BTU Load Sizing for Boiler versus Forced Air

Zman

It is nice that they are installing generously sized radiation. Be sure they plan to utilize outdoor reset so you rarely need 160 degree water. What model boilers are you considering? What are the turndown ratios?
If they are recommending a 120k model with 10-1 turndown, it might not be worth fighting with them. Have them set it up with outdoor reset and pipe it per manufacturers instructions and move forward.

Crissie

Crissie said:

STEVEusaPA said:

If you have usage data just tell us and I’ll use a simple app I made.
Just need to know your zip code and amount of btus in cubic feet, used from 9/1/20-9/1/21.

Zip Code is 60137. I'm giving you total Therms used from 10-6-2019 thru 9-7-20 (12 months) because I realized my HVAC system was making me ill last winter (2021) and I had a space heater going all winter, so there was supplemental heat and we would not have an accurate reading.

12 month Therm Usage: 1052
Furnace AFUE: 96.1
August Usage was 12.6 - this would be for cooking, hot water, etc. Multiple by 12 months = 151.2 Therms
Net 12 months estimated heating Therm usage = (1052-151.2) = 900.8

Not sure how I convert this to BTU's used for heat. I see a formula for a straight convert of therm to BTU, and I can factor in an additional 10% waste of my furnace, but how do I get to the heat load of the home? Should I just take one extreme winter month?

KC_Jones

Crissie said:

Zman said:

Ask them all how many linear feet of baseboard they intend to install. Take that number and multiply it by 550. That will give you the absolute maximum amount of heat your system can absorb. If they are quoting a boiler size larger than that, they have some explaining to do...

The spec'd 200 linear ft of Slant Fin baseboard, so using your 550 BTU / ft, it is 110,000 BTU's. Design water temp is 160 degrees. Not sure if this matters, I think it does. BTW, Slant Fin spec says 600 BTU / ft at water temp of 200 degrees.

When I originally asked about the boiler size of 175K BTU's, he said it is customary to size the amount needed for heat and then add 25% for future possible expansion. Correct or no? My guess is no.

When I asked about short cycling, he said the system design would prevent that. Is this correct, can you design a piping system that will prevent short cycling?

And what if you never expand?

He's guessing, then up sizing because he feels it's better to short cycle the heck out of the boiler than take a chance you will be cold.

If he's using 160 water temp, that's about 90k BTU of baseboard, so what's he planning to do with the other 85k BTU. BTW the amount he is oversizing too big for the house.

200' of baseboard giving you 90k output at 160 (for one chart I quickly pulled up), to me, is a good thing. He's oversizing the baseboard (again he apparently doesn't want to do math) so you could run at lower temps and get an efficiency boost during the entire year with an outdoor reset system.

A point of note, you said you are going with the Weil Mclain GV-90, my understanding is that is a cast iron, but with the secondary stainless heat exchanger it's somewhat like a condensing boiler. I looked up the specs and it can handle return water temps as low as 60. I'm not an expert on that boiler, but it sounds like you are interested in maximum efficiency without your concerns over the ModCon boilers. If that's the case your design needs to really be by the numbers, these guys that are guessing are just no good.

Unless they can produce actual calculations to you, they are guessing, and please don't let them pull the "I've been doing this for X years" stuff. That could just mean they've been doing it wrong for X years. You want someone who has experience, not repeating their first year 30 times. The second, in my personal experience is what is common, and you seem to be getting.

I would suggest the most dependable information is coming from this board, and just about everything the contractors are telling you should be thrown out the window.

STEVEusaPA

Crissie said:

I'm in 60137. I will try to get the usage therms / BTU's from Nicor, our natural gas company. But, I have a gas hot water heater, gas cooking, gas dryer. I will not be able to separate those from the actual heating. Should I take usage from Feb and then August, subtract the two and use the difference as the average worst heating month? Wouldn't the outdoor temp mater in this? Thanks

Just give me all the numbers, let's see what I come up with.

Crissie

Let me know if you need more usage data.

Crissie

KC_Jones said:

Crissie said:

Zman said:

Ask them all how many linear feet of baseboard they intend to install. Take that number and multiply it by 550. That will give you the absolute maximum amount of heat your system can absorb. If they are quoting a boiler size larger than that, they have some explaining to do...

The spec'd 200 linear ft of Slant Fin baseboard, so using your 550 BTU / ft, it is 110,000 BTU's. Design water temp is 160 degrees. Not sure if this matters, I think it does. BTW, Slant Fin spec says 600 BTU / ft at water temp of 200 degrees.

When I originally asked about the boiler size of 175K BTU's, he said it is customary to size the amount needed for heat and then add 25% for future possible expansion. Correct or no? My guess is no.

When I asked about short cycling, he said the system design would prevent that. Is this correct, can you design a piping system that will prevent short cycling?

And what if you never expand?

He's guessing, then up sizing because he feels it's better to short cycle the heck out of the boiler than take a chance you will be cold.

If he's using 160 water temp, that's about 90k BTU of baseboard, so what's he planning to do with the other 85k BTU. BTW the amount he is oversizing too big for the house.

200' of baseboard giving you 90k output at 160 (for one chart I quickly pulled up), to me, is a good thing. He's oversizing the baseboard (again he apparently doesn't want to do math) so you could run at lower temps and get an efficiency boost during the entire year with an outdoor reset system.

A point of note, you said you are going with the Weil Mclain GV-90, my understanding is that is a cast iron, but with the secondary stainless heat exchanger it's somewhat like a condensing boiler. I looked up the specs and it can handle return water temps as low as 60. I'm not an expert on that boiler, but it sounds like you are interested in maximum efficiency without your concerns over the ModCon boilers. If that's the case your design needs to really be by the numbers, these guys that are guessing are just no good.

Unless they can produce actual calculations to you, they are guessing, and please don't let them pull the "I've been doing this for X years" stuff. That could just mean they've been doing it wrong for X years. You want someone who has experience, not repeating their first year 30 times. The second, in my personal experience is what is common, and you seem to be getting.

I would suggest the most dependable information is coming from this board, and just about everything the contractors are telling you should be thrown out the window.

@KC_Jones You could not be more correct. I am finding that nearly all the HVAC companies focus on replacement, not complete system install and design, and they guess and upsize constantly. US Boiler even has an article example of this on their website - 'what happens when you increase the size of replacement boiler and your customer shows up in your office because the boiler runs for 10 minutes and then shuts down'. This business is not regulated here in the state of IL. Scary and negligent. This is how I ended up with a 115K BTU 96% AFUE replacement of a 90K 80% AFUE furnace, and have been living thru hell with it.

ChrisJ

A heating system that is too big doesn't just cost more.
It's also less comfortable, takes up more space and uses more fuel.

There's literally nothing good.

Extra baseboard is good as long as it's the same amount of extra everywhere in the building.
Extra boiler is bad. Bad bad bad.

As others have said, tell us what size rooms, windows, insulation, what's above and below (Cold attic, heated crawl space, cold basement etc) and we'll see what you actually need.

Slant Fin has an app for Android, and probably Apple for free where you can do all of this.

Hot_water_fan

Excellent, thanks for doing the legwork. Using a 65 degree base temp, there were 5,881 HDD between 11/6/2019 and 10/7/2020. Between those two dates, you used 975 therms and it looks like 12 therms/month for non-heating usage over those 11 months. So 975-(12*11) = 843 therms input, 843*.96 = 809 therm output. That's under 14k Btu/HDD. Using a design temp of -15, which is very conservative based on this time period, your heat loss is about 46k Btu.

If you want condensing, 200 ft of baseboard will definitely get you that. There's no reason to oversize for domestic hot water either.

It makes you wonder if a modulating furnace sized correctly, say 20k Btu Min output and 60k Btu Max output would improve comfort enough to skip the baseboard all together.

KC_Jones

Hot_water_fan said:

Excellent, thanks for doing the legwork. Using a 65 degree base temp, there were 5,881 HDD between 11/6/2019 and 10/7/2020. Between those two dates, you used 975 therms and it looks like 12 therms/month for non-heating usage over those 11 months. So 975-(12*11) = 843 therms input, 843*.96 = 809 therm output. That's under 14k Btu/HDD. Using a design temp of -15, which is very conservative based on this time period, your heat loss is about 46k Btu.

By those numbers the smallest quote was more than double, and the biggest is almost quadruple what you need.

I'd love to hear the double talk from a contractor to explain it.

KC_Jones

ChrisJ said:

A heating system that is too big doesn't just cost more.
It's also less comfortable, takes up more space and uses more fuel.

There's literally nothing good.

Extra baseboard is good as long as it's the same amount of extra everywhere in the building.
Extra boiler is bad. Bad bad bad.

As others have said, tell us what size rooms, windows, insulation, what's above and below (Cold attic, heated crawl space, cold basement etc) and we'll see what you actually need.

Slant Fin has an app for Android, and probably Apple for free where you can do all of this.

An extremely good point.

Rich_49

Crissie said:

Jamie Hall said:

If the air leakage is taken into account -- as it should be -- then the heat loss will be the same regardless of the heat source.

I'm a little concerned about the air tightness -- that house is much too tight (2 to 4 air changes per hour are the minimum recommended for indoor air quality. Unless the mechanical air exchange is through a heat exchanger, that needs to be considered.

I currently have a fresh air intake on the return side of my furnace that brings in 90 CFM, but only when furnace / ac runs. I am planning to put in an ERV with the boiler.

I wonder if there is anyone I could hire to build out a boiler design for me, even someone not local, just so I have something to use to get quotes. Is that common, to quote off a design?

If it were my house and I knew what I know I would contact this guy and LISTEN to the process . These guys will figure out what You NEED based on your issues . Ask for Doug Krahl http://www.americantemperaturecontrol.com/index.html

Crissie

Hot_water_fan said:

Excellent, thanks for doing the legwork. Using a 65 degree base temp, there were 5,881 HDD between 11/6/2019 and 10/7/2020. Between those two dates, you used 975 therms and it looks like 12 therms/month for non-heating usage over those 11 months. So 975-(12*11) = 843 therms input, 843*.96 = 809 therm output. That's under 14k Btu/HDD. Using a design temp of -15, which is very conservative based on this time period, your heat loss is about 46k Btu.

If you want condensing, 200 ft of baseboard will definitely get you that. There's no reason to oversize for domestic hot water either.

KC_Jones I gave you meter reading dates, it was actually 12-months of readings, not sure if this changes anything. What BTU size boiler should I be looking at. I wanted the Weil McCann GV90, it is cast iron conventional with secondary stainless steel heat exchanger to allow for condensing and exhaust thru PVC. Thanks!

Rich_49

Also try A Team Heating & Air conditioning 630 793 5233

https://ateamheating.com/

Rich_49

Crissie , What about your current system was making you ill ? What drove your decision to go hydronic besides a thought that blowing air was the culprit ?

Crissie

Rich_49 said:

Crissie , What about your current system was making you ill ? What drove your decision to go hydronic besides a thought that blowing air was the culprit ?

I am going to call you in a little while! My illness started 3 winters ago, would wake up in the am and feel dizzy, double vision, off balance. Dizziness would last for hours, had to leave the house. Could not figure out what was going on. This was in Jan - right at the peak of heavy heating season. In am, would be heating up my house from having turn the temp down at night. This winter, I realized it was happening only when the HVAC was running. Lots of research, torn out drywall to expose the ducts. Return ducts were tampered with, upstairs return ducts were disconnected, large 3" holes sucking Rockwool insulation, huge holes in supply lines, furnace oversized with too little return sucking air from ceiling / wall cavities and foundation.

Fresh air return is bringing in 90 CFM of cold air through return (dampered Honeywell system). That was solid pipe but insulation wrap was soaking wet. I think there is mold in ductwork, can smell it. This winter I used ozone - made a huge difference but not permanent. Combination of old (44 yrs) leaky ductwork, someone tampering with it, oversized furnace, and probably not correct ventilation of outside air. By April / May, it is much better, furnace not working as hard, me spending more time out of house.
In all, these homes with old ductwork and modifications made can be very dangerous.

Jamie Hall

Crissie said:

Rich_49 said:

Crissie , What about your current system was making you ill ? What drove your decision to go hydronic besides a thought that blowing air was the culprit ?

I am going to call you in a little while! My illness started 3 winters ago, would wake up in the am and feel dizzy, double vision, off balance. Dizziness would last for hours, had to leave the house. Could not figure out what was going on. This was in Jan - right at the peak of heavy heating season. In am, would be heating up my house from having turn the temp down at night. This winter, I realized it was happening only when the HVAC was running. Lots of research, torn out drywall to expose the ducts. Return ducts were tampered with, upstairs return ducts were disconnected, large 3" holes sucking Rockwool insulation, huge holes in supply lines, furnace oversized with too little return sucking air from ceiling / wall cavities and foundation.

Fresh air return is bringing in 90 CFM of cold air through return (dampered Honeywell system). That was solid pipe but insulation wrap was soaking wet. I think there is mold in ductwork, can smell it. This winter I used ozone - made a huge difference but not permanent. Combination of old (44 yrs) leaky ductwork, someone tampering with it, oversized furnace, and probably not correct ventilation of outside air. By April / May, it is much better, furnace not working as hard, me spending more time out of house.
In all, these homes with old ductwork and modifications made can be very dangerous.

Yup...

KC_Jones

Crissie said:

Hot_water_fan said:

Excellent, thanks for doing the legwork. Using a 65 degree base temp, there were 5,881 HDD between 11/6/2019 and 10/7/2020. Between those two dates, you used 975 therms and it looks like 12 therms/month for non-heating usage over those 11 months. So 975-(12*11) = 843 therms input, 843*.96 = 809 therm output. That's under 14k Btu/HDD. Using a design temp of -15, which is very conservative based on this time period, your heat loss is about 46k Btu.

If you want condensing, 200 ft of baseboard will definitely get you that. There's no reason to oversize for domestic hot water either.

KC_Jones I gave you meter reading dates, it was actually 12-months of readings, not sure if this changes anything. What BTU size boiler should I be looking at. I wanted the Weil McCann GV90, it is cast iron conventional with secondary stainless steel heat exchanger to allow for condensing and exhaust thru PVC. Thanks!

If 46k is the number, you want to get as close to that as possible. In a Weil Mclain GV-90, that would be the smallest one they make, and it would still be about 40% oversized.

This is what happens all the time, the equipment just can't get small enough. That's why ModCons are being pushed, some of those can go down to I believe it's 8k on the lowside. The other reason they are pushed is contractors like you encounter don't have to size anything and just depend on modulation to "fix" their inadequacies.

Remember that 46k, or whatever ends up being, is only for the very coldest day of the year, most of the winter you will probably be 2/3-1/2 that number. So the equipment, no matter what, is grossly oversized.

Now imagine what that 175k would do on a mild fall day when it's 50 outside and you don't need much heat at all.

Rich_49

Crissie said:

Rich_49 said:

Crissie , What about your current system was making you ill ? What drove your decision to go hydronic besides a thought that blowing air was the culprit ?

I am going to call you in a little while! My illness started 3 winters ago, would wake up in the am and feel dizzy, double vision, off balance. Dizziness would last for hours, had to leave the house. Could not figure out what was going on. This was in Jan - right at the peak of heavy heating season. In am, would be heating up my house from having turn the temp down at night. This winter, I realized it was happening only when the HVAC was running. Lots of research, torn out drywall to expose the ducts. Return ducts were tampered with, upstairs return ducts were disconnected, large 3" holes sucking Rockwool insulation, huge holes in supply lines, furnace oversized with too little return sucking air from ceiling / wall cavities and foundation.

Fresh air return is bringing in 90 CFM of cold air through return (dampered Honeywell system). That was solid pipe but insulation wrap was soaking wet. I think there is mold in ductwork, can smell it. This winter I used ozone - made a huge difference but not permanent. Combination of old (44 yrs) leaky ductwork, someone tampering with it, oversized furnace, and probably not correct ventilation of outside air. By April / May, it is much better, furnace not working as hard, me spending more time out of house.
In all, these homes with old ductwork and modifications made can be very dangerous.

The 2 guys I referred can both figure out what the exact issues are and offer REAL SOLUTIONS w/o grasping at straws . They both use a process that was derived out of being frustrated at programs and utility offerings that do not offer real solutions . The process is known as HVAC2.0 and involves a real process to eliminate problems that effect peoples health . You could possibly receive a recommendation to change a few things and some things redone , in the end , the decision you choose will be yours . They will actually educate you on the issues and let you decide the path for your house . They are quite capable of doing the hydronic work properly too if that is what you wish

sunlight33

The new Vitoden 200-W can go as low as 8.5k on the low side, and 120k on the high, 14:1 ratio.

STEVEusaPA

'Quick & Dirty'...assuming you didn't use any supplemental heat. I'd still confirm with a full heat loss.
I'd definitely go with a mod/con.

Crissie

KC_Jones said:

Crissie said:

Hot_water_fan said:

Excellent, thanks for doing the legwork. Using a 65 degree base temp, there were 5,881 HDD between 11/6/2019 and 10/7/2020. Between those two dates, you used 975 therms and it looks like 12 therms/month for non-heating usage over those 11 months. So 975-(12*11) = 843 therms input, 843*.96 = 809 therm output. That's under 14k Btu/HDD. Using a design temp of -15, which is very conservative based on this time period, your heat loss is about 46k Btu.

If you want condensing, 200 ft of baseboard will definitely get you that. There's no reason to oversize for domestic hot water either.

KC_Jones I gave you meter reading dates, it was actually 12-months of readings, not sure if this changes anything. What BTU size boiler should I be looking at. I wanted the Weil McCann GV90, it is cast iron conventional with secondary stainless steel heat exchanger to allow for condensing and exhaust thru PVC. Thanks!

If 46k is the number, you want to get as close to that as possible. In a Weil Mclain GV-90, that would be the smallest one they make, and it would still be about 40% oversized.

This is what happens all the time, the equipment just can't get small enough. That's why ModCons are being pushed, some of those can go down to I believe it's 8k on the lowside. The other reason they are pushed is contractors like you encounter don't have to size anything and just depend on modulation to "fix" their inadequacies.

Remember that 46k, or whatever ends up being, is only for the very coldest day of the year, most of the winter you will probably be 2/3-1/2 that number. So the equipment, no matter what, is grossly oversized.

Now imagine what that 175k would do on a mild fall day when it's 50 outside and you don't need much heat at all.

OMG - so you are saying I could put a 40,000 BTU furnace or boiler in and in theory it should be okay, it might struggle in -15 degree weather but overall should be okay? I am almost speechless. Why would a condensing boiler be better?

Crissie

STEVEusaPA said:

'Quick & Dirty'...assuming you didn't use any supplemental heat. I'd still confirm with a full heat loss.
I'd definitely go with a mod/con.

STEVEusaPA said:

'Quick & Dirty'...assuming you didn't use any supplemental heat. I'd still confirm with a full heat loss.
I'd definitely go with a mod/con.

Steveusa_PA Can you provide a link for that app? Is heat loss = to BTU's needed for boiler or furnace sizing?

KC_Jones

The quotes are getting long I'll just reply without quoting. The short answer is yes, but it also depends on a solid design, not only in the piping layout at the boiler, but also the baseboard layout and getting the amount to match the room by room heatloss.

The above heatloss is for the whole house. To properly design the system, you need the heatloss for each room so the amount of baseboard can be calculated. Without knowing that, it's again, a guess. This is where the slant fin app is so helpful as it's set up to do each room, get the calcs and then it will even tell you how much baseboard you need. I think it took me about 1 hour to do my whole house, but most of my windows are the same size so that sped it up a bit.

The contractors you are talking to just go big, and let the thermostat cycle the boiler like crazy.

I am far from the most knowledgeable on these systems, other far more knowledgeable people commenting here. But just to add, I'm not a professional, just a homeowner like you, and on some of these subjects it seems I know more than the professionals you have called in.

This is the kind of stuff I ran into on my steam system when the boiler needed replaced, so I did it myself.

Hot_water_fan

OMG - so you are saying I could put a 40,000 BTU furnace or boiler in and in theory it should be okay, it might struggle in -15 degree weather but overall should be okay? I am almost speechless. Why would a condensing boiler be better?

The caveat is that there's not infinite boiler sizes, so maybe 60/70k is the best you can hope for (of non-modulating options). Baseboard systems have low water content, so they will still short cycle if they don't modulate and/or you don't add some mass, like a small buffer tank.

STEVEusaPA

Crissie said:

Steveusa_PA Can you provide a link for that app? Is heat loss = to BTU's needed for boiler or furnace sizing?

Yes, in general, that would be the size of your equipment, but again, a full heat loss to confirm. I use this mostly for oil, basically because there are only a few different sizes for oil, and they only go so low. But it's been fairly accurate with gas, or because there are other gas appliances usually in a house, serves pretty well as a 'do not exceed' for sizing. The number is probably low because your house is so tight. But if it turns out to be the right size, it's the right size.

There are supply houses have the proper software, like Wrightsoft, and will do the design work for their customers. I send the plans to them, they do the design, but you do have to buy your equipment and material from them. There's no free lunch.

You can find 10:1 Mod/Cons that go from 8K to 80K.
----
It's a database I created using FileMaker, based on an article by @Robert O'Brien -it's been brought up many times on this forum.
You'd have to download FileMaker Go (sorry but IOS only) for free, and I could send you the file.

Or you can download a 30 day trial from FileMaker for desktop (Mac or PC), and I can send you the file.

But you really don't need it as you still need to do a room by room heat loss calc to properly size emitters and equipment, which will dictate, pipe sizes, flow rates, and on and on.

PS. It's not necessary to quote entire long posts, especially pictures. You can take someones quoted post, highlight all the areas not pertinent to your follow up question and delete them.

Crissie

Thanks so much, you all are so helpful!
If I went with a furnace, I'd have to replace my ductwork. Can't imagine the difficulty in properly sizing that. Funny, I have had a few contractors offer to quote on that, telling me that is absolutely the route I should take. I have never received one single quote for them, they leave saying I will have something in a few days. Weeks later, nothing. One did send a tech out who said i don't have enough room to replace it.

Crissie

Jamie Hall, SteveusaPA , KC_Jones and Hot_water_fan I just worked with the Slant Fin Heat Loss calculator. I used my main open concept living area, below are the metrics for it. The app called for 31,311 BTU of heat. Is this correct? If so, my house can't have a total load of 46K BTU, I still have two other levels that total about 1900 sq ft.


Main Level is Living, Dining, Kitchen, Foyer, Mud Room / Garage Entrance:
Size Room: 33’ x 24’ - 792 sq ft
8 ft ceilings
118 sq ft of glass (lots of windows and door glass). All double pane, vinyl.
3 doors – total 57.78 sq ft
Below is unheated basement.
Above is attic with R50.
Exterior wall length (2.5 sides) = 68 ft. R15 with 1 inch foam board on 28 ft of it
Shares a wall with a garage. A cold Partition wall. 23 ft
Used .023 for infiltration factor because not a full three sides are exterior walls, more like 2.6 side are exterior.
Used 71 degrees for indoor and 1 degree temp for outdoor (app won’t do negative temps, 1 degree is close enough).
I got 31,311 BTU’s needed for hear, needs 55 ft of baseboard.

hot_rod

The bid looks like a mod con boiler is being priced, based on efficiency numbers? If so most have a 10- 1 turndown, some claim 12-1. So the boiler output will modulate based on the building load. This is a huge plus for reducing short cycling. Ramp delay and other functions add even more adjustability.
An 80,000 mod con boiler can modulate down to 8,000

I'd select a boiler close to the required load. Upgrade the structure to reduce loads instead of oversizing for any size addition With a wide modulation range on a mod con boiler, a bit oversized is not a big deal. Most allow you to lock them down to a lower output.

KC_Jones

One point of clarification, the door glass is taken care of by the door factor, you don’t add that glass to the total, only actual windows.  The way you worded sounds like you added it.

With your inputs (had to guess on window sizes, but same amount) I’m getting about 24,000.

Crissie

KC_Jones said:

One point of clarification, the door glass is taken care of by the door factor, you don’t add that glass to the total, only actual windows.  The way you worded sounds like you added it.

With your inputs (had to guess on window sizes, but same amount) I’m getting about 24,000.

Here are the window sizes. Yes, I added the glass from doors into Glass for windows, but that was not a lot. What did you used for the basement, the only option I saw was an unheated crawl? Thanks so much
Window Glass Sizes Sq Ft of Window Glass

97" x 63" 42.4
104" x 63" 45.5
23" x 64" 10.2

KC_Jones

Wood over ENCLOSED UNHEATED space, it’s not really referring to a crawl space specifically.  Some crawl spaces are open to outdoors, some are not.  That option covers basements and enclosed crawl spaces.

I put those windows in and sitting at 29k now. So pretty close, could be the extra glass added for doors making the difference.

Remember this is worst case scenario.  The other heat loss from your actual usage is sort of an average, so if you only had a couple days at worst case, overall you will be lower, but the usage load calcs are a great reality check for what’s going on.

I think you are on track with this, looking good so far.

Crissie

KC_Jones said:

Wood over ENCLOSED UNHEATED space, it’s not really referring to a crawl space specifically.  Some crawl spaces are open to outdoors, some are not.  That option covers basements and enclosed crawl spaces.

I put those windows in and sitting at 29k now. So pretty close, could be the extra glass added for doors making the difference.

Remember this is worst case scenario.  The other heat loss from your actual usage is sort of an average, so if you only had a couple days at worst case, overall you will be lower, but the usage load calcs are a great reality check for what’s going on.

I think you are on track with this, looking good so far.

I just called Slant Fin and they said they haven't supported this app in years. Not sure if that makes a difference. Also, they were just sold to Mezteck.

Hot_water_fan

@Crissie if you're going with a boiler, you have limited minimum efficiency options that have max outputs below 65k Btu, so you don't have to put too much effort into this. If you're considering fixing the forced air, furnaces come in a few smaller sizes, and if you use a heat pump/furnace hybrid option, you can get a heat pump sized for your cooling load and that'll probably be small enough to do a good job of modulating during the winter. Bottom line, trust the gas meter.

Crissie

Hot_water_fan said:

if you're going with a boiler, you have limited minimum efficiency options that have max outputs below 65k Btu,

Will as 65K BTU Boiler provide more heat, let's say up to 75-80K BTU's if needed, or is that 65K the strict limit?

Crissie

Zman said:

It is nice that they are installing generously sized radiation. Be sure they plan to utilize outdoor reset so you rarely need 160 degree water. What model boilers are you considering? What are the turndown ratios?
If they are recommending a 120k model with 10-1 turndown, it might not be worth fighting with them. Have them set it up with outdoor reset and pipe it per manufacturers instructions and move forward.

They sized a 175K BTU. Looking at Weil McCann GV90, cast iron with secondary stainless steel heat exchange to allow for condensing and PVC exhaust. Are you familiar with them?

Crissie

KC_Jones said:

Crissie said:

Zman said:

Ask them all how many linear feet of baseboard they intend to install. Take that number and multiply it by 550. That will give you the absolute maximum amount of heat your system can absorb. If they are quoting a boiler size larger than that, they have some explaining to do...

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Called Slant Fin today and at the 160 degree water temp design, I would only get 450 BTU/ft. That is a Lot of baseboard!

Crissie

@KC_Jones @STEVEusaPA @Hot_water_fan @Jamie Hall I just ran every room in the house using the Slant Fin app and came up with the total of 81,474 BTU's. It might not be precise, I did not get the infiltration down correctly because the app just gave construction type options. Also, had to use 1 degree as the design temp.

If I understand correct, the difference with this and the actual use sizing you did above is the 81K BTU's is needed for the coldest 5% of days in the year when temps are below 5 degrees. Sometimes this is extreme in Chicago, with temps in the -10 degree range for several days. So the actual use is only an average of what was used, it is not taking into consideration the BTU's needed for the 1-5% of the time when we get extreme cold.

Another quick question. Are boiler that are sized for a specific BTU, like 65K, it that their top limit, or do they provide heat above that? Thanks!!

Jamie Hall

If it says it's rated for 65,000 BTUh, that's it.

Hot_water_fan

@crissie no, the 81k Btu is way too high unless there’s a non-gas supplemental heat source skewing the numbers. The gas usage is what you actually used. The 46k btu heat loss was calculated at a temp of -15. That’s not 5%, probably more like .1% of hours. This is mostly academic if you get a modulating boiler. 

Are boiler that are sized for a specific BTU, like 65K, it that their top limit, or do they provide heat above that?

That’s the top limit. Some boilers are 100% or 0% output. Others are 10%-100% of output. Either way the size is 100% output. 

Crissie

Hot_water_fan said:

@crissie no, the 81k Btu is way too high unless there’s a non-gas supplemental heat source skewing the numbers. The gas usage is what you actually used. The 46k btu heat loss was calculated at a temp of -15. That’s not 5%, probably more like .1% of hours. This is mostly academic if you get a modulating boiler. 

@Hot_water_fan I have an attached Sunroom with a PTAC unit in it, I turn it on when it get very cold outside or if I happen to be using it (possibly 4-5 days / winter). That gives off enough heat to warm part of my lower level, so perhaps that is coming into play, but I doubt it is much. The thermostat is on my main level which is where I spend most of my time and the furnace cycles constantly in very cold temps, creating way too much heat on the upper level and my lower level has concrete floors so it is always colder.