A Two-In-One Seasonal Alternative
Portable heat pump basics.
February 1, 2013 by Clark Michel
The time is right to review the technologies available to building owners, operators and maintenance supervisors for emergency, temporary and supplemental heating applications. There are a number of good portable heating options (see sidebar below) on the market for non-residential applications, but they are not as well understood as their portable air conditioning counterparts, and are therefore sometimes underutilized.
Portable heat pumps are the newest technology available, having come into use over the past 10 years or so. They look identical to the spot coolers that are so widely accepted for a host of applications, but with the addition of a heating function that is designed to be safer and more efficient than resistive electric heating. Unlike the portable devices described in the sidebar, portable heat pumps are unique in their ability to provide “two-in-one” cooling and heating functions – at an incremental cost of around 20 to 30 per cent higher than a same-size unit with cooling capability only. Because of their versatility and the fact that so many regions of the country have seasonal needs for both cooling and heating, portable heat pumps are regarded as having strong growth potential.
Applications range from spot heating and cooling of spaces ranging from office environments to shop floors to industrial processes. The units’ portability permits localized temperature controls and allows users to roll out the units at the end of a rental or lease period.
Portable heat pumps can address “cold spots” as well as personal preference for more cooling or heating than the general environment. Corner rooms, areas that receive less sunlight, or spaces that need extra heat due to lack of balance in the central system are other common applications.
Probably the biggest drawback to portable heat pumps involves their operating temperature limitations in the heating mode. Although a portable heat pump in the cooling mode can offer cooling relief in the most sizzling summer heat wave, the same portable unit in the heating mode cannot always be used as the primary heat source, since you can only extract so much heat from cold air with a mechanical heat pump system. The limitations for a portable heat pump, in the heating mode, require the surrounding air temperature to be at a minimum of 40F and in some cases 55F depending on the manufacturer of the equipment. As a result, the ambient climate must be temperate or another heating source must be available.
Within these limitations, portable heat pumps are an excellent option in several scenarios. These include supplemental heating in chilly lobbies and public spaces, restaurants, offices, conference rooms, manufacturing areas, special events, nursing homes and hospitals. Though portable heat pumps are not the answer for an unheated building in a very cold area, they can also be effective during a cold snap in a usually temperate region.
When a building’s central system is shut down or set back to a lower temperature for a night, weekend or holiday closure, heat pumps can deliver spot heating targeted only to the areas that need it – at a much more efficient cost than the energy that would be required to heat the whole building.
They are particularly suited to temporary heating during renovation or repair of the primary HVAC system: In a high-rise tower or other large building, HVAC equipment repair or replacement is typically performed on a zoned basis, usually floor by floor. Portable heat pumps provide an ideal way for contractors to fill the gap and keep occupants comfortable during partial HVAC service or refurbishment.
The portable units can be easily wheeled to different areas or floors as work progresses.
Portable heat pumps are convenient in retail locations because they need no extra wiring. The units plug into the wall, with no outside access needed through windows, making them safe and secure.
Portable heat pumps typically range from one to five tons (or up to about 60 000 Btuh) in capacity, are mounted on wheels and are designed to fit through standard interior doors. Units at the lower end of this range can typically run on a standard 115-volt circuit, but larger capacity units will require higher voltages to operate. Some units have the capability to perform at ambient temperatures of 40F, utilizing an expansion valve and an indoor/outdoor condenser, which provides for maximum efficiency through a balanced condenser air pressure. Some portables also have a built-in condenser plenum that draws warmer air from above the ceiling to increase heating output.
Other units use a cap tube system that requires minimum ambient temperature of 55F to operate.
Computerized controls that automatically switch from 70 000 Btuh indirect fired portable heater is used to heat a construction site. The return air is recirculated through the heater to form a closed loop.
Event tent is heated by bringing in fresh outdoor air through the portable heater and ducting inside. the cooling to heating function (or vice versa) are provided on some models, along with variable speed evaporator fans that automatically cycle down as the temperature in the space approaches set point.
Current model heat pumps are updated to operate using environmentally friendly R-410A refrigerant.
The most effective approach to installing a spot cooler is to use a return air plenum to draw heat from above the ceiling. The resulting heat transfer effect will bring more warm air into the space. As noted above, some portables come equipped with built-in plenums as standard equipment while other manufacturers offer it as an added-cost accessory.
If a return air plenum cannot be utilized, an alternative is to draw negative pressure into the space you are trying to heat. This will help to increase the net heating effect, but not as efficiently as a plenum. In the majority of applications, you will want to locate the unit within the space to be warmed. If this is not possible due to space constraints or noise concerns, the portable may be located outside the room and warm air ducted into the space to be heated. Keep in mind, though, that wherever the portable is located, the ambient temperature must be high enough for it to operate.
Finally, while the air-cooled heating and cooling portables described above dominate the market, water-cooled portable heat pumps are also worthy of mention. In these systems, water is used instead of air to remove heat from the refrigerant inside.
Water-cooled portables can perform in any temperature above freezing, offering greater operating range and much greater capacity than same-sized air-cooled counterparts, as it is possible to pull heat out of water more readily than out of cold ambient air. They are an excellent choice in the right conditions, but they are limited to use in areas where the municipal water supply is plentiful and economical, or in buildings with closed-loop cooling towers. The most common applications include emergency or supplemental heating in retail stores, restaurants, offices and other commercial spaces. <>
Clark Michel is vice president of Atlas Sales & Rentals, Inc., a distributor of portable HVAC equipment (www.atlassales.com).
Categories of portable heating equipment
Direct fired heaters (also known as construction heaters). With direct fired units, air is blown across a natural gas or propane flame and into the area to be heated. On an open construction site, nothing works more cost-effectively – however, there is an open flame that has some level of toxic emissions, so safety issues are of concern. For temporary use in well-ventilated open
areas, it is a viable choice and can help speed construction processes such as ground thawing, concrete and drywall curing, etc. These units typically feature very large capacities of 80 000 – 7 000 000 Btuh
Electric heaters, whether large or small, use resistive heat, which is very effective in cold environments but is the most expensive form of heat you can get. Large electric heaters with capacities of 34 000 – 512 000 Btuh are quite popular. They use a forced air fan to blow or duct fresh, warm air to the area to be heated, delivering clean, dry air with no emissions. Electric power may be an issue: These large heaters usually require three-phase 220 V or 460 V high voltage wiring, which may not be available – sometimes necessitating the use of a generator or an electrician. Much smaller electric space heaters are also used for spot heating, but due to the well-known safety issues (e.g., the potential for nearby drapes or furnishings to catch fire, or for office workers to singe a hand or foot on the exposed coils), these units are often prohibited for use in commercial buildings.
Indirect fired gas heater is suited to indoor or outdoor use. Indirect fired heaters have been gaining in popularity due to the combined benefits of enhanced fire safety, no harmful emissions, and the ability to heat outdoor or indoor areas including construction sites, manufacturing/industrial areas and tents. These powerhouses generate a high amount of heat (ranging from about 100 000 – 1 000 000 Btuh capacity) and are user-friendly. They are used in a wide range of applications – from keeping NFL players warm on the bench, to making congregations more comfortable in drafty churches when the central heating system needs a boost due to frigid temperatures outdoors. Indirect fired heaters incorporate a heat exchanger with a natural gas line or propane or kerosene tank that heats up the air. Fresh air is heated and then blown or ducted into a building or other area, with no worries about open flames or toxic emissions as experienced with direct fired heaters. Indirect heaters also offer advantages over electric heating units: They are easier to operate, are self-contained and convenient, and operate with low power consumption, so generator power is not required.
To determine the net heating effect that will be delivered, the simplest calculation is to take the cubic footage of the area to be heated and divide by 60. This will give you the general cfm requirement for the portable heater. The climate zone in which you are located, insulation factors, and desired temperature to be delivered to the space may increase or decrease your heating demand. You will also have to allow for the impact of other heating sources in the building (e.g., the central heating system) and heat loss that may be generated by windows, lack of insulation, etc. Taking these factors into account, along with the available power supply, you should be able to arrive at a fairly accurate estimate of the size and/or number of portables needed for the application.
SELECTING A PORTABLE EQUIPMENT SUPPLIER
A reputable distributor of portable equipment should be able to lead you through the above steps and provide sizing, technical and installation assistance. When selecting a supplier, ask:
• What kinds of equipment are offered? Look for a supplier who carries both portable heating and cooling equipment from multiple manufacturing sources, so you can get unbiased advice on the best unit for your application as well as year-round assistance.
• Ask what accessories are available and whether they are included in the basic rental price.
• Does the supplier have a national accounts program? If you manage multiple facilities, look for a supplier who offers a program tailored to national account customers. You may be able to save time and money with an integrated program that includes a single point of contact within the portable equipment supplier organization for all your service needs, a customized emergency response plan and the ability to accommodate your company’s ordering system.
• What other support and/or certifications are available? Ask what kind of technical support is available from the supplier.
• Ask if they follow green initiatives such as the use of environmentally-friendly refrigerants, etc.
• Does the supplier have a large inventory of locally-stocked equipment and a proven track record in emergency response?