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Question DetailsAsked on 12/3/2016

If My gas fired boiler is 60% efficient, does that mean that I get 60 BTUs our for every 100 BTUs put in?

I have been told that if I replace that 60% efficient gas boiler with a high efficient boiler I could cut my heat bill by 70%. The numbers do not add up to me. This installation is in a 1950s church building.

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2 Answers



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Answered 3 years ago by Member Services


In most simplistic terms yes, but in reality either Yes and No depending on where your unit is located and how it is vented. Technically it means 60% of the energy from the gas burned goes into the circulated liquid under certain factory test conditions, and 40% goes into the exhaust gases or is radiated from the boiler to the surrounding area. It also generally assumes the gas burned in the pilot (if you have a standing pilot) is wasted - but there are different test standards and variations allowed which, like with auto EPA estimated gas mileage, can result in a confusing or misleading picture - and of course, manufacturers use the rating method that makes their device look the best.

Now the reality - in a boiler especially but also to an extent in forced air systems, generally most of the pilot light heat goes into the heat exchanger fluid (air or water as applicable) and keeps it warmer than building temp - so most of that heat is actually not "lost", at least during the heating season, it just make the fluid warmer so the furnace/boiler has less heating to do before the thermostat kicks back off at the end of the cycle. Also, especially in basement or tuck-under garage or in-house (within the conditioned space) utility room boiler locations, as long as it is shut down (typically to pilot only to reduce rusting/condensation) in the air conditioning season (so waste boiler heat does not have to be removed by the A/C), a goodly portion of the "waste heat" radiated from the boiler and about half to three quarters (very roughly) of that going up the flue with the flue gases is actually recovered in building heating. To truly figure efficiency one would have to measure both the gas flow (input BTU's) and the airflow and temperature of the exhaust gases over many hours at several heating season outside air temps to figure the true energy input and waste heat release to the atmosphere.

In many cases, like with my tuck-under garage and centrally located (roughly middle of house) boiler and flue ducting, most of my boiler and water heater "waste" heat actually goes to heating the conditioned space - making up energy that would otherwise have had to be provided by the burner firing. In my case, because we do not have air conditioning and rarely open windows (a year-around cool climate) and with a day-long heating season about 9 months long and nighttime heating season about 10-11 months long, a vast majority of my unit's "waste" heat is actually used in productively heating the house. In the "warm" months just the pilot alone is enough to keep the thermostat happy and keep the house up to 68-70 degrees, it just keeps the zone valves open much longer each cycle than if the boiler was up to full temp. My garage, in the heating season, generally does not kick on the boiler-fed unit blower / heat exchange radiator serving that space until outside temps get down to near freezing - stays at the 55 or 60 degree minimum I set (when not working in there) just from the "waste" heat from the boiler and water heater, and in cold conditions the waste heat significantly reduces the "run time" of the unit heater - so direct gas savings from that almost year around. I figure my unit is about 90-95% efficient in terms of the amount of gas heat actually used in productive "conditioned space" heating (normal exhaust heat temp to outside at roofline is about 80 degrees), so in my case only about 10 degrees over conditioned space temp - so a high efficiency unit would not gain much if any energy efficiency in my case.

Of course, if predominately a warm area like SoCal or the deep south where the furnace generally only works on colder nights for a few hours and most days you use natural window ventilation or A/C for cooling, then much of the "waste heat" from your boiler would truly be wasted in overheating the building during the days - so in that case a higher efficiency unit which does not radiate heat when off could save more and potentially also reduce A/C costs during days when the boiler and the A/C are each functioning at different times of the day. Desert and high altitude areas commonly have this issue of heater and A/C fighting each other part of the day - heater heats the house to desired temp at night or on cloudless days (high radiation losses from the house) but then during the hotter day the A/C kicks on, and the continuing "waste heat" from the steam or hot water heating unit become an additional load for the A/C to have to remove heat from. And of course in this situation, with simplistic thermostats sometimes the A/C and furnace are on different thermostats because it cannot handle both units being "active" at the same time, so they can actually both end up running at the same time if there is not enough temperature difference allowed between the two settings. I have seen cases where furnace and A/C were both blasting full power essentially continuously because the furnace thermostat was set at a higher temp (say 70 degrees) than the A/C setting (say 65 degrees), so each was fighting the other to reach its assigned temperature. In those sort of cases, forced air heat (no significant continued boiler heat when off), a fancier control system, or high-efficiency pulse-fired units can sometimes pay off - as can units that allow the boiler temp to drop way down (to say 70-80 degrees) when there is no demand, rather than keeping it at the normal 160-210 degrees 24 hours a day regardless of whether there is any demand. Some units can be adjusted to that operating mode, some not - though doing that does guarantee condensing conditions in the boiler flues, so there is a risk of shorted unit life due to rust unless the unit is a direct-vent "condensing" unit (high efficiency) by design.

Anyway, once you decide if you are getting useful recovery of much of the "wasted" heat from your boiler or if maybe the waste heat is unwanted and causing more A/C operation or making the building uncomfortably warm most of the time it is not seeing demand but still hot, you can guesstimate potential savings from there. If your current boiler is 60% efficient, it is probably at least 25 or more years old - so basically under current energy code you will be into probably a 78-81% minimum efficiency unit anyway (85 or 90% if Green Star certified) for a replacement unit - so about 25-33% potential reduction in gas use IF the waste heat is not producing useful heating.

To figure your possible energy savings, you should look at your utility statements for annual gas consumption, subtract off estimated gas usage for all other gas devices (water heater, range, clothes dryers, gas lights, etc) based on the energy labels on them - or google for typical annual consumption numbers if no label or go to manufacturer website for the energy usage info from the owners manual, to get down to what amount of gas the boiler only uses. Or if building use is similar year-around and the boiler is shut off after heating season is over, just compare heating versus non-heating season gas monthly consumption numbers (for months where only one or the other is operating, not a mixture) to see how much the boiler is contributing to the overall usage. Typically about 70-90% of usage depending on how many appliances are gas - toward lower range if gas dryer and range and barbecue and such, higher range if only boiler and water heater is gas and everything else is electric or if a colder or longer heating season area. Gas utilities commonly give data on typical percentages for their locale. Sometimes even less than 70% is building heating contribution if warmer area and furnace does not run much, if you have inordinately high hot water usage (have gym or workout room showers or such), and/or if you have separate gas heating for a sunroom or garage or pool or hot tub or such, some of which can become your major demand in warmer climate cases.

Then multiply that number of annual boiler-consumed gas units (therms, cubic feet, hundred cubic feet (ccf) or whatever unit your utility bills in), and multiply that by the gas cost per unit to get your annual gas cost for the current unit. Note you are multiplying by the gas cost only (including any taxes or regulatory fees applied to the gas usage itself), NOT including the basic monthly service charge or any regulatory fees or such which are fixed regardless of gas consumption. Utility can help sort out which are variable with gas usage and which are fixed monthly charges if the billing does not make it clear. Fixed charges do not come into the picture because they occur regardless of gas usage and almost certainly will not change with a new unit.

Then to figure the new unit cost, take its efficiency (with consideration to whether the "waste heat" is actually wasted or not) and divide it into the above number and multiply by the current efficiency. For instance, with a 95% efficient unit its gas use cost will be 60%/95% times the current usage - or 63% as much - a maximum 37% savings potential. This assuming all the "waste" heat is actually wasted - say if boiler is outdoors or in a detached utility room or unheated garage (which would be real risky with a boiler because of pipe freezing risk). Alternatively, if the radiated waste heat from the boiler is productively used when it is "live", maybe 10-20% of the wastage goes out the stack - so true heating efficiency in that case might be 100% minus 4 to 8% (total gas consumption used minus the 10-20% of the total 40% wastage which goes out stack) so you might currently have 92-96% effective efficiency - in which case even an extreme high efficiency (and very expensive) unit would save you at best a couple of percent on gas usage, and a normal high-efficiency (90-95% efficient) unit might potentially not save anything measureable at all, because the higher efficiency would basically just mean less "waste heat" heating the building and more from the more efficient burning in the new unit.

In some cases your gas utility or a state/local energy conservation program will help you with the calculation - there are also various life-cycle cost calculators on the web (try several to be sure the one you are using is not wacky) at government energy and EPA websites, gas and electric utilities, heating equipment manufacturers, and energy conservation sites that allow you to put in comparative efficiencies, local gas costs, annual gas usage now, etc and figure the annual savings and payback period for a higher efficiency unit. Jsut be aware that many at manufacturer and energy conservation organizations tend to overly optimistic because they are promoting total energy conservation (higher efficiency) even though it may not be economic. The EPA and GreenStar site tools appear to be overly biased for that reason, as are some appliance manufacturer sites.

As to cutting your utility bill by 70% - even assuming he meant cutting the gas consumption cost part only (not including fixed monthly portion, which is commonly around $30-40), not the total bill, that would require a new unit efficiency of 200% - which only a heat pump operating in a warmish winter environment (generally above 45-50 degrees) or pulling from ground or water body heat source could do, because a direct fired boiler cannot exceed about 99.5% efficiency - and that at great purchase cost. [Calculation - new efficiency, to save 70% on current gas usage cost, would mean new unit usage would be 30% of current usage - so 60%/new % = 30%, so new % has to be equal 200% - or 0.6/2.0=0.3]. Efficiency of 2 is nothing to a heat pump - the breakpoint between standard and "high" efficiency, under federal standard, is a Seasonal Heating Performance Factor or HSPF (amount of heat you get divided by amountof electricity to generate it) of 8 - so a higher-end standard efficiency heat pump (if your winter outdoor temps are fairly consistently on the warmer side) could potentially use about 0.6/8 = 0075 or 7.5% of current usage in the best case - but can also be more expensive if the auxiliary heating unit or elements have to operate much, especially if you gas or electric is expensive.

A heat pump can commonly easily do this as long as the average heating season heat source (outside air usually) averages a few degrees above the point where supplemental electric heat cuts in. Of course, if supplemental electric or gas heating will be needed with the heat pump unit (below about 40 something they can't pull signfiicant heat out of the outside air, so their air handler needs a supplemental heating unit attached to it), then the calculation becomes more complicated and requires looking at the average monthly temps and monthly gas usage, as well as the relative efficiencies of the heat pump and proposed supplemental heating source, and the relative costs of gas versus electricity. Also, heat pumps generally run around double or more a conventional furnace in initial installation cost, and maintenance costs are higher and the life is shorter by about half or so, so the life-cycle cost calculation commonly shows that unless the heat pump is primarily providing air conditioning (cooling), they commonly are not economic even over the long run in areas needing either long-term or significant cold snap heating.

In your case, a heat pump has the additional drawback that they tend to be easier to configure and install with forced air heating than with circulated water systems, and because the output heat tends to much cooler and more suitable to forced air use, you can readily get into a situation where the unit has to be way oversized to turnn out enough heat for the heating loops so the fluid stays warm enough to be effective at the end of the loop. Gets expensive and complicated in a hurry trying to do that - though also very expensive to convert to forced air, especially with large-volume buildings like traditional (high ceiling) churches, where radiated hot-water radiator or baseboard (or in-floor) heating is much more effective at concentrating the heat dzown where the people are, whereas forced air systems tend to lose a lot of their heat to the higher elevations in a building.

And particularly in a church environment, even though presumably the period over which life cycle cost is figured will be the estimated life of the unit (little or no risk of moving out part way through its life and missing out on the benefits), primary considerations may include the reliability of the unit (simple electrical-controlled not electronic, standing pilot gravity venting so about 80-85% efficiency probably most reliable, with the building presumably vacant a substantial portion of the time so a failure might not be noticed) and up-front cost (again, simple gravity vented - probably what you have now - stack to roof) without any modifications needed for installation may control the decision even if the numbers show a high-efficiency unit MIGHT be cheaper in the long run, just because money for a $10,000 or more unit (depending on church size) is plain not available.

Other factors to consider in the decision might include total heating load (heat pumps are generally not recommended for high heating load demand if a large building), whether even if you have generally low heating load you occasionally get winter storms that would demand more heat than a heat pump could put out even with reasonable supplemental heating (though supplemental gas-fired direct vent unit heaters in areas needing it might pick up that demand), and electricity reliability and whether you have a generator.

On that - areas with a lot of electrical storms tend to fry heat pumps pretty quickly because it is not as feasible to put good lightning and surge protection on them like you can on a gas fired boiler because of the difference in power load - ditto to areas with a lot of power outages, because heat pumps tend to dislike unexpected power losses (especially short-term flickers that do not allow the unit to cool down before restarting) more than boilers. Also, if you have a small emergency generator it can generally handle the boiler load easily (commonly only a few hundred watts electric demand when running), but would take a large one to run a heat pump (similar to or when heating much more than an A/C load), so if you have unreliable power that could be a major consideration.

Ditto to availability of high-quality heat pump maintenance in you are if small town or rural area - most HVAC techs can service and repair an ordinary gravity-vent gas furnace or boiler OK - a significantly smaller percentage an electronic controlled high-efficiency direct-vent furnace or boiler, and even far fewer a heat pump, so that can be a significant consideration - both from the aspect of repair/service availability and the typically much higher cost for heat pump system service and repair.

One other consideration - if the unit might be serviced or worked on by a handyman or janitor or commercial or diocese or such building maintenance staff, then the simpler the better - because inexperienced maintenance people and heat pumps tend to not mix well. Ditto if a clergy member might be the primary person doing any adjustments or use of it.

Guess you can guess my bent by this time - that unless you are in a very high gas cost area, have very cheap and reliable electricity, or have really cold conditions; a normal 80's % efficiency boiler is likely to work fine and be cost effective - though you would have to run through the numbers to see what specific brand and efficiency lakes sense for you if you are looking at replacing a currently functional unit just for energy savings, or if considering several different efficiencies for replacing a dead unit.

PRofessionally, if this is a large unit (so you have gas bills of many hundreds of $ a month) it might pay to have an Energy Efficiency expert (Energy Efficiency Auditing is the Search the List category) help with the assessment. IT is also VERY commonly true that going with a standard efficiency economic and simple heating unit and spending some limited amount of money attacking the primary heat loss locations can be far more $ efficient than a more efficient heating or cooling system. I have seen commercial buildings (which a church would be considered) where one might have gained 10-25% efficiency with a very expensive very high efficiency HVAC system cost a LOT of bucks up front, but actually gained that much or more at a fraction of the cost by judicious insulation and air gap sealing - in many cases just a few hundreds of $ of air gap sealing, local insulation on very poorly insulated spots, and weatherstripping can equal the savings of many thousands in greater HVAC system efficiency.

You can find a number of previous similar questions about furnace/boiler replacement and the trade-offs between high-efficiency and standard efficiency units in the Home > HVAC link in Browse Projects, at lower left.

Answered 3 years ago by LCD

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