Wednesday, January 15, 2014

Cold Calls – “Our Electric Coils Aren’t Working”

With the record cold that has gripped the country, the last thing one wants to hear is that the heat isn't coming on! So, we looked into the issue and have found that in most cases, this complaint stems from the design engineer not understanding some basic facts regarding the proper use of a VAV reheat air terminal device.

The typical complaint we get on electric heat is that it doesn't come on because the airflow safety switch isn't “making”. There are several issues that need to be understood:

1. Energizing: With today’s DDC systems, there is always low voltage available and electric heat is energized through electronic relays. In the past, these were typically wire wound electromagnetic coil relay devices, triggered and powered by low voltage current. As of recent, we are employing solid state relays, which although more expensive, are silent. With older pneumatic systems, the circuitry was all high voltage, which sometimes caused issue with arcing across the contacts in the safety “sail switches”.

2. Safety: UL specifications (and logic) require that the electric heat not operate unless there is sufficient air flow across the coils. After the main relay that energizes the electric coils, there is an airflow proving device and an overheat “high limit” device and on Single Duct units, there is a manually re-settable high temperature thermal cutout.

3. Airflow Proving: The air proving device used in all Krueger boxes is a “sail switch”, which is actually a combination of a membrane and micro-switch. This is used as a differential pressure assembly with only the high side connected to a pickup in the air stream. The low side is typically open to the control enclosure. On some airflow switches, there is no connection tap on the low side. If the switch is installed in a relatively air tight enclosure, this can be a problem, so Krueger has elected to only use higher cost switches with a low side tap so we can reference the differential pressure outside the cabinet if desired.

4. Air Flow Pickup: The single point pickup probe is located at the point of highest airflow in the heater assembly. The single tube pickup is actually a “total pressure”, rather than a velocity pressure device, which would require two tubes.

5. The mechanics of this type of sail switch precludes a tight operating range specification, with the one Krueger uses having an 0.05” stated response, but with an accuracy of +/- 0.02”. We find that most operate (make) at about 0.03”. The published minimum airflows in the Krueger catalog develop less than 0.01” velocity pressure at the probe location, so Krueger requires that there be at least 0.03” downstream pressure in the airflow path to the room to get the necessary total pressure to engage the airflow switch.

So, knowing the “rules”, we can now look at the issues they raise. If a single duct VAV box is set to have heat come on at minimum VAV box airflow settings, it is very unlikely that downstream pressure will be as high as 0.03. If it were, and if the box is set at the typical 25% of cooling maximum air flow, the downstream pressure would 16 times higher at maximum flow! For a typical single duct VAV box electric heater, this is about 0.4”. Few systems are set to operate with downstream pressures this high.

This is compounded by the damper position in the VAV box. Even though we require a longer box when using electric heat, we still see a higher velocity pressure at the pickup location at partially closed damper positions than when the damper is fully open. With dynamic air handler system pressures being set so that at least one box has a fully open damper (as required in some codes), it is possible that a perimeter heating VAV box may have a fully open damper, but then it may not “make” even at the Krueger stated minimums. A balancing damper set to provide some back pressure may be required in some extreme cases.

In most cases, it shouldn't be a problem, that is, if the engineer is following good practice (and most codes) and limiting discharge temperatures to 90°F. This will usually require heating airflows to be higher than 25% of cooling maximums to meet the heating load requirements. DDC controllers make having a “dual minimum” as simple as just setting it up. (Older pneumatic systems require complex plumbing to achieve dual minimum control). The ASHRAE 90.1 energy standard, typically referenced in codes, now allows for reheat up to 50% of design cooling air flows if one starts at 20% and limits discharge temperatures and uses VAV heating. Of course, Krueger’s LineaHeat electric heat was designed with this as a standard feature should one add a temperature probe located downstream of the heater.

So in summary, a VAV box with electric heat will operate properly when the heating airflows are high enough to provide sufficient total pressure at the heater. This can usually be accomplished with sufficient airflow to meet the 90°F discharge limitation. If lower airflows are required, providing some static pressure resistance downstream may be needed.

Authored by: Dan Int-Hout, Chief Engineer Krueger