, my technician informed me that the heating loop Delta - or temperature drop  was at 3 degrees F. He asked if the system could be undersized and I assured him that it was not. I explained that if it was undersized, the water temperature would not be at 200 degrees F and the Delta-T would not be so close. Most systems are designed for a twenty-degree Delta-T. The low temperature drop made me realize that the piping was not surrendering its heat to the building.  It must be something in the distribution end. When he asked what to do, I was stumped and offered, Try closing the balancing valves on the reheat coil until you get a 20 degree drop across the coil. Both he and the director of maintenance were skeptical. He called back in a couple minutes and offered that the discharge and room temperatures  had miraculously risen.  They changed all the coil balance valves for the same temperature drop and the building heated to the setting of the thermostats. They reduced the boiler temperature back to the proper setting of 180 degrees F. One of the things I have learned about this industry is that no matter how much you think you know, it can humble you and that is what I love about it. I did some research on this and found that it is not  a rare phenomenon. If the velocity of the water through the pipe or coil is excessive, it will not surrender its heat, resulting in comfort complaints. It does not make sense but I have seen it happen.  

On a similar project, we were called to an apartment building with fin tube radiation that was complaining of inadequate heating in several of the suites. The system had an enormous circulating pump that was pumping the hot water to the apartments.  The piping system had about 10 zone valves on the returns from the apartments. When we arrived, the system sounded like a cat howling. Several of the zone valves were closed. When I manually opened the zone valves, the howling went away and the apartments that did not heat were suddenly heating. We also noticed that the fin tube radiation enclosure had a manual damper that was almost closed on top. I opened all the dampers wide and felt the heat convection immediately. I also instructed the tenants that couches and chairs in front of the radiation would impede the heat to circulate and should be moved. They refused saying that the furniture would not work anywhere but where they were placed it.   

The last building had a combination boiler with an internal copper coil that was used for the domestic hot water for the facility. The boiler was warm but would not heat the domestic water loop. The domestic water side was piped in primary secondary.  We partially closed the globe valve on the domestic water coil until the temperature increased. Once the temperature rose, the loop started heating. The client was happy and so were the people in the shower.  If the system is not heating with wide open valves, perhaps it is time to try closing them. Check the temperature drop across the loop. Most systems were designed for a 20 degree drop. If it is less than that, the system may not be giving up its heat. This domestic water boiler was designed for a 100 degree rise.

The following are some rules of thumb that you can use for verifying the size on Hydronic systems for steel pipe based on a 20 degree F delta T.

Steel Pipe

Pipe Size

Maximum Flow GPM

Btuh

1/2"

2

15,000

3/4"

4

40,000

1"

8

80,000

1 1/4"

16

140,000

1 1/2"

25

220,000

2"

50

450,000

2 1/2"

80

850,000

3"

140

1,300,000

4"

300

3,000,000

6"

850

8,500,000

8"

1,800

18,000,000

10"

3,200

32,000,000

12"

5,000

50,000,000

 

The following are some rules of thumb for flow in copper tubing

Copper Tubing

Tube Size

Maximum Flow GPM

Btuh

1/2"

1 1/2

15,000

3/4"

4

40,000

1"

8

80,000

1 1/4"

14

140,000

1 1/2"

22

220,000

2"

45

450,000

2 1/2"

85

850,000

3"

130

1,300,000

 

 The following are rules of thumb for PEX tubing

PEX Tubing

Tube  Size

Maximum Flow GPM

Btuh

3/8"

1.2

12,000

1/2"

2

20,000

5/8"

4

40,000

3/4"

6

60,000

1"

9.5

95,000

 

Want to learn more? Check out my books, Lessons Learned in a Boiler RoomLessons Learned: Connecting New Boilers to Old PipesLessons Learned: Servicing Boilers, and Lessons Learned: Brewing with Steam.