Cost of AC/Furnace - what is best time to replace?

[Beets]

My 20 year old furnace works - I'm thinking about replacing it with a furnace/AC because I would like AC.

Is there any time of the year that is any better than another to be looking for furnace/AC unit? I'm thinking of costs.

Second question: has COVID or the very hot summer we just had driven up costs? I can wait, so thought I would ask.

 

[Breplum]

Not really a best time.
FYI, a friend nearby has bids on a replacement system, including $3k USD for asbestos abatement, and all new ducts for a 2k sq.ft. home.
Never had A/C before so there is electrical needed. Total cost average from four different contractors: $25k USD. That is for two stage, 96% AFUE furnace, single stage A/C.
Note: We are in a very high cost real estate market here.

 

[Sponsor]

 

[Beets]

Yikes! That's crazy!

 

[WorthFlorida]

Yes SF, Chicago, NYC are notoriously have higher cost than most of the USA but it can still get $$. What type of furnace do you have? If you have a forced air system your 1/2 there. If you have a hydronic system, then what Breplum stated a lot of items must get checked off.

If you have a forced air system the duct work may be too small for AC. It depends when the home was built. New HE AC units get some of the SEER numbers by moving more air at a slower rate than 30-40 years ago. With a forced air heating system the AC coil is placed on top of the furnace. If you have gas heat air handler it is mostly self contained with both the heating plenum and the AC coil. There is no single way to go or a simple swap out.

In Florida, replacing an AC unit, a new air handler and the outside condenser using the same ductwork and copper lines it's about $5,000 for a 3.5 ton unit. If you want AC only, mini split systems maybe more reasonable in cost since no duct work is needed. Mini splits are also heat pumps so you get a more efficient heat source for your heating season and may have a faster payback time. One advantage is if the main furnace goes out in the middle of winter, you'll still have heat.

 

[Fitter30]

Don't be surprised that there is a shortage of equipment just like appliances. Best to buy fall not cold enough for emergency furnace replacement not hot enough for ac.

 

[Reach4]

My 20 year old furnace works - I'm thinking about replacing it with a furnace/AC because I would like AC.

Is there any time of the year that is any better than another to be looking for furnace/AC unit? I'm thinking of costs.

Second question: has COVID or the very hot summer we just had driven up costs? I can wait, so thought I would ask.

COVID-driven shutdowns.

For costs, consider one or more mini-splits for AC together with the current heating system. I would get one that does heat too, for flexibility.

 

[Dana]

My 20 year old furnace works - I'm thinking about replacing it with a furnace/AC because I would like AC.

Is there any time of the year that is any better than another to be looking for furnace/AC unit? I'm thinking of costs.

Second question: has COVID or the very hot summer we just had driven up costs? I can wait, so thought I would ask.

There have been supply chain problems for lots of HVAC equipment due to COVID, so even after you've selected equipment that is a good fit for your loads you may STILL have to wait, unless you're willing to compromise and take what's available. There has also been labor shortages combined with a construction boomlet, which has driven quoted prices up even for "plan-B" equipment selections (at least in my neighborhood. YMMV.) That said, if you can spec the equipment and put it out for competitive bid, and not all contractors have added another decimal place to the quotes, and not all contractors have the same overhead cost to cover. On one project I specified earlier in the summer out of 4 proposals the most expensive bid came in at slightly more than 3x the lowest bid, which was priced only slightly above pre-COVID expectations.

Since you have time to wait you also have time to assess your cooling and heating loads carefully. Odds are high that your existing equipment is more than 1.5x oversize or even more than 2x, which pushes it out somewhat beyond the best-comfort cliff. While multi-stage and modulating furnaces can deal with some of that, modulation ranges aren't infinite, and it doesn't take much thumb on the scale with a 2-stater to have even the LOW fire stage exceed the required capacity for the 99% load.

Picking right-sized equipment is necessary for optimal comfort & efficiency. For the necessary heating capacity it's easy to use your existing system to MEASURE the 99% design load with reasonable precision using wintertime fuel use and very-local heating degree-day data, then apply an appropriate oversize factor. For determining the 1% cooling load using a Manual-J type calculator (or two, if using online freebie calculators, which aren't always very accurate without some massaging) should at least get you in the right ball park. Comparing the calculator's estimated heating load against the measure fuel-use load is one way to assess whether you've use sufficiently aggressive input assumptions. Usually the calculator's estimate will exceed the fuel use derived number by more than 10%, which is fine. But they differ by more than 20% it's worth revisiting what parameters were entered into the calculator.

 

[Dana]

If you have a forced air system the duct work may be too small for AC. It depends when the home was built. New HE AC units get some of the SEER numbers by moving more air at a slower rate than 30-40 years ago. With a forced air heating system the AC coil is placed on top of the furnace. If you have gas heat air handler it is mostly self contained with both the heating plenum and the AC coil. There is no single way to go or a simple swap out.

In Florida, replacing an AC unit, a new air handler and the outside condenser using the same ductwork and copper lines it's about $5,000 for a 3.5 ton unit. If you want AC only, mini split systems maybe more reasonable in cost since no duct work is needed. Mini splits are also heat pumps so you get a more efficient heat source for your heating season and may have a faster payback time. One advantage is if the main furnace goes out in the middle of winter, you'll still have heat.

In Manitoba the latent cooling loads are low, and the sensible loads aren't insane, so the "ton per 750 feet of floor area" or whatever rule-of-thumb approaches commonly used in the southeastern US are out the window. I see square foot/cooling load numbers in relatively balmy (compared to most of Mantitoba) maritime Maine come in at a ton per 2000' or more every time. Unless there is a panoramic west facing floor to ceiling window the sensible loads in the Canadian midwest aren't that bad either, but it's important to run the numbers.

It's also painfully common to find 150,000 BTU/hr furnaces serving 60,000 BTU/hr design loads in cold climates, with similarly upsized ducts. If that turns out to be the case here a right-sized low cfm multi-stage furnace may work just fine with the right-sized AC for that house. But without some real load numbers and a better description of the ducts we're just guessing.

Of course right sized mini-splits might be the easiest and most economical approach. Mini-splits don't have infinite modulation range either (though most are pretty good), so right sizing is still important.

 

[Beets]

Thank you. House is forced air, natural gas furnace. Alberta. House is 20 years old. Weather pretty similar to Manitoba which Dana mentioned. House is pretty decently insulated. 2000 ft^2 up and 2000 ft^2 down. Walkout bungalow, with almost no windows that get direct sun. Neighbors are always impressed with how cool my house is despite not having AC.

Sounds like I need to get some quotes. Thank you.

 

[Dana]

Thank you. House is forced air, natural gas furnace. Alberta. House is 20 years old. Weather pretty similar to Manitoba which Dana mentioned. House is pretty decently insulated. 2000 ft^2 up and 2000 ft^2 down. Walkout bungalow, with almost no windows that get direct sun. Neighbors are always impressed with how cool my house is despite not having AC.

Sounds like I need to get some quotes. Thank you.

That's a 2000' walk-out basement ~1/2 of which is below grade, under another ~2000' fully above grade? Odds are pretty good that only the upper floor would need any AC, and it's likely to come in at about 1-1.5 tons of cooling, not 2. There are not many central air systems that go lower than 2 tons (24,000 BTU/hr at AHRI rating conditions), so even the smallest in class central air might be suboptimally oversized.

There are ducted mini-split heat pumps that go smaller, but they are not designed for and don't don't tend to mate well with typical gas furnaces. The (Canadian vendor) Dettson's Chinook series modulating gas furnaces' air handlers are designed to work with their modulating Alize mini-split heat pump compressors, and they may have a reasonable pairing for the loads at your house. Dettson installations are rare (all but unheard of) in my USA neighborhood, but a contractor familiar with their lineup is perhaps easier to find in yours. The model sizing steps & modulation ranges of the Chinook make them much easier to right-size than typical 1-2 stage gas furnaces from other vendors, especially at the lower heating loads of 21st century construction home. But there are other potential solutions to consider once we know your 99% & 1% design loads.

Unlike Manitoba or Maine in Alberta the latent cooling (humidity) loads are uniformly negative, so VERY high SEER equipment would work even under light cooling loads without humidity levels building up in the house. In that climate ventilation alone is enough to manage indoor humidity sources such as breathing/bathing/cooking.

What is the 99% outside design temperature and 1% design temp at that location, approximately? Alberta's listed design temps start on p.32 (p38 in PDF pagination). (Typical 99% temps in much of AB is -15F/-26C, 1% temps ~80F/+27F. I know it gets at lot colder and a lot warmer than that at the annual extremes- those are just the 99th and 1st percentile hourly temperature bins, meaning only 1% of all hours in a year (or about 87 hours/year) are colder or warmer than those numbers. Calculating the heat load at the 99% temperature and applying ASHRAE's recommended 1.4x oversize factor covers the load all but the 25 year temperature extremes (which don't linger for days or even many hours), and a 1.2x oversize factor for cooling would be plenty, (unless you're planning on having a dozen guests over for an indoor dance party on the hottest afternoon of the year.) A typical ~30-32 SEER /13-3HSPF high wall coil type 1 ton mini-split (tested & rated at 12,000 BTU/hr at 95F outside) can deliver about 15,000 BTU/hr of cooling at AB type design temps.

 

[Beets]

My natural gas bill notes how many GJ of energy I used over each billing period. I've converted that to BTU/h and plotted that below:

If I understand correctly, we want to know how many BTU/h my furnace consumes at -26C? To estimate that, I downloaded the thermostat data which tracks indoor and outdoor temperature. When we hit -26C, the temperature difference (dT) would be 47C because indoor temperature is around 21C. None of the average dT's were that low during any of the billing periods. See plot below.

To estimate energy consumption at -26C (or a DT of 47), I created a cross plot with data from the above two plots. I then drew a straight line extrapolation to see what energy usage would be at DT of 47:

At a DT of 47C, I would expect the furnace to use about 50,000 BTU/h. If I multiply that by 1.4, then a furnace of 70,000 BTU/h would be good.

BUT, my old furnace only had a 80% efficiency and I'm replacing it with something that is 95% efficient. Thus, I could go down to 60,000 BTU/h. Does that sound right?

Several questions/concerns. On the cross plot, the data point at 40,000 BTU/h looks to be an outlier. However, it happened during the coldest month we had this year, so I don't feel good about ignoring that data point. The other thing I noted is that my thermostat tracks the amount of time the flame is on. I have a single stage furnace with input of 99,000 BTU/h. I figured I could multiply this by the number of hours the furnace is on (according to my thermostat) and that it would line up with the gas bill. It doesn't. Gas bill says I'm using 50% more gas that that calculation. Does it make sense that the input to my furnace could be 50% higher than name plate? What do you do when there is uncertainty? Size up?

 

[Beets]

I was thinking about this further. We had a cold snap in February 2021. I plotted some data below. What you can see is that during the day I kept the house at 23.5 C and at night I dropped to 20C (blue line). Outside temperature is also plotted in red. The lower panel shows how many minutes per hour the furnace was on. What you see is that when the set point changed from 20 to 23.5C, it took around 2.5 hours for the furnace to get the temperature up there. At around the -25 to -27C range, the furnace could hold that temperature running about 25% in steady state. Can we tell anything about how right (or wrong) sized the furnace is from this chart? It can get to -40C on rare occasions here.

 

[WorthFlorida]

It's good data but I'm not sure if it can determine sizing since there are many factors that determines it. When you can, do the same data collection with a steady temperature setting. You may find by lowering the thermostat and up again that it may not or may not be saving energy. For a long run such as a few days or a week not being home, etc. will save energy, but a daily changed can be up for debate. The 2.5 hours getting up to temperature not only heats the air but all the furnishings, walls and floors takes heat.

 

[Beets]

Thank you, I don't have any data with constant temperature, but I agree the energy savings aren't likely as high as one would think. I do like cooler house at night though.

I'm a bit surprised there hasn't been more chatter about the charts/data. I suspect it is pretty rare for someone to have actual data, so perhaps I have thrown everyone for a loop

 

[Dana]

My natural gas bill notes how many GJ of energy I used over each billing period. I've converted that to BTU/h and plotted that below:

It's not clear if that is the instantaneous peak BTU/hr at some indoor & outdoor temperature (probably not) which is what we are looking for, or if it's the average over the number of hours in a month(?). Without accurate average heating degree-day temperature data an average use per month isn't going to tell us much.

If I understand correctly, we want to know how many BTU/h my furnace consumes at -26C? To estimate that, I downloaded the thermostat data which tracks indoor and outdoor temperature. When we hit -26C, the temperature difference (dT) would be 47C because indoor temperature is around 21C. None of the average dT's were that low during any of the billing periods.

The BTU/hr at 21C indoors, -26C outdoors is the relevant number. And it has to be scaled by the efficiency of the unit, not the input BTUs, since it's the net output that went into the heating of the house- the rest went up the flue. Eg, a burn rate of 50,000 BTU/hr into an 80% furnace only delivers 0.8 x 50K= 40,000 BTU/hr into the house, and if using ASHRAE's 1.4x scaling factor the right size would be a furnace with 1.4 x 40,000= 56,000 BTU/hr.

To estimate energy consumption at -26C (or a DT of 47), I created a cross plot with data from the above two plots. I then drew a straight line extrapolation to see what energy usage would be at DT of 47:

At a DT of 47C, I would expect the furnace to use about 50,000 BTU/h. If I multiply that by 1.4, then a furnace of 70,000 BTU/h would be good.

BUT, my old furnace only had a 80% efficiency and I'm replacing it with something that is 95% efficient. Thus, I could go down to 60,000 BTU/h. Does that sound right?

Again it's the OUTPUT BTU/hr, not input BTU/hr that's relevant. So scaling a 40K design load by to 56K means a 95% efficiency x 60K-in (= 57K out) furnace would be about right.

Several questions/concerns. On the cross plot, the data point at 40,000 BTU/h looks to be an outlier. However, it happened during the coldest month we had this year, so I don't feel good about ignoring that data point. The other thing I noted is that my thermostat tracks the amount of time the flame is on. I have a single stage furnace with input of 99,000 BTU/h. I figured I could multiply this by the number of hours the furnace is on (according to my thermostat) and that it would line up with the gas bill. It doesn't. Gas bill says I'm using 50% more gas that that calculation. Does it make sense that the input to my furnace could be 50% higher than name plate? What do you do when there is uncertainty? Size up?

That's why you need to run the math on gas use against heating degree-day data from a nearby weather station during the coldest months only (to reduce the distortions of hot water use and passive solar gains). If you keep it ~21C indoors use base 18C HDD weather data. You will find that the BTU per HDD numbers will fall in a fairly narrow range when using colder month only data. Converting BTU/degree-day to BTU per degree-hour is simple arithmetic (divide by 24), which is a linear constant. The zero point is the HDD base (I'm recommending using 18C based on your indoor design temp), since that is the presumptive point where internal gains from mammalian & avian occupants and background energy use (water heater standby, refrigerators, other 24/7 electrical plug loads) keep the place at a fairly constant temperature without direct heating. So you're extrapolating from a linear rate- the heat that needs to be supplied by the furnace increases by your derived constant with every degree below 18C (not the full difference between indoor outdoor temperatures).

While the performance of the building envelope isn't perfectly linear with delta-T, it's close enough. The performance of insulation increases with higher delta-Ts, the performance of windows falls, but within the temperature range of interest it's going to close enough for sizing the equipment. Only very-high performance houses would call for better thermal modeling to be used.

I'm a bit surprised there hasn't been more chatter about the charts/data. I suspect it is pretty rare for someone to have actual data, so perhaps I have thrown everyone for a loop

The loop I've been on is more about binge watching NetFlix on the couch with the dog waiting for the pandemic to subside.

The shorter term thermostat duty cycle and temperature information is useful, but usually not quite as good as accurate as longer term averages. Still it looks pretty reasonable. Looking at ONLY the duty cycle sections after the house has cooled off to a stable 20C the average outdoor temp is about -28C, and the duty cycle is about 25 minutes/hr = 42%.

The output of an 80% 99KBTU/hr furnace is 79,000 BTU/hr, so a 42% duty cycle means it's using 0.42 x 79K= ~33,200 BTU/hr. With a presumptive balance point of 18C and an outdoor temp of 4C that's 18C- -28C=46C heating degrees and a heating constant of 32,200/46C= 700 BTU/degree-hour indicating a pretty tight house- that's pretty good for a 2x6 framed walkout- I would have expected somewhat more (but not 2x).

So at a design temp of -26C outdoors you would be at (18C - -26C= ) 44 heating degrees, for an implied load at that temperature of 44 x 700 BTU/hr= 30,800BTU/hr. Scaling that by 1.4 (ASHRAE) puts the right size furnace output at 1.4 x 30,800= 43,120 BTU/hr. That means a 50K condensing furnace(x 95% efficiency= 47.5K out) is about right but at (47.5K/30.8K=) 1.67x still slightly oversized, thus 2-stage or fully modulating would be recommended.

Try this method, to gain better psychological comfort and (usually) better accuracy. It's conceivable that it may turn out that 40K furnace would be a better fit, if the longer term analysis shows slightly lower design load numbers.

 

[Beets]

I sure thought I had responded, but I don't see my response. Thank you for all the calculations. If I had taken my estimate of 50,000 btuh/h and multiplied it by the 0.8 efficiency, I would have the same answer as you.

I've read the article at the other website. I would like to understand why a smaller system is more comfortable? When it gets below -20C, I do end up with cold areas in the house. So I get that a furnace running 80% of the time is better than one running 40% of the time for keeping temperature more uniform. However, when it gets below -20C, I run the fan 100% of the time to ensure that I have enough air movement to keep condensation and ice from building up on the windows. Thus, I don't understand how a smaller furnace helps with comfort. I know that constant cycling of a furnace is not good, so smaller helps therein. Mainly curious about the comfort improvements.

Would a larger furnace have a better blower? Maybe that is more important for keeping windows clear of condensation? And yes, dropping humidity helps as well.

Thanks again!

 

[Dana]

I sure thought I had responded, but I don't see my response. Thank you for all the calculations. If I had taken my estimate of 50,000 btuh/h and multiplied it by the 0.8 efficiency, I would have the same answer as you.

What, arithmetic in Canada works the same as in the states? Who knew!

I've read the article at the other website. I would like to understand why a smaller system is more comfortable? When it gets below -20C, I do end up with cold areas in the house. So I get that a furnace running 80% of the time is better than one running 40% of the time for keeping temperature more uniform. However, when it gets below -20C, I run the fan 100% of the time to ensure that I have enough air movement to keep condensation and ice from building up on the windows. Thus, I don't understand how a smaller furnace helps with comfort.

Running 20-22C room temperature air around during the "off" cycles doesn't move much actual heat to the cold rooms that have dropped to 18C, and it generates a wind-chill/draft effect that is uncomfortable to humans. With a condensing burner it would be delivering 45-50C air 80% of the time, which is much more pleasant. Most 80% burners would be delivering ~60C air when running, so at a 40% duty cycle it's a hot blast followed by an extended chill.

Also, at a 40% duty cycle if the on cycles are relatively short (<< 10 minutes) the rooms at the ends of the duct runs may only be getting full temp air out of the registers for a small fraction of the cycle, since it has to both purge the tepid-air volume of the duct, and raise the temperature of the interior walls of the duct before reaching a steady state temperature. When running an 80% duty cycle that's not an issue- the duct walls and static air in the duct stay much closer to the steady state temp between cycles.

Would a larger furnace have a better blower? Maybe that is more important for keeping windows clear of condensation? And yes, dropping humidity helps as well.

Thanks again!

A larger furnace delivers more cfm, but if it's running a shorter time it isn't helping, just making more noise while delivering more wind-chill due to the higher exit air velocity.

A smaller furnace does more for reducing window condensation than a larger furnace. The larger furnace makes it worse, due to the lower duty cycle. Usually the registers are under (or blowing toward) the windows. Blowing warm air toward the windows on an 80% or higher duty cycle raises the surface temperature far more than blowing warm air at it 40% of the time (or blowing 23C air for 60% of the time) can.

It's not at all intuitive, but bigger is the OPPOSITE of better except during the coldest cold snap of the century (which may or may not occur during the lifecyle of the equipment.) A 1.4x oversize factor is enough.

If you have access to the numerical data behind those graphs over many winter nights you should be able to crunch the numbers to a fairly high accuracy on both the balance point temperture (which should be used as your heating degree-day base), and the linear BTU/degree-hour constants. (Degreedays.net used to have a downloadable widget somewhere on their site for figuring that out as well.)

 

[Beets]

Thank you for the explanation. It makes sense!