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Unitary Heating Equipment – Acronyms
As mentioned in a previous short, acronyms are rampant in our business. I think more on the BAS side of things than anywhere else (catch that?), however there are plenty on the mechanical side of HVAC (how about that?). Alright, enough of the BS, let’s get to the point here. Unitary Heating Equipment, as mentioned earlier in this series of “shorts”, is a term that encompasses a broad array of similar equipment. We start with the basic unit heater (UH), and move through all of its variations: electric unit heater (EUH), hot water unit heater (HWUH), and gas-fired unit heater (GUH). Oops, I forgot one…steam unit heater (SUH). So all of these aforementioned heaters are those that get suspended from structure, as in a warehouse or unfinished space, and are controlled by either a wall-mounted thermostat or an integral (unit-mounted) thermostat.
Then come the cabinet unit heaters (CUH) and wall heaters (WH, not to be confused with water heater!). So a hot water cabinet unit heater may be tagged as HWCUH, but that’s kind of a mouthful, and if the only type of CUH on the project is hot water, then the HW can be omitted. Same goes with the wall heaters. If there are only electric wall heaters on the job, then the design engineer may choose to omit the E and just call them out as WH.
Finally we get to the baseboard heating equipment. Maybe a baseboard heater is in the form of a radiator, and then it’s tagged as BBD. Whether it’s hot water, steam, or electric may be indicated in the acronym, however as stated in the last paragraph, if there is only a single type of BBD, then the prefix will likely be omitted. Now, if the baseboard takes the form of a pipe and strips of metal (fins) that are spaced closely together in such a way as to conduct heat from the pipe and transfer it to the surrounding air, then we are apt to call it fin-tube (FT). Fin-tube generally being of the hot water variety, we omit the prefix and simply call tag it FT.
So there you have it. Oh yeah, one more thing to mention. Of course none of these tags do any good unless followed by a number to individualize each and every piece of unitary heating equipment. Sometimes design engineers will tag multiple pieces of same-sized equipment with the same tag number, instead of giving every piece of equipment its own unique indicator. This is preference-based, but could lead to problems and is something to watch out for. Now, go out and tag your world!
Mixing box or external dampers?
Ever seen an air handling unit on a set of plans? Good, that’s a start. Seriously, I’m talking about an air handler designed to serve either a single large zone (CV unit), or many zones (VAV unit). These types of air handlers will typically be modular in design, meaning that you may have several sections or modules that make up the whole. I’m talking built-up systems here, not packaged equipment. For instance, an air handler serving a gymnasium may have a supply fan section, a return fan section, a coil section, and perhaps a mixing box section. This last item is the topic of discussion, and a potential source of confusion, I’ve found.
As often as not, the design engineers will show the built-up air handling unit with “external” outside air, return air, and exhaust air dampers. Dampers that are shown to be mounted in the ductwork that connects to the unit. Which is fine. Except that the unit manufacturer offers this mixing box as an option. And the contractor responsible for procuring the unit decides to order it with the mixing box. Provided that this gets through the submittal process unflagged, what inevitably happens is that the control systems contractor, if responsible for the motorized external dampers, will purchase the dampers and furnish them to the sheet metal contractor for installation, as per the plans.
In this scenario, the unit, mixing box and all, gets installed, and the external dampers get installed as well. Which leaves the controls contractor with a question: where to install the damper actuators? Or more basically, how did this happen?
What I’ve seen in this situation, after an RFI into the design engineer gets answered, is that the mixing box goes unused, and the external dampers become the recipients of the damper motors. The mixing box dampers are mechanically propped open, and the box becomes a costly extra, with no functional value.
The moral of the story? Quite simply, communication is the key. We all need to work together to make sure that these kinds of things don’t happen, as we’re all in this together, and we all need to turn a profit in order to stay in the game. In this scenario, in order to not place the blame on anyone in particular, I fault the equipment vendor for including something that he should know is an option and may not be required, the mechanical contractor purchasing the unit for not recognizing the external dampers on the plans, and the controls contractor for not recognizing the mixing box on the equipment submittal and bringing it to the attention of the mechanical contractor.
FPB in Unoccupied Mode: Unit Heater!
Series and parallel fan-powered boxes (FPBs) are VAV boxes equipped with fans. A series FPB puts the fan in series with the “primary air damper”. The fan runs continuously in the occupied mode of operation, and draws air from both the primary air damper and the return air plenum, mixing the two airstreams before delivering the air to the space. If the primary air damper closes down to its minimum position, as dictated by a decrease in space temperature below setpoint, then the fan draws most of its air from the return air plenum, and the FPB’s heat is activated to heat the space. A parallel FPB puts the fan in parallel with the primary air damper. The fan won’t run unless/until the primary air damper closes down to a minimum position. At that point the fan turns on, draws mostly plenum air, and delivers it to the space through the FPB’s heater, which is engaged.
The fan powered box provides a means for the terminal units to heat the space served, up to setpoint, in addition to being able to cool the space served, down to setpoint. All this during the occupied mode. So what happens during the unoccupied mode? Well, the primary air system, that which delivers 55-degree air to all the terminal units (VAVs and FPBs), shuts down. The VAVs on the system become functionless. But the FPBs continue to be active, and operate to maintain a reduced “night setback” space temperature setpoint. When the temperature in any given zone served by an FPB drops below the unoccupied mode setpoint, the FPB fan energizes, and the FPB’s heater activates. The air drawn from the fan and through the heater is from the return air plenum. The air propagates the space, heating it up as it passes through, and makes its way back to the return air grilles, and up into the return air plenum. All in all, when all is said and done, the FPB is nothing more than a unit heater during the unoccupied modes of operation, in terms of functionality. I like to cut to the chase when explaining unoccupied mode FPB operation, by saying “works just like a unit heater!”. Usually prompts a question or two, but it certainly gets us to the point a lot quicker!
Taming the Beast that is Steam
Steam is often a difficult medium to control. This is especially true with larger steam capacities. Consider a large make-up air unit with a steam coil. Sounds simple enough. Put a single control valve on the coil and modulate it to maintain discharge air temperature setpoint. Problem is, with larger steam capacities, and consequently with larger steam control valves, control of the steam rate is much more difficult to “dial in”, resulting in a control valve that constantly hunts, and a discharge air temperature that swings widely above and below setpoint.
A method of control that has been around for a long time, is that of 1/3, 2/3 valve control. The premise is that we can better control the steam rate with two smaller valves, sized accordingly. One valve is sized for 1/3 the steam capacity, and the other is sized for 2/3 the capacity. The valves are “sequenced”: the smaller of the two valves modulates open first, in an effort to achieve and maintain setpoint. As demand increases, the smaller valve reaches its fully open position, and the larger valve begins to modulate open. This translates into tighter setpoint control, especially during periods of light demand, when the smaller valve is the only one modulating.
Tip of the Month: This method of maintaining setpoint in a steam application is tried and true, yet is not the only way of doing it. An upcoming “short” in a future column will discuss another method of “Taming the Beast”, so stay tuned!
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