How To Deal With Ramped Up Loads
By AdeyEngineering Green Technology Hydronics Energy Efficiency Residential Buildings
Jevons' paradox is alive and well in high performance homes.
The industry’s public relations and marketing campaign to push higher performing homes appears to be working, at least for a percentage of the population, but even consumers adopting “green” and sustainability are proof that W.S. Jevons was right–at least on some scale. Jevons was an economist and philosopher who foreshadowed several developments of the 20th century. He argued that technological progress was not to be counted on to reduce society’s energy consumption.
There are many examples of Jevons’ paradox at work in housing, including disproportionate floor areas (relative to needs), inaccurate or no load calculations,1 oversized equipment/components and the increasingly common monster-sized kitchen range hoods.2 Well thought out energy paths are frequently trumped by ramped up cooling and dehumidification loads, heating of make up air for kitchen exhaust or unique potable water heating demands.
The following is an example of a project where comfort, durability and low maintenance objectives were a top priority, and where the philosophy of sustainability was headed in the right direction until it took a slight detour.
The 4500 ft.2 (418 m2) home and carriage house, which is located in the Great Lakes area, is constructed with insulated concrete block with an additional layer of exterior insulation. Accounting for the windows and doors, the vertical enclosure is a weighted R=18 hrft2F Btu (RSI=3.17 Km2/W), the slab is insulted to R=10 hrft2F Btu (RSI=1.76 Km2/W) and the attic to R=60 hrft2F Btu (RSI=10.57 Km2/W).
Working with the client, we were able to base our loads for east, south and west windows on triple pane, argon filled with a Cardinal glass coating configuration of LoE366/clear/LoE180, U value=0.14 Btu/hrft2F (0.79 W/m2K), solar heat gain coefficient (SHGC)=0.24 and visible transmittance (VT)=0.54. North side windows had LoE180/clear/LoE180 glass to allow for greater visible light transmission of a VT=0.67, U=0.15 Btu/hrft2F (0.85 W/m2K) and SHGC=0.51. In similar buildings the builder has demonstrated leakage rates of 1.5 ACH50 (approximately 0.1 ach) or lower.
Sensible heating flux from the radiant panel averaged approximately 11 Btu/hrft2 (35W/m2) and cooling sensible flux was at approximately 7 Btu/hrft2 (22 W/m2). Additional sensible cooling is provided by the ventilation system (see Figure 1). The flooring is to be light carpet in the basement, tile for all wet areas and wide wood plank flooring in all other areas.
The occupant conditioning system is embedded radiant
either in concrete or subfloor/wall panel systems. From the first run of calculations, the weighted return fluid temperature for outdoor heating design conditions at 6F (14C) was set to 94F (34C) and at minimum load to 76F (24C) to enable higher boiler efficiencies and minimize VOC emissions from subflooring.3 Flow velocity was established based on conservative delta ts and pipe diameter selections for the cooling approach so that the circulator saw similar head losses in either the heating or cooling mode.
The cooling system was set for mean fluid temperatures of approximately 53F (12C) for dehumidification at outdoor design conditions of 87F (31C) and 72F (22C) WBT and then mixed up to 66F (19C) for the radiant floors. Wherever rogue zones needed higher or lower temperatures radiant walls were added to increase the surface area. This prevented the system from operating unnecessarily at higher or cooler temperatures and eliminated the need for additional control systems.
A 118 MBH (34.6kW) wall hung high efficiency boiler (B1 in Figure 2) connected to a 25 USg (97L) buffer tank (Tk2), will deliver heated fluid for the radiant floors and heating coil (HC1) for the kitchen exhaust and make up air (F1 and F2 in Figure 1). Likewise a five-ton (17.6kW) reverse cycle air cooled chiller (CH1, Figure 2) will be used to maintain 50F (10C) in a 50USg (189L) buffer tank (Tk1) to feed the radiant floors and walls and for the cooling coil (CC1, Figure 1) for the kitchen make up air.
Due to the deliberate low operating fluid temperatures in heating and somewhat higher temperatures in cooling, both of the coils in the kitchen make up air system are ‘oversized’ relative to traditional coil selection procedures. This oversize is a onetime capital cost with the lifetime benefit of lower operating costs.
As a result of HVAC configurations and building geometries, mechanical room one, which contains the boiler, is located in the main house. It is approximately 75 ft. (15 m.) away from mechanical room two in the carriage house, which holds the cooling equipment. The two pipe changeover system uses a four way reversing valve (RV1) to flow a 40 per cent propylene mix either from the boiler and buffer tank to mechanical room two in heating or from the chiller and buffer tank to mechanical room one in cooling.
The space ventilation system is a three speed HRV (HRV1) that pulls its outdoor air (O/A) through the heated and chilled water coils (HC1, CC1), which are installed as part of the kitchen make up air system. In the normal space cooling and ventilation mode, the cooling coil provides dehumidification of 150 cfm (71 L/s) of OA. The cooled, lean and filtered (MERV 11) supply air (S/A) is reheated via the exhaust air (E/A) passing through the heat recovery core in HRV1 before being distributed directly to each space. EA is extracted either directly from each room or via undercut doorways or jump ducts depending on design flow rates and room use. Timers in the bathrooms and kitchen/dining room engage the HRV’s high speed capacity (400 cfm [189L/s]).
A few items to note: as the home is to be very tight, the infiltration load and thus the latent load is essentially limited to the incoming ventilation air plus moisture released to the space from the occupants (two adults, two children), plus moisture released during meal preparation, home cleaning and bathing. Since the cooling coil’s primary purpose is to dehumidify make up air for the 900 cfm (425 L/s) kitchen exhaust fan, space cooling and ventilation air passing over the coil even at high speed will be at a low velocity of approx. 150 fps (46 m/s) and even lower during normal operation. Because of the chosen fluid temperatures and wide operational band on the coils, constant fluid flow with a modulating two way injection control valve with actuator having a wide rangeability were selected. A linear valve is preferred for heating due to the low mean fluid temperature and larger delta t; and an equal percentage valve for the cooling coil due to the somewhat higher mean fluid temperature and smaller delta t.
The design challenge of significance was integrating the 900 cfm (4256 L/s) kitchen exhaust into the system. There were two approaches. The first was for separate heating and cooling coils in a make up air unit independent from the HRV. The second, and chosen method, was to size the makeup air coils for the kitchen exhaust system and use the same coils for the space ventilation systems. In the latter application the cooling coil provides dehumidified air to both systems. If necessary, the heating coil can provide wintertime preheat for the HRV. Additionally, if aggressive dehumidification is required when the occupants are entertaining or during unusually extended wet outdoor conditions, the HRV can be switched from its 150 cfm (71 L/s) to 400 cfm (189 L/s) capacity with 250 cfm (118 L/s) air bypassed (R/A Bypass), and recirculated across the cooling coil. The analysis of three air flow rates over a single coil is necessary to understand the space ventilation supply air and kitchen make up air conditions and the necessary valve/damper/control strategy to achieve the designed performance.
After it was all s
aid and done, the makeup air heating coil load was 83 MBH (24.3kW) and the cooling coil was 48 MBH (14.1kW) compared to the 48 MBH (14.1kW) space heating load and 39 MBH (11.4kW) space cooling load (figures are approximates). As is the case with many modern projects, it is not the space load driving the plant size but rather the make up air for the kitchen.
Why would the client not drop the big gas fired range and its required monster exhaust hood, which would allow for a smaller induction unit with a smaller hood at about a third of the load?
We asked the same question. The family frequently entertains large groups of people. They struggled in their previous home to prepare meals for large gatherings and their hearts were set on gas. C’est la vie.
On other projects we have been able to convince clients to drop the big gas ranges, which tightens up the entire mechanical system. As they say, some days you are the pigeon and other days the statue. More often than not we are finding we have to set aside our personal objectives for sustainability and meet the subjective needs of our customers. But given the choice between a poor building with extraordinary loads and a good building with extraordinary loads – we will always take the latter over the former. <>
Robert Bean, who is president of Indoor Climate Consultants Inc., is a Registered Engineering Technologist in building construction through the ASET and a Professional Licensee in mechanical engineering through APEGA.
See Robert at the Modern Hydronics-Summit 2015 on September 10, 2015. For more information visit www.modernhydronicssummit.com.
1 See HPAC Feb, 2015 (www.hpacmag.com, archives), Heat loss/heat gain considerations.
2 See HPAC June, 2013 (www.hpacmag.com, archives), Ignore Pressure Differentials At Your Peril: The unintended consequences of monster-size range hoods.
3 See HPAC January 2012 (www.hpacmag.com, archives), Together Forever: VOC Emissions Modelling