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

Wednesday, January 8, 2014

Top Ten HVAC Predictions for 2014


1. LEED V4, released last fall, includes a reference to either AHRI 885 or the ASHRAE Handbooks to prove compliance to acoustical requirements. The handbooks are missing a critical path table for the sound transmitted from a plenum noise source into a space. The incoming chair of TC 2.1 has indicated he will get the missing ceiling table included in the Handbooks. I predict he will fail in this effort for at least three years and that the 885 handbooks will be the only easy path to compliance. (One can hire an acoustician, of course, and TC 2.6 is populated with them).

2. Installed linear diffusers will still fail to have pattern controllers set prior to balancing. If adjusted after balancing, rebalancing is required. It is the design engineer’s responsibility to provide the adjustment information. The installing contractor should be designated as the one responsible for setting the pattern deflectors.

3. ASHRAE Thermal Comfort Standard (55-2013) has been modified to include Normative (mandatory) and Informative sections so that it can be referenced directly in codes. ADPI will be included by reference to the ASHRAE Handbooks, which will be updated to show the relationship between air distribution and thermal comfort.

4. There will still be no new published, peer reviewed, energy savings data for any of the “energy saving” systems (VRF, displacement, underfloor, or chilled beams), but engineers will continue to claim energy savings compared to overhead systems, to get LEED or Energy Star ratings.

5. The AHRI / ASHRAE study on whole system energy use of fan powered boxes will start to be integrated into the Energy Plus and possibly Trane Trace and Carrier HAP energy models under an AHRI research program.

6. The number 1 reason for not renewing the lease in high rise buildings will continue to be “occupant dissatisfaction with the building environment” (ie: comfort). It has been so for the past 20 years, so this is likely a ‘gimmee’. One can hope I miss this one.

7. The market for HVAC components will again be up about 5% with pent-up demand slowly coming back.

8. VAV overhead air distribution will continue to be (by far) the most used system in new buildings. As loads decrease, it will become more apparent that the ventilation load in the interior is the predominant building load. ASHRAE RP 1515 will open a lot of eyes. The report from the ASHRAE MTG on Advanced Air Distribution will be circulated showing paths to optimize system design.

9. While many VAV box schedules will continue to show design discharge temperatures in excess of 90°F, a number of Engineers will finally get it. (ps: Hot air rises!)

10. Sadly, I predict that the Cowboys will continue to wallow in the mud of disappointment.

Authored by: Dan Int-Hout, Chief Engineer Krueger

Thursday, January 2, 2014

Scoring on My Top Ten HVAC Predictions for 2013

1. LEED 2013 will be released in the fall, and it will include a reference to AHRI 885 to prove compliance to acoustical requirements. Done - 10 points

2. Balancers will continue to ignore the need to adjust linear diffusers before balancing. If adjusted after balancing, rebalancing is required. This is the design engineer’s responsibility, of course, so they need to require adjustment is they specify adjustable slots. I predict this will be ignored. (It’ll be an easy 10 points). Yup - 10 points

3. ASHRAE Thermal Comfort Standard (55-2010) will be modified to include Normative (mandatory) and Informative sections so that it can be referenced directly in codes. This is already partly accomplished. ADPI will be included in ASHRAE standard 55 as a means of predicting compliance. In progress - 7 points

4. The updated ASHRAE UFAD Design Guide will be published in 2013. Done - 10 points

5. We will see no published, peer reviewed, energy savings data for any of the “new” systems (displacement, underfloor, or chilled beams), but engineers will continue to claim energy savings compared to overhead systems, to get LEED or Energy Star ratings. Sadly, still no published validation - 10 points

6. BOMA will continue to state that the #1 reason for not renewing the lease in high rise buildings is “occupant dissatisfaction with the building environment” (ie: comfort). (It has been for the past 20 years, this is likely a ‘gimmee’.) Still the top reason - 10 points

7. The market for HVAC components will again be relatively flat with local ups and downs. Still flat - 10 points

8. VAV overhead air distribution will continue to be (by far) the most used system in new buildings. As loads decrease, it will become more apparent that the ventilation load in the interior is the predominant building load. ASHRAE RP 1515 will open a lot of eyes. Yup - 10 points.

9. The majority of VAV box schedules will continue to show design discharge temperatures in excess of 90F, in spite of the ASHRAE handbook stating this will guarantee non-compliance to Standard 55’s vertical temperature stratification limitation, and without compensating increases in ventilation, as required by Standard 62.1 (and code in most states). Again, this is sadly true - 10 points

10. Sadly, the Cowboys will continue to disappoint their fans. (Again) I’m tempted to give this bonus points, but I’m in a giving mood – 10 points.

97 Points total. Nostradamus has nothing!

(It’s easy to score high if you grade your own predictions….)

Authored by: Dan Int-Hout, Chief Engineer Krueger