BTU Load Sizing for Boiler versus Forced Air

Crissie

I have a tri-level home that has been on forced air and am trying to transition to hot water boiler heat using Slant Fin baseboards. I got 4 quotes from different contractors and the house Boiler BTU sizing is different for each.
Had a blower door test done and the house is very tight, 25% tighter than code requirements for new construction. It has upgraded insulation everywhere.

A replacement forced air furnace would be sized at 80,000 BTU furnace at 95% efficiency.
Quotes on boilers ranged from 120,000 BTU's to 175,000 BTU's, all at 91-95% efficiency.
Only one boiler quote took into consideration the tightness of my house and used the blower door test. He was in the mid-range of sizing.

How do I know what is right?

Jamie Hall

A BTU is a BTU, and it doesn't matter whether it is coming to you from scorched air or hot water. If you are thinking of using Slant/Fin's baseboards, why not use Slant/Fin's excellent heating loss calculator here: https://www.slantfin.com/slantfin-heat-loss-calculator/. It will even give you the length of baseboard you need for each room...

On the various contractors' numbers. As I say, a BTU is a BTU, and if you have been happy with the ability of your existing furnace to heat the house, that gives you a pretty good guess as to how much heat you need -- which suggests that none of those contractors performed a proper heat loss calculation. Do your own and see what you get...

pecmsg

BTU’s don’t know the difference between forced air or HW. Did that furnace cycle on and off during cold windy weather?

STEVEusaPA

As @Jamie Hall said a heat loss is paramount. Especially for high efficiency equipment. 80K may be too big too.
80k may be too big too.

Crissie

As @Jamie Hall said a heat loss is paramount. Especially for high efficiency equipment. 80K may be too big too.
80k may be too big too.

There originally was a 90K BTU 80% efficient furnace. That was replaced with a 115K BTU 95% efficient furnace 9 years ago. It cycles constantly, way too big. Guess I will try to do my own heat loss calculation and BTU need sizing. Helps to know that BTU is BTU regardless of type of heat.

EBEBRATT-Ed

@Crissie

I wouldn't be surprised if you ended up in the 60-70K range

What you really need is a contractor that knows what he is doing. Yours don't sound like top shelf to me.

Check "find a contractor" on this site and post your location. Someone may have a recommendation .

A contractor is the most important decision

Crissie

Jamie Hall said:

A BTU is a BTU, and it doesn't matter whether it is coming to you from scorched air or hot water. If you are thinking of using Slant/Fin's baseboards, why not use Slant/Fin's excellent heating loss calculator here: https://www.slantfin.com/slantfin-heat-loss-calculator/. It will even give you the length of baseboard you need for each room...

On the various contractors' numbers. As I say, a BTU is a BTU, and if you have been happy with the ability of your existing furnace to heat the house, that gives you a pretty good guess as to how much heat you need -- which suggests that none of those contractors performed a proper heat loss calculation. Do your own and see what you get...

Boiler contractors are telling me the BTU sizing of forced air furnace is different than that of a boiler. Is this not correct? Thanks

Canucker

It is not correct. It sounds like you found a couple of contractors that know how to do things but not why they do them. 

Crissie

Canucker said:

It is not correct. It sounds like you found a couple of contractors that know how to do things but not why they do them. 

Four different Contractors. All sized the boiler BTU need.
1st Contractor. Boiler size of 111 BTU's
2nd Contactor. Boiler Size of 120 BTU's
3rd Contractor. Boiler Size of 170 BTU - but that included hot water
Fourth Contractor. Boiler size of 175K BTU - but the sized the heat load for the house at 121K BTU's. Said he adds 25% for future expansion (like hot water). Said the overall design will prevent short cycling. Circulation pumps, Primary and secondary loop. reverse return, Tekmar indoor / outdoor temp control.

I prefer the fourth contractor because they are flexible and not stuck on a condensing boiler (which I prefer not to use). I actually had two additional contractors come out to get quote on it, so I called 6 contracting companies in total. The other two were too busy to take on the job this fall.
Guess I should get some more input from another contractor.

pecmsg

No
All 4 contractors GUESSED! 

The 4Th is the worst. Any future Sq Ft should be so tight a candle could heat it unless your adding 50% more!

KC_Jones

If your house is as tight and well insulated as you claim, 175k could quite effectively heat a house of about 7-8000 sq ft., maybe bigger.  How close to that are you?

Also using the boiler for hot water does not change the boiler sizing.  It does impact control strategy in that the hot water would be on priority.  If there are concerns about available hot water then put in a bigger indirect tank.

Use the slant fin app and find your actual load and work from there.

EBEBRATT-Ed

@Crissie

Who ever is telling you that hot air furnace BTUs are different from hot water BTUs doesn't know what they are doing. Others have mentioned this above. A BTU is a BTU

Also as @KC_Jones mentioned you don't add anything for heating domestic hot water.

Your contractors are guessing because they are lazy.

Bigger is always better doesn't work anymore, it causes other operational issues


For instance you may come up with a heat loss of 91,000 btu. Is their a boiler made with that exact output? Probably not. 100,000 would be close enough in that instance.

Do your own heat loss with the Slant Fin app, that's the first step in any hot water or warm air job

Then the search for a good contractor

CMadatMe

I'm a little confused. If you had a blower door test, how did you not have a heat loss done at the same time.? The blower door test is useless without it. Its like you paid to get an infiltration factor without the complete math formula to make it worth anything.

GroundUp

In addition to everyone else saying that at least 3 of them are too high, I will add that heating the DHW adds nothing to the boiler size unless it's a combi unit which you've already stated you are not interested in. A 120k unit may be acceptable, but something closer to 80k sounds a whole lot more reasonable given your forced air furnace sizing

CMadatMe

GroundUp said:

In addition to everyone else saying that at least 3 of them are too high, I will add that heating the DHW adds nothing to the boiler size unless it's a combi unit which you've already stated you are not interested in. A 120k unit may be acceptable, but something closer to 80k sounds a whole lot more reasonable given your forced air furnace sizing

A combi unit does not add to the heating sides btu/hr output - The heating side is still limited to the flow rate across the heat exchanger. With a Combi unit you are sizing individually to the DHW need and the heating need.

Crissie

KC_Jones said:

If your house is as tight and well insulated as you claim, 175k could quite effectively heat a house of about 7-8000 sq ft., maybe bigger.  How close to that are you?

Also using the boiler for hot water does not change the boiler sizing.  It does impact control strategy in that the hot water would be on priority.  If there are concerns about available hot water then put in a bigger indirect tank.

Use the slant fin app and find your actual load and work from there.

My house is 2800 sq ft with a 350 sunroom that has a Ptac wall unit. All sizing should have been done on the 2800 sq ft. Yes, it is really tight. It needs mechanical ventilation to add fresh air. The blower door test was recommended by a home performance consultant. It was more of a air leakage test, he went around with an infrared camera to show me where air leakage is occurring. Since then I air sealed my attic, so if anything it is tighter. I recall the report saying it take 5 hours to exchange the air in my house. I wasn't impressed with the blower door test at all. Am incredibly frustrated. Have spoken with many large to medium size HVAC companies in my area. Top rated. It should not be that hard.

To add to this, I had began last March by looking at Mitsubishi ductless minisplits. I had several companies size those units, all did the same thing. The all took measurements, notes on window sizes, insulation and all came back with 120 BTU (two 60 BTU outdoor units or more). The ones that had more BTU's were sizing for zero degrees.

I will say my gas bill for last Jan was $120. That includes a $20 flat fee, cooking, hot water, heat and dryer.
Thanks for all the feedback, I am going to calculate my load. I feel like I need a mechanical engineer because this is a large investment and I am not knowledgeable enough to ensure it is right.

Jamie Hall

No, you don't really need a mechanical engineer. What you do need is a really competent contractor. Where are you located? We may know someone...

Crissie

GroundUp said:

In addition to everyone else saying that at least 3 of them are too high, I will add that heating the DHW adds nothing to the boiler size unless it's a combi unit which you've already stated you are not interested in. A 120k unit may be acceptable, but something closer to 80k sounds a whole lot more reasonable given your forced air furnace sizing

It was a combi unit for the one who quoted it, quoted 170 BTU. Would the system water temp or GPM make any difference in all of this? The 4th contractor is sizing for a 160 degree water temp through all baseboard Slant Fin. Weil McCann GV90, cast iron with a secondary stainless steel heat exchanger. I in Chicago.

Crissie

Jamie Hall said:

No, you don't really need a mechanical engineer. What you do need is a really competent contractor. Where are you located? We may know someone...

Chicago area. Thanks

The Steam Whisperer

My previous home was a 3200 sq ft. two story, with about 2800 sq ft from 1908 or so in Northern Illinois Heavily insulated (2 x 6 walls)and air tightened using the Canadian standards for air tight drywall systems. We had around 800 sq ft of glass, 600 of which was the original windows ( weatherstripped pretty well). Heat loss was about 12 to 15 btu/sqft so we only needed about a 50,000 btu/hr output boiler.
Contrary to many views on this website, a btu is not a btu. The DOE has studies showing the greatly increased air leakage of home with typical newer forced air systems compared to homes with other types of systems. Now if you have really tightened up the home, then this is not nearly as much of a factor. Also , if you are using a system that modulates the output, it should also reduce heat loss by limiting hot air stacking at the ceiling. Radiant heat sources are usually your best bet for efficiency, since a radiant floor systema can reverse the typical temperature gradient from floor to ceiling, making the large ceiling area cooler for lower air leakage and conductive heat loss.

Jamie Hall

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.

Crissie

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?

pecmsg

The find a contractor on this site

and

Also Here
https://www.myhomecomfort.org/find-a-contractor/

GroundUp

CMadatMe said:

GroundUp said:

In addition to everyone else saying that at least 3 of them are too high, I will add that heating the DHW adds nothing to the boiler size unless it's a combi unit which you've already stated you are not interested in. A 120k unit may be acceptable, but something closer to 80k sounds a whole lot more reasonable given your forced air furnace sizing

A combi unit does not add to the heating sides btu/hr output - The heating side is still limited to the flow rate across the heat exchanger. With a Combi unit you are sizing individually to the DHW need and the heating need.

Tell us you've only seen 1 combi boiler without telling us you've only seen 1 combi boiler.....

Crissie

<Slant/Fin's baseboards, why not use Slant/Fin's excellent heating loss calculator here: https://www.slantfin.com/slantfin-heat-loss-calculator/.

Web App doesn't work, it freezes. Download links do not work. Any other app available out there? Thanks!

The Steam Whisperer

Jamie, air leakage rate is largely tied to the operation of the furnace fan. In typical systems that the DOE tested the air leakage rate of the structure doubles when the furnace fan kicks on. If the fan is not in operation the presence of ductwork leads to about a 10% increase in air leakage. This all means that there are a lot less btu's needed at design with systems other than typical ducted systems which should be running the furnace fan continuously on these days. Much better sealing of the ductwork and home probably greatly reduces this leakage.

Jamie Hall

Crissie said:

Download the application to your PC -- the link is near the bottom of the page. Smartphones... aren't always.

Jamie Hall

The Steam Whisperer said:

Jamie, air leakage rate is largely tied to the operation of the furnace fan. In typical systems that the DOE tested the air leakage rate of the structure doubles when the furnace fan kicks on. If the fan is not in operation the presence of ductwork leads to about a 10% increase in air leakage. This all means that there are a lot less btu's needed at design with systems other than typical ducted systems which should be running the furnace fan continuously on these days. Much better sealing of the ductwork and home probably greatly reduces this leakage.

Um... perhaps. But may I ask where the extra BTUs from the leakage go? Of course if he leakage is in an unconditioned space I can see it, or if the system creates excessive positive or negative pressure...

pecmsg

The Steam Whisperer said:

Jamie, air leakage rate is largely tied to the operation of the furnace fan. In typical systems that the DOE tested the air leakage rate of the structure doubles when the furnace fan kicks on. If the fan is not in operation the presence of ductwork leads to about a 10% increase in air leakage. This all means that there are a lot less btu's needed at design with systems other than typical ducted systems which should be running the furnace fan continuously on these days. Much better sealing of the ductwork and home probably greatly reduces this leakage.

Ducts leak
wall and ceilings leak. 

We’re never going to calculate the exact #. 

A properly installed abs sealed duct system will loose no more then a piped hydronic!

CMadatMe

GroundUp said:

CMadatMe said:

GroundUp said:

In addition to everyone else saying that at least 3 of them are too high, I will add that heating the DHW adds nothing to the boiler size unless it's a combi unit which you've already stated you are not interested in. A 120k unit may be acceptable, but something closer to 80k sounds a whole lot more reasonable given your forced air furnace sizing

A combi unit does not add to the heating sides btu/hr output - The heating side is still limited to the flow rate across the heat exchanger. With a Combi unit you are sizing individually to the DHW need and the heating need.

Tell us you've only seen 1 combi boiler without telling us you've only seen 1 combi boiler.....

A little confused. The difference between a boiler and combi is simply a motorized valve and an additional heat exchanger. Once the valve moves into the heating position it's simply a boiler operating at a 12PSI system pressure.

Hot_water_fan

https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new

Use this, you already have the usage data. Bet you come in under 80k. 

STEVEusaPA

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.

EBEBRATT-Ed

@STEVEusaPA

Think you jumped a year there

STEVEusaPA

EBEBRATT-Ed said:

@STEVEusaPA

Think you jumped a year there

Thanks. Fixed.

Crissie

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.

I'm using 11-6-2019 thru 10-7-2020 because last winter I realized my HVAC system was making me ill and had a space heater going all winter.

Zip Code: 60137
Total therms Used over 12-months: 1052
July therms Used: 12.6 (cooking, hot water, etc.)
Net 12-month heating therms used: (1052-(12.6 x 12))=900.8
Furnace AFUE = 96.1 (but I had a combustion gas test done on it and it was only providing 88% efficiency.)
Therms used in coldest month of Jan 2020 less cooking / hot water (199.2-12.6) = 186.6

Hot_water_fan

With a well air sealed house, water heating will be bigger portion than typical so important to include. Remove the summer months, but just for fun, include them as well in a second calc. You can use February, but if possible use a full 12 months and subtract 12 x August from the total therms. Outdoor temp matters, but it’s a linear relationship. So we can take therms/HDD and just apply it to design temperature. 

The Steam Whisperer

Jamie, you've got the idea. I haven't seen any particular studies ( that I remember) but some typical areas I've seen that are liklely leakage pints are when returns are run without ductwork and use stud or joist spaces and those spaces aren't sealed. Another leakage place I suspect is the large gaps around registers in exterior walls and ceilings. Out of balance ductwork systems that are pressurizing or sucking down pressures are going to cause increased leakage.

Pecmsg..... Unless you install a radiant floor forced air system, you will still see increased air leakage in well sealed forced air systems due to higher temperatures at ceilings.....Increased temperature differential between the ceiling surface and attic will always increase air leakage unless the surface is perfectly sealed or the attic is pressurized. Also, with radiant floor systems running much lower ceiling temps than forced air, conductive transfer through the ceiling materials decreases. One of the largest suppliers in Chicago has radiant floor in thier main warehouse and ceiling temperatures run about 10F less that the air temp at the floor. This is with high ( 15 ft?) ceilings.

All btu's are not equivalent......just read the research data.

Canucker

The Steam Whisperer said:

Jamie, you've got the idea. I haven't seen any particular studies ( that I remember) but some typical areas I've seen that are liklely leakage pints are when returns are run without ductwork and use stud or joist spaces and those spaces aren't sealed. Another leakage place I suspect is the large gaps around registers in exterior walls and ceilings. Out of balance ductwork systems that are pressurizing or sucking down pressures are going to cause increased leakage.

Pecmsg..... Unless you install a radiant floor forced air system, you will still see increased air leakage in well sealed forced air systems due to higher temperatures at ceilings.....Increased temperature differential between the ceiling surface and attic will always increase air leakage unless the surface is perfectly sealed or the attic is pressurized. Also, with radiant floor systems running much lower ceiling temps than forced air, conductive transfer through the ceiling materials decreases. One of the largest suppliers in Chicago has radiant floor in thier main warehouse and ceiling temperatures run about 10F less that the air temp at the floor. This is with high ( 15 ft?) ceilings.

All btu's are not equivalent......just read the research data.

I know this is probably being pedantic but I think we're unnecessarily muddying the waters. IMO a btu is a btu but not all distribution systems are equivalent. It's part of the reason I think AFUE is such a poor measure of overall efficiency.

Zman

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...

Crissie

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?

Jamie Hall

"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?"

no, and no