Hot Water Heat 1

[Icanfix]

The last 5 years of my career were spent in mainting the HVAC for a large sewage plant. (what an experience)

We had just moved into a newly constructed office building, (no commissioning!). My only training in HVAC was one semister in college, talking to father-in-law, who owned a heat and refrigeration business.

The building had direct fired absorption chillers, an air handler for each floor, and zoned reheat coils. The chillers also supplied the heating hot water in the winter. The air handlers had outside air dampers, and return air dampers that were linked together. The controls were electronic, and pneumatic mixed.

A couple of things that I noticed after I became familiar with the system was that there was no provision for exhausting the air from the building. Thus the outside air provision for the economy mode couldn't bring in enough outside air to cool the building in the spring and fall. Some of the air handlers were marginal in size for the anticipated future planned loads.

A real problem that I had was that the air handlers would kick out on low temperature in the winter. The colder the outside temperature the more frequently the outages. The information that was passed on to me by the inspecting engineer was that the hot water temperature had to be lowered to 110 degrees F to minimize the outages.

The heating coils in the air handlers were two section and stacked. The only thing that I could figure was that with the normal low water flow, and poor mixing of the air was that stratification occured in the heating coils. The upper coil would be favored by the water flow.

What I did was to reset the minimum outside air damper position. This basically eliminated the trips. But that certainly was not a good fix, since it cut back on the ventilation air into the building.

Another problem was that when the chillers responded to a demand of greater than 50% there was surging in the stack, and much condensation in the winter. The HVAC tech finally installed a relief dampers in the exhaust riser from each chiller.

The other major fault was that the heating hot water temperature was supposed to be controlled by the building management system based on the heating load. It never worked because when it increased the return water to mix with the hot water it would restrict the flow through the heater and shut it down on low flow.

The long and short of the problems in the building would have been addressed by a commissioning. All the equipment was decent quality. The sequence of operation was good, only the some of the controls could not do their job. This would have shown up in a commissioning. Had this been done the building's climate control would have been excellent and with much less attention.

 

[lzh007]

>The information that was passed on to me by the inspecting >engineer was that the hot water temperature had to be >lowered to 110 degrees F to minimize the outages.

I still can not understand the reason of the outage. Is it becasue some anti-freezing protection? If you can make the outside air and return air mix better, things may change.

>The other major fault was that the heating hot water >temperature was supposed to be controlled by the building >management system based on the heating load. It never >worked because when it increased the return water to mix >with the hot water it would restrict the flow through the >heater and shut it down on low flow.

I think the supply temperature should be controlled by the heater, flow rate through heater should be constant. the mixing valve is to maintain the temperature drop of the unit. Because lots of ways to control a system, it is hard to say.
Knowledge of Chiller is limited. Sorry.

 

[Icanfix]

<I still can not understand the reason of the outage. Is it becasue some anti-freezing protection? If you can make the outside air and return air mix better, things may change>

What was happening was that the AHU was kicking out on freezestat. The hot water flow through the coil was controlled by a series valve. The outside air damper would close down to 15%. The discharge air temperature was read by an averaging element. The controls would open the hot water valve enough so that the average temperature was at the set point. But at low flow conditions the upper coil was favored by the little hot water flowing. So the freezestat kicked the AHU out because the bottom coil was at the 38 degree set point.
After I read some more topics on this site, I figured that the coils should have had full flow all the time, while a mixing valve controlled the temperature of the water through the coil. So that series valve was a poor design.

< The other major fault was that the heating hot water >temperature was supposed to be controlled by the building >management system based on the heating load. It never >worked because when it increased the return water to mix >with the hot water it would restrict the flow through the >heater and shut it down on low flow.>

The heater did control the water temperature that it generated. That was the problem. The 3 way mixing valve would try to control the temperature in the rest of the loop. When it recirculated the hot water in the loop it starved the heater, which would shut down on high temperature. The designers solution was to hold the valve open.

The chiller was an interesting machine. The neat thing was that it would generate both chilled and hot water simultaneously. This saved a lot of complaints in the locker rooms. Which needed reheat in the summer, because of the ammount of ventilation required. The duct ran at 55 degrees, which really cooled the locker rooms off.

 

[lzh007]

In the first case, I think you still need a small pump to keep constant flow thru the MAH, not only a 3 way valve.
About the heater, without a PFD drawing, I can not say too much. I am surprised that the heater does not have its own pump to keep the flow rate. maybe the 3 way valve was installed in the wrong place.

 

[Icanfix]

I think that I understand about the pump on the MAH. It would circulate the water through the coil and the 3 way valve. The valve would modulate to allow enough heated water into the MAH loop to satisfy the demand. But with a good flow of water there would be no chance for stratification.

Sorry I don't know what PFD is? Unless it is the control diagram.

I think that you are suggesting that the same type of looping would work at the heater also?

I'll just talk about the heating for now. There are two loops. One for the MAHs and one for perimeter heat and reheat coils in VAV boxes. Both of the loops were fed from two pumps in parallel. The building was 4 storys with provisions for two more storys to be added. Each MAH had a balancing valve to compensate for elevation, and maximum flow. The MAHs each had a controller card that communicated with a Johnson Metasys. The parameters in the MAHs could be controlled from the metasys computer. The MAHs discharged into the main duct for its floor. The discharger temperature was nominally set at 55 degrees summer and winter. The perimeter and reheats were controlled by individual wall thermostats.

The hot water would pump through the loops and return to the heater inlet. The discharge of the heater would flow back to the hot water recirculation pump. The three way valve in question was mounted in the return line just before the water was fed back to the heater inlet. What it was supposed to do was let the return water bypass the heater and dump back to the inlet of the pumps. I think that the heater wasn't considered as it had its own temperature control. The designer forgot that there was supposed to be a minimu flow through the heater.

The design problems, and the start up problems are because there wasn't anyone on the owners side that really knew HVAC, therefore the problems were inherited by the owner.

Sorry I keep trying to write in past tense, because I retired last year. The job was so challenging for me that I really loved it. Maybe by writing about it will help someone else.

Thanks for your time and interest.

 

[lzh007]

It is strange. The Flow rate through the water heater must be constant! Otherwise, minor change of water flow will result in frequent trip. I do not understand the meaning of the 3 way valve here.
About the MAH, if some anti freeze solution can not be used instead of water, I will keep the water through the coil stable while use a by pass section or change the air flow through the coil to control the outlet air temperature.

Make sense?

 

[Icanfix]

The water heater was is a York Paraflow direct fired absorption chiller. It supplied both the heating and cooling for the building. It had a 10:1 turn down modulating burner, and it would safely shut itself down. Then it would take about an 30 to 40 minutes to start generating good heat.

The 3 way valve was supposed to control the temperature in the hot water heating loops based upon the buildings heating demand. Kind of like a reset controller. What it was supposed to do was divert the cooler return water to the inlet of the circulation pumps rather than letting it go through the heater to bring it back up to full temperature. In the process of doing that it would decrease the flow of water through the heater causing the heater to turn off on high water temperature. Then the time lag for it to restart.

The trips were in the air handlers. Yes if the air handlers had been installed with a bypass section it would have eliminated the problem. However when I inherited the maintenance of the HVAC, the building was already accepted by the owner. And yes they could have sized the system so that glycol could have been used. Unfortunately they didn't. I also think that they wanted the size of the mechanical rooms kept as small as possible so that they could use the space for other purposes.

 

[ChasBean1]

Icanfix,

Good post. It sounds like it was a beast that was tough to tackle - I've seen a similar, large plant with multiple HVAC issues. Each tree was designed pretty well, but the forest was a mess!

 

[Icanfix]

That is a beautiful discription of the equipment. The original plant was put in service in about 1940. Still has some of the original HVAC equipment. As the plant expanded the design philosophy changed. Most of the expansion was added on as individual pieces rather than any grand scheme.

Like I said in the orginal post, it was the most interesting five years of my career. It was fun, mainly because of the challenge. I was finally blessed with a couple of HVAC techs who knew what trees were supposed to be like. I learned so much from them.

 

[lzh007]

I am sill a little confused about the MAU.
If the equipment is selected properly and the air flow is contant, freeze protection should not be triggered so often. I think it is manufacturer's problem.
About the water heater, I am sure the 3-way valve was installed in the wrong place. it should control flow to the system but not to the heater.

 

[Icanfix]

I'm sorry I didn't mention that the MAU had a duct pressure controller on it. The air flow is controlled by a pneumatic controller that operates an inlet damper on the fan. The fan is a backward inclined double inlet type fan. The inlet damper was modulated to satisfy the amount of air that was being brought into the zones to cool them if they became too warm.

Yes I think the 3 way valve was in the wrong place, but I couldn't figure out where to put it so that it could control the loop temperature without changing the flow through the water heater.

I finally figured out that the freeze protection was triggered by poor mixing of the outside air and the return air at the inlet to the filter box. The mixing chamber was only about 3.5 foot wide by 10 foot long by 7 or 8 foot high.

Had the coils been set up so that they would get a constant flow of water rather then have the flow controled by discharge temperature the coils wouldn't have stratified. with the low flow.