Please help me decide between traditional or mod-con replacement boiler

Coldax

Hi all, I’m a homeowner seeking advice on whether to purchase a replacement ‘traditional’ boiler or a high-efficiency for my home in New Jersey.

It’s a contemporary home built in the early 80’s and is 3400 SF, 4 bedrooms and 2 full baths. There are about 90 windows and the large living room and kitchen have two story ceilings. The current boiler is a propane Utica M-300-AGB 21 rated at 300,000 BTUH input, and 236,000 D.O.E. heating capacity. This serves 3 zones with a total of 287 linear feet of exposed finned baseboard. This boiler was installed in the late 80’s as part of an oil-to-propane conversion.

I have lived in the home for 4.5 years and I average using 1800 gallons of propane each year, and I keep it on the cool side in the winter (to everyone in the family’s dismay) in order to save some money.

DHW is provided by a 75 gallon propane Bradford White that is about 7 years old. I have not experienced any DHW issues so far, and when I turn off the boiler after the heating season has ended the use of propane drops considerably.

It is obvious even to me that the boiler is too big for the application, outdated and wasteful. So I have been doing my research and so far I have had about 10 different local contractors come in for estimates. Almost every one has suggested a ‘mod-con’ replacement of the Utica and the Bradford, sealing up the flue and venting to the exterior (which I don’t really want to do for aesthetic reasons). The quotes for mod-cons have exceeded my budget by a large margin. I also understand they may have yearly maintenance costs.

One vendor’s suggestions was refreshing – he said keep the Bradford because it works, was expensive to install, has life left, and is not wasteful. He suggested I replace the Utica with a ‘traditional’ boiler, a Buderus Logano G234X/55 and keep the flue. This was more in-line with my budget. I thought that made sense so I received a similar style quote for Burnham x205, 175,000 BTU. These are about 84% efficiency.

The last vendor come listened to my request for a traditional but suggested I go with a Navien NCB 240 at 94% which he could vent out the existing flue using PVC. He was about 40% higher. I am skeptical that the Navien can provide for all the heat and DHW water needs of the house and family. I also need to test my water for hardness, and would need to install an acid-reducer so as not to destroy my pipes.

I like the concept and elegance of the Navien but I feel that the Buderus or Burnham will be more reliable, and the 10% efficiency difference will take many years to recoup and could even be lost due to maintenance and shorter life span.

After having so many vendors in I like these three the best. They are not salesmen, but rather the owners of each company and would oversee the installs. They are local and have been in business for a long time with great reputations to uphold.

I would appreciate your opinions on my choices and let me know if further information is needed.
Thanks, Coldax

Jamie Hall

The very first step in deciding is to figure out the heat loss of the house. The very first step. Your contractors should be able to do that for you -- but you could also download the Slant-Fin calculator and try it. You'd probably be at least close.

Then the second step is to figure out what temperature the existing baseboard would have to run at to deliver that much heat. Not that it would have to run that hot all year, but you'd be able to get a feel for how much of the year it would have to run at various temperatures.

This is important, because a mod/con boiler -- which, as you have observed, is more expensive and takes more maintenance -- will only give you the higher efficiency so cheerfully advertised if the return temperature from your radiation is less than 140 degree F. Anything higher than that, and it's no more efficient than a conventional boiler -- such as the Buderus or Burnham, both of which are built like tanks and will last a long long time with any sort of care and routine maintenance.

I would definitely keep the separate water heater. Combination units have some attractive points, but it's remarkably difficult to get one which is properly sized for both the domestic hot water and the heating loads.

I have to admit that I prefer simplicity -- and so I'd go conventional and not worry about it.

EBEBRATT-Ed

Others will differ from my view I am sure. I vote for a traditional boiler. The MD CONs are great but expensive. The savings in operation in my view is due to MOD CON modulating the flame to match the heating needs, reduced water content which reduces jacket loss and taking combustion air from outside rather than inside the building. Mod cons need service like anything else but the repair parts are perpriatory and cost $$$$

First thing is an accurate heat loss of your home. Your old boiler is oversized for the amount of baseboard you have. The 175,000 you mentioned matches the baseboard.

But measuring the baseboard is not the correct sizing method with hot water...a heat loss should be done. 175,000 btus for a decently built 1980s house seems like you shouldn't need that much.

None of the mod cons are going to give you the advertised 94, 97 whatever % when running baseboard.

At least you have whittled it down to 3 decent contractors. Get something with decent controls and oa reset

Gordy

I really think you answered your own question.

A budget is a budget get what you can afford. Stay traditional, and get a room by room heat loss done, and an emitter survey room by room.

In knowing whether each room has more than enough emitter in your case fin tube, you can still lower supply temps some what with a traditional boiler coupled with outdoor reset. Providing some system efficiency gain, and more comfort with proper boiler return water protection. You may not need 180 degree water all season.

I would have them stub out a future indirect zone for when the water heater may die so you can do DHW off the boiler with a indirect.


If your heatloss were 50 btus a square foot (very doubtful) that would be 170k output boiler.

How many zones is the system now?

JohnNY

You had 10 contractors estimate your job? What were you looking for that you didn't get after speaking to 3 or 4 of them?
Just curious.

Gordy

This is a fin tube output chart. Depending on your present flow rate, and AWT. With 287 feet of fin tube you are not putting more than 150k into your home if the AWT is 170 degrees. You are running three zones. Do they ever all call at the same time?

150000 btu /3400 sf = 44 btus sf. Still a very high load for 80’s construction.

With 90 windows keep in mind insulating window treatments can really enhance in reducing heat loads.

Robert O'Brien

Given the cost disadvantage of propane vs natural gas, I would lean toward the mod/con. A substantial portion of the savings is in the modulation part rather than the condensing part. If you don't condense for a portion of the season,so what? You'll condense for most of it.

http://www.fcxalaska.com/PDFs/BrookhavenBaseBoard.pdf

Gordy

I guess when I read the initial post the struggle seems to be initial cost, the cost of maintenance, and ROI for deciding on using a Mod/Con. When I see this I fear that the routine maintenance gets put off by the owner, and the boiler finds a short life span.

We have read of many claims here of switching to a mod/con saving 30% on fuel minimum. I’m subjective to the claims as it depends on how the 30% savings was achieved. Was it partly because the right size boiler was finally installed? Was part of it modulation, was part of it system changes in pumps, and piping?, was part of it ODR?, and the last part condensing?

Most of that comes from the complete package a Mod/con offers in its controls which are far more superior to a typical CI boiler. The rest comes from a compitent installer that understands the technology the boiler offers, and uses it to its full capability. This includes the system piping practices.

If the OP’s budget allows for, and they are fixed on wanting a mod/con by all means let’s get them there.

The OP must also remember that the ROI is only on the difference in price between a CI boiler, and a Mod/Con boiler not the total cost since they are getting a new boiler one way or the other. It would be different if the Client has a functioning boiler, and were contemplating switching to a Mod/Con for the sole reason of saving money on fuel consumption over their present boiler.

Gordy

The most efficient boiler is one that is off. To do that the envelope needs attention where high heat losses are.

From there it’s matching the boiler to the load with a proper sized boiler. In a perfect world with a modulating condensing boiler with a low enough low end modulation, proper zoning size, and proper emitter sizing it would never shut off. It would throttle the fire, and thus water temps to the system to perfectly match the heat load with outdoor temps all heating season.

Coldax

Jamie Hall said:

The very first step in deciding is to figure out the heat loss of the house. The very first step. Your contractors should be able to do that for you -- but you could also download the Slant-Fin calculator and try it. You'd probably be at least close.

OK I have the app and am working on it. But no contractor so far has offered to do one. They are all coming in around the 175K BTU figure more or less.

Coldax

Gordy said:

I would have them stub out a future indirect zone for when the water heater may die so you can do DHW off the boiler with a indirect.

How many zones is the system now?

So you are saying that either Burnham or Buderus can create DHW? There are 3 zones.

Gordy

Yes. Any boiler can using an indirect water heater.

Gordy

Coldax said:

Jamie Hall said:

The very first step in deciding is to figure out the heat loss of the house. The very first step. Your contractors should be able to do that for you -- but you could also download the Slant-Fin calculator and try it. You'd probably be at least close.

OK I have the app and am working on it. But no contractor so far has offered to do one. They are all coming in around the 175K BTU figure more or less.

Be sure to use the correct design day temperature for your location.

Coldax

JohnNY said:

You had 10 contractors estimate your job? What were you looking for that you didn't get after speaking to 3 or 4 of them?
Just curious.

Well for one thing, an actual quote. Two vendors never bothered to provide me with a quote, and I called them back each twice to ask for it! What's up with that?

My first quote was for Energy Kinetics - they are made a few miles from my home - but that is running double the cost of the Burnham/Buderus. I really like that system.

Some vendors are clearly trying to rip me off (shocking, right?) by overcharging for lower end units. Salesmen on commission. Don't they realize I can easily find out the retail pricing and do basic math? I'm getting better at picking them now, going local with long histories and master plumbers, etc. I am also learning more and asking better questions. I'll bring in another 5 if I have to, just to learn more and view other manufacturers.

Coldax

Gordy said:

This is a fin tube output chart. Depending on your present flow rate, and AWT. With 287 feet of fin tube you are not putting more than 150k into your home if the AWT is 170 degrees. You are running three zones. Do they ever all call at the same time?

150000 btu /3400 sf = 44 btus sf. Still a very high load for 80’s construction.

With 90 windows keep in mind insulating window treatments can really enhance in reducing heat loads.

Yes they often call at the same time, also DHW. Most of the windows/tall ceilings are in the living area and kitchen. Just by design there are no window treatments there, it would ruin the look. This home was built by a home builder, many windows are custom sized and cut, and there are walls of windows. The bedrooms are more traditional and have treatments, but the windows are tall. The kitchen has an island with 2 toe kick blowers and that really helps in that room.

EBEBRATT-Ed

@coldax, Unfortunately most contractors will not do a heat loss unless pushed into it and some have no clue how to do one. Why would they spend more time estimating the job when they can get the job without doing a heat loss? That is how they think.That doesn't mean you don't need one. The numbers do not lie. Why by more boiler than you need??

Your oversized now with 300k input and I would be pretty sure you don't need 175K that your baseboard shows.

the fact that you know how any feet of baseboard you have tells me that's how a contractor sized the boiler. At least that's better than replacing size for size but still not right.

Gordy

I don’t disagree that it is a daunting task picking the right boiler, and a competent installer. Especially for a homeowner who knows little about what is involved with the whole process to get a desirable outcome.

Price isn’t always an indicator of you get what you pay for as far as the installation process.

Feel free to keep asking questions!

kcopp

What are you paying for LP gas?
What is the cost of oil there?
Would oil be an option if you already have a chimney?

HomerJSmith

I like Gordy's responses, he's so sensible.

This is the way I look at it. A whole house and zone by zone heat calc would be the way to go. It would give you some idea of your needs.

The questions I would ask is do you have enough heat emitters to satisfy the heat loss that you will be experiencing in the dead of winter, at what water temperature, and if glycol is going to be used.

If you have a ModCon running at 180 deg to satisfy your heat loss , you have gained nothing and a conventional boiler would be the better choice. But, if you have abundant baseboard that can run at a lower water temperature go for the ModCon. A ModCon that can modulate will save big buckos every year.

As a personal example, I have a 3200' house in an area that gets very cold in the winter. We can get as much as 20' of snow, thankfully that happens only rarely. But, when I built my house, I had the foresight to install an extravagant amount of baseboard.

When I switched over to a ModCon from a conventional boiler, I had enough baseboard to run at a lower water temperature.

I have a Munchkin 140M that is derated 15% because of altitude and it does space heating and domestic hot water. I cut my heating costs in half, from $300 to $160 a month.

My advice is to find out what your heat loss is and see if the baseboard is abundant enough to run at a lower water temperature. I calculate baseboard at 450 Btu/ft at 180 deg, not at the Lab rating of 500 Btu's. If it is, then, a ModCon would be my choice even tho it's not a perfect match.

As far as money goes, you pay up front for a ModCon or you pay later in increased fuel cost with a conventional boiler. With a conventional boiler that cost is every year that that boiler is in operation. PS, I see the cost of boilers and labor going up in the next few years, so action now is prudent.

By the way, a ModCon just doesn't slip into a conventional boiler's footprint. There are added piping and labor costs in replacing a conventional boiler with a ModCon.

Coldax

kcopp said:

What are you paying for LP gas?
What is the cost of oil there?
Would oil be an option if you already have a chimney?

The last fill up (I have a 1,000 gallon tank) last spring was $2.49 US. Has been pretty stable for the past 4 years. No natural gas where I live. This house used to be oil I really don't want to go back.

Coldax

Thanks everyone so far for the comments, I really appreciate it.

Gordy

Coldax said:

Gordy said:

This is a fin tube output chart. Depending on your present flow rate, and AWT. With 287 feet of fin tube you are not putting more than 150k into your home if the AWT is 170 degrees. You are running three zones. Do they ever all call at the same time?

150000 btu /3400 sf = 44 btus sf. Still a very high load for 80’s construction.

With 90 windows keep in mind insulating window treatments can really enhance in reducing heat loads.

Yes they often call at the same time, also DHW. Most of the windows/tall ceilings are in the living area and kitchen. Just by design there are no window treatments there, it would ruin the look. This home was built by a home builder, many windows are custom sized and cut, and there are walls of windows. The bedrooms are more traditional and have treatments, but the windows are tall. The kitchen has an island with 2 toe kick blowers and that really helps in that room.

So my question is. Is there any problem areas, or zones that have trouble heating?

If the original installer did his math right there shouldn’t be. Also most time installers play it safe, and install more than enough emitter. Sometimes not. Maybe because of poor calculations, or the room with your emitter type just didn’t have enough wall space.

In older homes than yours, usually the home has ample emitter, and maybe the homes envelope was upgraded by insulation then the emitter becomes oversized which lends itself well to lowered AWT in the system which helps get the mod/con more efficient.

Coldax

HomerJSmith said:

The questions I would ask is do you have enough heat emitters to satisfy the heat loss that you will be experiencing in the dead of winter, at what water temperature, and if glycol is going to be used.

If you have a ModCon running at 180 deg to satisfy your heat loss , you have gained nothing and a conventional boiler would be the better choice. But, if you have abundant baseboard that can run at a lower water temperature go for the ModCon. A ModCon that can modulate will save big buckos every year.

As far as money goes, you pay up front for a ModCon or you pay later in increased fuel cost with a conventional boiler. With a conventional boiler that cost is every year that that boiler is in operation.

When the heat is running it feels great, but I can literally see the needle dropping on the propane tank gauge. It's like putting a 426 Hemi into a Honda Accord. Even a light foot sucks down the fuel.

No vendor has suggested gycol so far.

I live in central New Jersey. It gets cold - it can go weeks without going above freezing - but it's not like northern New England with massive snow falls and sustained sub-zero temps.

There seems to be adequate baseboard. It's 3/4 inch.

HomerJSmith

With a 300k boiler I have my doubts that any calculation was made. In the olden days, a plumber would come in and oversize the boiler, read the manufacturer's instructions and still get it wrong.

Today, with all the real technical schooling things are different. There is an engineering basis for what we do and why we do it that way.

Adequate baseboard is one that is mathematically calculated.

BennyV

The first thing you need is a heat loss calc. You have about 160k worth of baseboard, depending on the exact type, but you heat loss is most likely somewhere in the 90-120k range. You need to figure that out first. Also do an energy audit to see if you can improve insulation, air sealing, low flow shower heads, all that sort of stuff. That stuff is always cheaper and has a better ROI than any mechanical device.

Secondly, with LP gas in a large house, a mod-con is an absolute no brainer, not just for the condensing, but also for the modulation, especially since you have several zones. Further, a mod-con is less critical in sizing, especially since there are models with 10:1 or 15:1 TDRs, so if they put in a 150k Navien, for example, it can modulate all the way down to 10k, so at that point, who cares if they got the sizing exactly right? If the load is actually 100k, it will just modulate and do it's thing. You will recoup the price of a mod-con much faster on LP than on NG due to pricing. Oil is cheaper per BTU, but it's loud, dirty, smelly, and you can claw back some of the price difference with a modern gas mod-con that will outperform the best oil boilers by a significant margin.

In terms of hot water, you should plan ahead, and have the electrical and piping connections put in and capped off for an indirect. The tankless units are difficult to size, and have water quality issues, etc. If you don't do it now, I'd look to move to an indirect in 3-5 years, as they are very efficient and reliable, and you can utilize your boiler for both loads in that case, and still have thermal mass in the hot water system so that you don't have to kick the burner on every time you wash your hands like with a combi. If you really dig the whole tankless thing, and then I'd do a separate tankless water heater that's not also your heating boiler, but an indirect is preferable to that approach. Depending on your hot water usage, and electric heat pump could also be a very efficient way to make hot water, it's cheaper than propane, but can't keep up if you have a large family with high hot water demands.

Lastly, in such a warm climate, I'd look at your AC system and see if either ducted or ductless heat pumps could provide some of your heat. Like you've said, it doesn't get that cold that often in NJ, and since you don't have NG, heat pumps have a huge cost advantage over LP or oil.

Coldax

Using the app I get a heat loss of 13,004 BTU/HR. Does that sound correct?

Gordy

Negative. That might be a larger room. Unless you meant 130,000 btu.

At 25 btus a sf your load would be 85k that’s an average construction home. With all your windows you may be higher just to give you an idea.

Can you tell us how many linear feet of base board are on each zone?

Do you use setbacks ritually?

You have enough emitter to absorb 150,000 btus which is not to say that this is a real life number. Your some where between 85, and 120k.

Gordy

@BennyV, I wouldn’t say installing a mod/con is a “no brainer”.Not until some hard numbers surface. The OP has a budget let’s remember, and respect that . If a mod/con was a definite affordable decision then the OP would have started off with which mod/con brand. Not ci, or mod/con, and then reflect on the extra cost of a mod/con.

HomerJSmith

13,004 BTU/HR, you say, hell you could heat that with a candle.

That's an obvious mistake. Re do the calcs for an accurate determination. Use several programs and compare the results.

Even Gordy drops a zero now and again and he's damn near perfect.

Gordy

Rich McGrath with Lanagans heating, and plumbing may be in your neck of the woods. He also offers system design assistance.

BennyV

13,000 BTUs is not right unless the house has foot thick double walls and triple pane windows, which it clearly doesn't. I'd guess that output was 130k.

There's lots of nice 15-150k or 10-150k mod-cons out there that would fit nicely in that application, with the ability to run an indirect a few years down the road when the water heater needs to be replaced.

Gordy said:

@BennyV, I wouldn’t say installing a mod/con is a “no brainer”.Not until some hard numbers surface. The OP has a budget let’s remember, and respect that . If a mod/con was a definite affordable decision then the OP would have started off with which mod/con brand. Not ci, or mod/con, and then reflect on the extra cost of a mod/con.

With LP gas and a large house, the mod-con is going to have a lower TCO by a significant margin, and is a no-brainer if it's actually how the house is heated (i.e. not using heat pumps, wood stoves, etc). TCO is all that really matters, not up front cost. Unfortunately, most people don't sit there and do the math out on things, whether it be cell phone installment plans, gas guzzling cars, or heating boilers.

EDIT: Not meant to point to the OP, he's ahead of 99% of people in the country by asking this type of stuff.

EBEBRATT-Ed

@BennyV said, Oil is cheaper per BTU, but it's loud, dirty, smelly, and you can claw back some of the price difference with a modern gas mod-con that will outperform the best oil boilers by a significant margin.

Not so sure about that. The newer oil boilers are pretty nice if properly maintained.

I would also suggest that many in Eastern MA. affected by the recent gas explosions that have no heat and will have no heat until mid December would love to have an oil boiler now.

But not picking a fight, everyone has their own opinion and everything has it's place

Gordy

Up front costs do matter if the owner has to borrow money for interest to have their mod/con instead of a ci boiler.

Just by having the “right size” CI boiler with an outdoor reset strategy will already trim fuel consumption. In my opinion that is the starting point of how much a mod/con saves in comparison. Couple that with yearly maintenance which off sets ROI, and a shorter possible life span than a conventional boiler that would off set ROI. About the time the mod/con has paid back its cost over run your ready for a new one while a CI boiler will still be going for another 10-15 years. Pure speculation of course since mod/cons don’t have a real substantial longevity record yet.

HomerJSmith

Gordy, I'm kinda dumb. How would an ODR work on a CI boiler with a fixed firing rate? I can see a motorized mixing valve set up, but that is for mixed temperatures. The firing rate would be the same, but I can see a reduction of firing at reduced demand temperatures with a mixing valve setup which would save on fuel. hmmm.

Gordy

ODR on a CI boiler will only get you so far with high temp radiation, and return water protection. However in low load parts of the season it can get supply temps down in the 155 supply return 135 degree region. If the boiler doesn’t have to fire to 180 then that is less fuel burned even though it’s not a modulationg boiler. Claims can be as much as 11% savings.

Coldax

I used the heat loss calc on the US boiler site. I get 100,854 for 60 temp diff and 119,008 for 70.

The Navien being suggested is 199,000 BTU and the Burnham/Buderus are around 175K.

I am still leaning towards the Burnham/Buderus, despite all the compelling statements above, for a few reasons:

• Initial extra cost of Navien will take about 7 years to recoup assuming LP stays the same and there are no other costs associated with ownership, like labor to repair a warrantied part. I'm not including routine maintenance which I will do for either boiler I choose.
  
• I'm having my water tested for hardness this week and I might have to install a softener system so that the Navien works properly. I don't even know what that would cost would be, plus ongoing expenses.
  
• I understand that I may have to install a acid reducer so the condensate does not destroy drain pipes.
  
• I have my doubts that the Navien will be able to supply heat and DHW simultaneously.
  
• Every time I use small amounts of DHW, like hand washing, will trigger the Navien to fire up. I assume that is minimal but it is still a usage.
  
• My current DHW heater still works well and was expensive so why rip it out? I haven't calculated that loss into things. When it fails I would consider Rinnai or similar for instant.
  
• Burnham/Buderus is only 82-84% (just 10% less than the Navien), but the tech is tried and true and initial cost is 40% (or more) less.
  
• I place a value on things that are simple and low maintenance. I also think that it will be easier and cheaper to maintain and repair vs. Navien.
  

I have not yet decided, I'm still reading and learning, and I also have one vendor with an outstanding estimate. He suggested Lochinvar but I don't know the model yet.

Coldax

Gordy said:

Can you tell us how many linear feet of base board are on each zone?

Do you use setbacks ritually?

I have remeasured the heated portion of the house and it is actually 3,150 SF. The larger number in my first post is the total I took from the property appraisal. The difference is taken from the lowest level (it's a bi-level) where there is an unheated mudroom and workshop that leads to the garage. This is where the mechanical room is. A benefit of the current boiler is the ambient heat to this area! I don't think the Navien would throw off much.

For each zone: 100 feet of baseboard serving a total volume of (17,640); 99 (6,032) and 88 (9,280). This is SF x height. The first zone is the living/kitchen with 2 story vaulted ceilings.

Yes I have three 5+2 programmable thermostats and I set them to our schedules and to compensate for the volume differences. The house traps enough heat so that I can set it down to where the boiler does not run at night yet we sleep comfortably.

KC_Jones

Coldax said:

I used the heat loss calc on the US boiler site. I get 100,854 for 60 temp diff and 119,008 for 70.

The Navien being suggested is 199,000 BTU and the Burnham/Buderus are around 175K.

Educating yourself is a great first step, the second step is feeling comfortable telling a contractor the boilers are too big.

Finding one that will properly install the proper sized boiler is your next hurdle. Stick to your guns, don't accept the "I've been doing this for XX years" line when it comes to sizing.

Have you tried the find a contractor link on this site? I know there are some excellent people in NJ for hydronic heating.

@Rich

Gordy

Keep in mind setbacks can get you into trouble with a closely matched boiler on the coldest, or colder of days.

Even though there is a design day temp used for heat loss calculations in the geographic location , and systems are designed around, and boilers sized by such. Deep setbacks can cause systems to have a hard time reaching set point on coldest days, or colder.

I’m not trying to tell you to oversize the boiler, but to watch setbacks with a right size boiler. Especially when outdoor temps fall below the design day number.

hot_rod

Gordy said:

Keep in mind setbacks can get you into trouble with a closely matched boiler on the coldest, or colder of days.

Even though there is a design day temp used for heat loss calculations in the geographic location , and systems are designed around, and boilers sized by such. Deep setbacks can cause systems to have a hard time reaching set point on coldest days, or colder.

I’m not trying to tell you to oversize the boiler, but to watch setbacks with a right size boiler. Especially when outdoor temps fall below the design day number.

Good point, that cold snap back east, below design for a few weeks, about 3 winters ago had a lot of cold complaints, heaters running non stop, falling behind. With so many sizes and wide modulation size carefully.

The question becomes how much if any fudge in the calc these days?

Gordy

Yes climates can change in geographic areas.

Design day in my area is -4, but there have been winters that -27 with out wind chill have happened. Much below design For a couple weeks at a time.

These events seem periodic. 1950’s, late 70’s early 80’s had some tough below average temps.