Plumber69
In the Trades
Replaced an old belt driven 110btu furnace with a high efficiency 60btu. Small house. How much savings are expected
New Furnace Savings?
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Gas burner, propane, or oil?
What was the input & output BTU ratings on the old furnace?
What is the input & output BTU of the new furnace? A "high efficiency" furnace is anything 90% (output divided by input) and up efficiency, but there's a significant difference between 90% and 98%.
Assuming the old one was rated 78% and had degraded to 75% with corrosion on the heat exchanger or dirty burners, and the new one is 95% it only takes 75/95 the amount of fuel (about 80%)to deliver the same amount of heat to the ducts, a 21% savings. If the cfm of the blower is significantly lower the duct leakage would be lower too, with somewhat lower distribution losses due to both lower pressure and lower temperature air.
The efficiency of the blower motor may be higher too, saving on power use despite a higher duty cycle/longer run times.
Unlike hydronic boilers, low mass hot air furnaces don't take an efficiency hit from being oversized until/unless the oversizing factor is truly huge and the thing is short cycling like crazy.
So depending on the steady state efficiency numbers and other factors it could be anywhere from 10% to 25% fuel savings, probably not 30%, and definitely more than 5%.
What was the input & output BTU ratings on the old furnace?
What is the input & output BTU of the new furnace? A "high efficiency" furnace is anything 90% (output divided by input) and up efficiency, but there's a significant difference between 90% and 98%.
Assuming the old one was rated 78% and had degraded to 75% with corrosion on the heat exchanger or dirty burners, and the new one is 95% it only takes 75/95 the amount of fuel (about 80%)to deliver the same amount of heat to the ducts, a 21% savings. If the cfm of the blower is significantly lower the duct leakage would be lower too, with somewhat lower distribution losses due to both lower pressure and lower temperature air.
The efficiency of the blower motor may be higher too, saving on power use despite a higher duty cycle/longer run times.
Unlike hydronic boilers, low mass hot air furnaces don't take an efficiency hit from being oversized until/unless the oversizing factor is truly huge and the thing is short cycling like crazy.
So depending on the steady state efficiency numbers and other factors it could be anywhere from 10% to 25% fuel savings, probably not 30%, and definitely more than 5%.
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Plumber69
In the Trades
natural gas. 10% isnt very much. furnace was from 1980ish
One fed with outside combustion air would be more efficient. Inside combustion air sucks cold air in from outside at the cracks.
If the old furnace efficiency had fallen to 56% (probably has not), improvement would be better.
Most furnaces of that era where 80% steady-state efficiency, but with a standing pilot ignition only 78% AFUE. The degradation to 75% efficiency estimate I used was probably realistic.
Care to share the specs on the new furnace?
If it's 95% steady state you'd be looking at about a 20% reduction in fuel use.
If it's only 90% it would still be a 16-17% reduction.
It would only be as low as 10% if the initial steady-state efficiency of the old furnace was in the mid-80s.
The hit from infiltration by using conditioned space air for combustion rather than ducted air is "in the noise". The total cfm of the intake air even with an atmospheric drafted 110K furnace with 30% dilution on the combustion air plus dilution of the exhaust with indoor air at the draft hood is still pretty small. Air handler driven room-to-room pressure difference infiltration and duct losses will always add up to a bigger number than the combustion air + exhaust dilution air.
If the old furnace didn't have a flue damper so that the flue was pulling air into the dilution hood even when the burner wasn't firing that could add up to several percent on total fuel use, but not 10. Most heating appliances had automatic flue dampers by 1980, but not all.
In looking over some of the models in this study, it would seem that some furnaces of the 1980s had nameplate AFUE efficiencies well below 78%. The Lennox G12QE382-10 was no better than 72.5%, the Rheem RGOA100CER was a mangey 3-legged dog with fleas at 65%(!), and the Trane TUS060A936AO wasn't much better at 68.4% (really!?!).
The in-situ efficiency testing showed most of them operating at below spec, largely due to higher temperature/lower air flow than spec. See Tables 20 & 21.
If the original ducts were undersized or marginal for the 110KBTU beast they are probably still going to be amply sized for the 60KBTU high efficiency unit that replaced it. If that's the case the actual savings may beat the name-plate efficiency ratios, because the older version never ran at it's stated efficiency due to duct conditions. If the original ducts were right-sized for 110K furnace it was probably operating at or near it's nameplate efficiency, and the improvement would track the nameplate efficiency ratio.
The in-situ efficiency testing showed most of them operating at below spec, largely due to higher temperature/lower air flow than spec. See Tables 20 & 21.
If the original ducts were undersized or marginal for the 110KBTU beast they are probably still going to be amply sized for the 60KBTU high efficiency unit that replaced it. If that's the case the actual savings may beat the name-plate efficiency ratios, because the older version never ran at it's stated efficiency due to duct conditions. If the original ducts were right-sized for 110K furnace it was probably operating at or near it's nameplate efficiency, and the improvement would track the nameplate efficiency ratio.
