The Technology and Economics of Higher Efficiency Residential Gas Heating Systems
The Center for Energy and the Urban Environment (formerly the Minneapolis Energy Office) promotes and coordinates the installation of higher efficiency replacement furnaces and boilers. This report presents a review of residential heating equipment suitable for use in one to four family buildings.
One way to reduce residential energy use is through the replacement of the existing system with one of higher efficiency. In the case of gas heating, the existing furnace or boiler is likely to be of the natural or atmospheric-draft type with a standing pilot. One type of higher efficiency gas central heating system is basically a conventional unit, but with a damper to reduce heat losses through the vent when the burner is not operating and an intermittent ignition device (IID) instead of a continuously burning pilot. A furnace or boiler equipped with a damper and IID, we call a DI system. A mid-efficiency (ME) system, also equipped with an IID reduces heat losses further by reducing excess air drawn through the vent during combustion, virtually always with a fan, and releasing combustion products at a lower temperature than in conventional systems. Nevertheless, the flue gas temperature in ME systems is substantially above the dew point of the combustion products. For further efficiency increase, the flue gases are cooled down to temperatures close to their dew point in the near-condensing (NC) systems or below the dew point in fully-condensing units. This last category is referred to as a high-efficiency (HE) heating system. NC and HE units are also equipped with intermittent ignition devices.
ME, NC, and HE units are sufficiently different from conventional systems that new problems have to be dealt with. This newer equipment does not depend on natural buoyancy to vent combustion products, and includes pressure and/or temperature sensors to ensure proper venting. Because the flue gases exist at lower temperatures, conventional rules of vent design (diameter, length, material, termination, etc.) are no longer valid, and new guidelines have been developed. New heating systems are classified and labeled as one of four vent categories, that depend on whether the vent is positive or negative in pressure, and whether or not the flue gases leave the unit substantially above their dew point temperature.
Conventional, DI, and some ME units have a negative vent pressure and also release flue gases at 350°F or more (the dew point is 140°F for natural gas combustion products when there is no excess air) and make up the Category I heating systems. Category I units can be vented into an existing chimney provided it is not unlined masonry and has a size within the acceptable range. A category I heating system can be common-vented with another appliance in the same category, such as a conventional water heater (equipped with a draft hood); and indeed, common venting is desirable whenever possible. Vent tables have recently been developed for category I appliances, both when they are vented alone and when they are common-vented with another unit. ME units, with an induced-draft fan located at the outlet to the furnace or boiler, push the combustion products out with positive vent pressures and make
up category III. Because of the positive vent pressures, the vent needs to be gas tight, and a heating system in this category cannot be common vented with another appliance. Since the chimney draft is no longer needed, the heating system can be vented through a sidewall. Manufacturer's recommendations for vent sizing and installation need to be strictly followed. Failure to do so has resulted in many problems in the venting of ME heating systems.
NC and HE units also generate positive vent pressures and need to be vented alone. Since flue gas temperatures are relatively low, the vent material needs to be resistant to the corrosivity of the flue gas condensate. NC units use vents made of high-temperature engineering plastics or special corrosion-resistant stainless steel. HE units vent at much lower temperatures, and polyvinyl chloride (PVC) or chlorinated polyvinyl chloride (CPVC) have proved highly satisfactory. Manufacturer instructions on vent installation need to be strictly followed. The venting of ME, NC, and HE heating systems is discussed in Chapter 3.
Early models of NC and HE units ran into problems from heat exchanger corrosion, traced to the presence of chloride (from bleach and other household products) in the combustion air. This has resulted in the disappearance of NC furnaces, as well as special designs in the few remaining NC boiler models to make sure that there is no condensation (by keeping boiler return water temperature above dew point). Specialty stainless steels high in molybdenum and chromium content, such as AL 29-4C, are used in the construction of the heat exchanger where condensation is likely to occur in HE systems. These alloys have been tested to be highly resistant to chloride contamination of combustion air, and excellent manufacturer warranties give confidence to prospective purchasers. Earlier, defective models were modified at manufacturers' expense. Moreover, there have been no "recalls" of models distributed after 1985. Future models may use high temperature engineering plastics or ceramic materials and should be satisfactory, provided they are offered for sale by reputable manufacturers and backed by excellent warranties. The use of alternative materials should reduce the cost of HE equipment and make them more affordable.
One problem with HE boilers (as opposed to furnaces) is that the return water temperature may be too high to permit significant condensation to take place. The laboratory efficiency testing is carried out at a "return" water temperature of 120°F, and the rated (annual fuel utilization) efficiency is just over 90%. For houses without outdoor reset control, the return water temperature is likely to be 140°F or more. Except for installations with such reset control, we propose that fuel savings be estimated on the basis of an effective efficiency of 88%. Experience with condensing (HE) boilers in Europe, with outdoor reset control, have shown that return water is below the dew point for most of the winter and large energy savings are measured. The special problems of condensing heating systems are discussed in Chapter 2.
ME, NC, and HE heating systems require much less air for combustion and dilution than atmospheric combustion units that are equipped with a draft diverter or hood. Also the presence of safety devices that shut down the unit when it is not venting correctly makes a special combustion air intake unnecessary. Other exhaust devices such as fireplaces and powerful exhaust fans could result in excessive house depressurization and prevent other vented combustion devices from venting correctly. A fireplace and a conventional domestic water heater are particularly vulnerable. The combustion air intake, required by code to accompany the installation of the replacement heating system, may not be sufficient to avoid problems. The time of an energy audit, prior to heating system specification, is an excellent time to diagnose a house for venting hazards. We propose a sequence of venting safety tests, based on Canadian research results and procedures, to look for excessive depressurization and vent failures, and to prescribe corrective measures. The issues of combustion and make-up air are discussed in Chapter 4.
Defects in ducts, in houses with warm air heating, can increase energy use and lead to indoor air quality problems. In Chapter 5 we outline appropriate diagnostic procedures and remedial action, best carried out after the heating system has been installed and other duct modifications (adding registers, etc.) have been made.
All categories of higher efficiency heating systems are designed to have low off-cycle losses, so that their seasonal and steady state efficiencies are virtually identical, and there is no energy penalty from over-sizing. The one significant exception is for HE boiler installations where over-sizing may increase return water temperature and reduce condensation. Over-sizing can increase interior temperature swings leading to discomfort. Moreover, an oversized heating system will have more on and off cycles. Cycling increases the possibility of condensation and of condensation / evaporation cycles in ME furnaces and boilers and their vents, and in non-condensing parts of HE units. Condensation / evaporation cycles lead to the concentration of the acidic condensate and increase corrosion risk. A simplified methodology is proposed for the sizing of replacement heating systems based on the previous year's gas use data. The sizing of higher efficiency heating equipment is considered in Chapter 6.
A field performance evaluation of ME and HE furnaces in the Province of Alberta gives considerable information on the frequency of problems encountered by heating systems in these categories installed before 1986. Component failures, particularly in the sensitive pressure and temperature sensors, were the most frequent problems and led to the heating system shutting down. This was also the most common type of problem in European HE boiler installations. Manufacturers have since developed more reliable components. Improper installation was also common and highlights the need for better training. Noise problems were the next most frequent problem, and manufacturers have tackled this through modifications in the fan, changes in installation techniques, or even new furnace designs. Some of the problems were related to venting, especially of ME equipment, and the guidelines for venting developed by the gas industry and recently released should help to reduce venting problems. Training of installers remains the most important issue in avoiding problems. Chapter 7 summarizes the experience gained in the field from formal studies as well as by installers of heating equipment.
The principal benefit of energy-efficient heating systems derives from their efficiencies relative to existing conventional systems. Although new equipment has a rated annual fuel utilization efficiency (AFUE) label, the comparable figure for the existing unit is not readily available. We have used the labeled efficiency of the new unit and measured fuel savings in a number of heating system replacement studies to deduce an effective AFUE of the pre-existing heating systems that averages 68%. At present, there are insufficient data to determine if the labeled AFUEs of the new units are being realized in actual installations. Until such data are available, we propose to use the labeled AFUEs and a 68% efficient existing system to estimate fuel savings. Such an estimate will be consistent with measured energy savings in several studies. The exception is the HE boiler where, unless steps are taken to ensure that the return water temperature is low enough to permit flue gas condensation, we have proposed an effective AFUE of 88%. Recent changes in United States Department of Energy (USDUE) regulations require that furnace (but not boiler) efficiencies be reported assuming that the unit is installed in a space isolated from the heated indoors. Since Minneapolis installations are invariably in basements, we have proposed increasing the rated value for furnaces by one percentage point for estimating energy savings.
AFUE only considers fuel use, which is only a part of the energy input to a heating system. The auxiliary electricity input can be substantial, particularly for many ME and HE furnaces. One index that takes electricity use into account is the Seasonal Energy Utilization Factor (SEUF) which considers the heat input to the house from the auxiliary electricity input, and the relative prices of gas and electricity. The calculation of SEUF requires knowledge of the electric power input (watts) to the unit and the fuel input rate (Btu/h). The SEUF effectively downgrades the AFUE to reflect the electricity use. It is simple to use, and changes in SEUF are related to cost savings in the same way that changes in AFUE are related to fuel savings. Unfortunately, heating system power input data are currently not reported in a consistent manner by the gas industry, although they are likely to be in the near future. Meanwhile we take typical ranges of power input and furnace and boiler heat input to relate SEUF to the corresponding AFUE. This analysis shows that electricity input should not greatly affect equipment choice in boilers, but the situation for furnaces is very different. Furnaces require much more electric input than boilers, particularly for the air-distribution blower. ME and HE furnaces generally use significantly more, though recent innovations have led to the commercialization of HE furnaces that are highly electricity efficient. Consequently, electricity use is found to be at least as important as simple AFUE in choosing among different models. All aspects of energy use and efficiency measures are considered in Chapter 8.
The cost effectiveness of a energy-efficient heating system depends not only on the energy cost savings but also on the installed cost of the new system and the economic criterion used in comparing alternatives. The annualized life-cycle cost is shown to be a simple indicator that permits an economically rational choice among different alternatives, including the early retirement of a working system. A simple computer program, that takes the drudgery out of the calculation is presented in Chapter 9 and the method is illustrated with several examples for furnaces, boilers, using typical installation costs from earlier ThermoSense bids.
The reduction of housing costs should not only consider all shell and heating
system options available to the homeowner, but also make sure that problems related to house structure and the health of the residents are not overlooked. A procedure has recently been developed for considering house shell measures and side effects in an integrated procedure. This procedure, called MWX90, which was developed for the Minnesota Weatherization program and applicable to any cold-climate housing, considers heating system retrofits but not replacement. The present study is an excellent complement in that higher efficiency heating systems are the principal focus. In Chapter 10, we outline a procedure that combines the MWX90 procedure with the steps involved in picking and installing a new heating system. A combined procedure permits a better distribution of conservation investment among the alternative measures applicable to a given house as well as addressing related environmental problems. Such a combined procedure is ready to be field tested and could lead the way to a new era in residential energy and the environment.
The four Appendices consider some of the issues in detail: the first (Appendix I.) reviews our knowledge of the corrosion resistance of condensing heat exchanger materials; Appendix II. describes techniques for sizing replacement heating systems using past billing data; Appendix III. reviews data to make an estimate of the annual fuel utilization efficiency of existing heating systems. The final Appendix (IV.) presents a brief review of alternative criteria for choosing among heating systems, and the reasons for the method preferred in this report.
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The Technology and Economics of Higher Efficiency Residential Gas Heating Systems