First time sizing boiler options and EDR

Jack M

I'm trying to come up with a backup plan for when my boiler goes. When I measure the EDR of my radiators I get a total EDR of 225 (based on online references). There are two upstair bedrooms that currently have their radiators removed. I would like to add some minimal amount of radiator capacity to each of those two small bedrooms (not a lot) and double the size of one radiator on the first floor from 25 to 50 EDR to accomodate a small (insulated) addition.
If the tag on the boiler says NET I=R=B RATING STEAM 396 SQ. FT. is that boiler too large and I need to consider the next size down? Is the radiator EDR supposed to matched to the stated boiler nameplate "net EDR output" and this number includes the 1.33 SF? Not sure if the loss from piping is added in.

BobC

Ignore BTU rating, use the sq ft of steam. If you have 225 now and you want to to double one rad to 50 and add 2 small ones, Buy a 50 and another 25, you can use existing 25 upstairs for one of those small rooms. That means you need another 50 EDR for a total of 275, that puts you right in the range of a three section rated at 280 sq ft or similar sized atmospheric boiler.

The boiler sq ft includes a 33% pickup factor so you can forget about adding more.

Bob

Fred

The Sq.Ft. of steam rating on your new boiler should match, as closely as possible to the total radiator EDR in your house. (Current plus any additional radiators you actually install).
That 396 Sq. Ft. of steam on that rating plate is NET and the boiler has a 33% overhead factor built into the boiler (over and above that 396) to cover the header, mains and run-outs. You do not want to add anything to the Boiler Net Sq. Ft. rating for overhead.

Jack M

The next size down is MST288 69,000 BTU Output, 288 Sq. It runs a tiny .75 gph burner. Would it be able to get enough hot water off of something this small for an indirect?

BobC

A tank type electric or gas water heater runs 30-40,000 BTU so unless you need a tremendous and continuous supply of hot water I think you will be ok. Others will chime in if they think I'm wrong.

Bob

Jack M

BobC said:

....... you need another 50 EDR for a total of 275, that puts you right in the range of a three section rated at 280 sq ft or similar sized atmospheric boiler.
Bob

275 and 280 are terrifyingly close. I've read Dan's descriptions in article written here (an if The Art of Steam) regarding the terrible inefficiencies that result from undersized boilers. There must be many contractors that have this same concern. Nobody wants to fall short of the mark. What is that boiler just cranks away forever and can't overcome the latent heat loss from the pipes. It would be a disaster of monumental proportion. Larger boilers may cost more to plumb (twin risers) but the unit cost difference is not significant.

Fizz

Just curious, your current boiler rating is 396, how is system performing considering oversized boiler?

Jack M

The current boiler is a Crown UPCS4 Boiler ( Dunkirk) rated at 315 square feet of steam (75,000 btu hot water) running on a Becket RWB burner (like a Beckett AFG) with a .85 nozzle. The boiler is from the 60's and often referred to as a "dog" with a less than complimentary intent. The pressure on the boiler is rarely 1 psi. (American Standard 3 column sectional standing radiators). This is a1500 square foot 2 story New England farmhouse built in the 1920’s. The house is 4” balloon framing with newer window and very little insulation.
I have no complaints about the comfort level of the house's current heating system. It can take a while for the rooms to heat up but that may have been the result of a deeper setback. The temperature swings in the spring and fall are a little extreme as the thermostat shuts down but the radiators keep dumping heat. The domestic hot water loop in the boiler is terribly inefficient way to heat water. Over the last few years, oil usage has averaged 1000 gallons a year. The house sits empty 5 days a week for 8 hours while everyone is at work or school. We have an annual service contract with the fuel delivery company but as I have come to learn recently that service is only for the burner/boiler (nothing to do with the steam system).

Jack M

With all due respect ( I really mean that/ I'm still learning) in the old days the coal boiler ran 24/7 (this house's previous boiler ran on coal/ still has an original hand fired coal stove). I can see how running the .75 gph burner 24/7 would provide enough steam to fill the radiators. But .75 times 24 equal 18 gallons a day. And what about the shoulder seasons of spring and fall? How would you turn the thermostat up to take a chill out of the air on a cool Spring morning (for a hour or so). Wouldn't it take forever? And as soon as it was warm we would walk out the door to go to work and all of the system's latent heat ($) would just waffle around the basement and empty rooms all day providing comfort to no-one. During the day the house would be heated naturally as outdoor temperatures rise. I'm sure I'm missing something but I thought Dan spelled it all out here:


https://heatinghelp.com/systems-help-center/why-undersized-steam-boilers-waste-fuel/

ChrisJ

When I calculated my insulated piping losses, the best I could come up with was around 2,000 btus. It was certainly no where near 31,000 btu which is where 33% would put me.

Currently I am around 10% larger than my radiation but I wish the boiler was even smaller now, but that can't happen.

My system typically runs @ 0.75 to 1" of water column and heats every room perfectly.

When it comes to an oil fired boiler, my understanding was they could be downfired quite a bit by a guy who knows what he's doing. I also don't know how an indirect plays into everything but if it was me I wouldn't go any larger than the MST288.

For your current radiation you need 54,000 btus and the MST288 does 92,000. For 275 sqft that's still only 66,000.

I think 26,000 is likely plenty for an indirect and piping loses. 38,000 with your current radiation.

This is assuming all piping is correct AND insulated well. You may also need to tweak your venting.


Please keep in mind I'm not a professional, just a homeowner obsessed with steam.

ChrisJ

Don't take my word for it, this is a good read on the subject.

Taking Another Look at Steam Boiler Sizing Methods
https://heatinghelp.com/systems-help-center/taking-another-look-at-steam-boiler-sizing-methods/

vaporvac

If you have 275edr and install the MST rated for 288 that would mean you're at least 13edr over-sized, not including the 33% additional included for pick-up. I don't understand why you think that's undersized. When DanH writes about undersizing this isn't what he means. You would be perfectly matched with plenty of excess for HW. The V-stat should only be used as an upper limiting device. The less pressure you build the better and that's determined by correct sizing. Every ounce above the minimum necessary is costing you money.

I have twinned boilers (240/200btus), and after I reach temp from a setback the 200btu boiler drops out altogether on subsequent firings, and continues to do so when just maintaining temp. I've never had more comfortable or efficient heat. You're lucky you can use the MST. C

Jack M

That's an interesting read Chrisj. Looks like you are running on natural gas. Does NG allow you to modulate the burner? How long does it take you to get heat out of the radiators from a cold startup, what about between cycles?

ChrisJ

No no modulating available. As far as time on a typical winter day I get heat to my radiators in 2 to 3 minutes. On a really cold night I've seen 90 seconds. A cold startup is 15 to 20 minutes to get the boiler steaming and then 4 to 5 minutes until heat at radiators. This isn't much different than before I put the smaller burner in.

See the real beauty is how little pressure I build and how quiet my vents are. I can also do large recoveries without problems.

I can assure you my system doesn't waste fuel

ChrisJ

The object Is to heat the house and not produce more steam than your radiators can condense. Having extra doesn't do anyone any good.

Steamhead

Have you done a heat loss on those rooms to see how much radiation they actually need?

nicholas bonham-carter

If the thermostat is left at the desired temperature constantly, you can use a lower setting, than if you are "turning it up to take the chill off".
In this steady state of temperature, a properly sized boiler, with a good thermostat, and a well balanced/vented system will produce enough steam to fill a couple of sections of the radiators on the cool spring day, or all the sections on a sub-zero night. This will produce real comfort, as you absorb the radiant heat from the rads.
Contrast this with a forced air system, which only heats the air, and where you either feel hot, or chilly.
Just like driving a car on a long highway, you should gradually get up to the speed limit, and use cruise control to keep it there, (Except when there is ice on the road!!!).
Sizing the boiler to the radiation will always yield the proper results.--NBC

Jack M

Steamhead said:

Have you done a heat loss on those rooms to see how much radiation they actually need?

I did a heat loss for the house.

I used this tool:

Jack M

nicholas bonham-carter said:

If the thermostat is left at the desired temperature constantly, you can use a lower setting, than if you are "turning it up to take the chill off".
In this steady state of temperature, a properly sized boiler, with a good thermostat, and a well balanced/vented system will produce enough steam to fill a couple of sections of the radiators on the cool spring day, or all the sections on a sub-zero night. This will produce real comfort, as you absorb the radiant heat from the rads.
Contrast this with a forced air system, which only heats the air, and where you either feel hot, or chilly.
Just like driving a car on a long highway, you should gradually get up to the speed limit, and use cruise control to keep it there, (Except when there is ice on the road!!!).
Sizing the boiler to the radiation will always yield the proper results.--NBC

I can follow the logic of this in the middle of winter. Don't mess with the thermostat.
But in the Spring the outdoor temperature could be 60 degrees all day long. I set thermostat at 66 and leave for 8 hours; during those eight hours the boiler would cycle on and off any number (X) of times to make up for all the heat loss out the leaky windows to keep the house at temp (66) and not dropping to 60. Probably 20 minutes each cycle because the each boiler run would be to spread out (in time) from the last cylce (the house is not cooling as rapidly).
If on the other hand, I turn the boiler off, it will not run at all during the eight hours. When I arrive home the house's indoor temp has dropped to 60 degrees (the outdoor temp). The boiler is turned on and after the first 20 minute cycle the rooms are headed back up to the desired 66.

In the second senario, the boiler ran once. In the first senario (steady state) the boiler ran 1 plus X times. So I saved X. Is there an error with this logic?

What if the windows and doors were left open for those eight hours during the day?

Fred

The biggest error in this logic is that, if the house is 66 when you leave home, the typical house (at least including mine) probably takes more than eight hours to drop to 60, on a 60 degree day. My boiler probably won't even come on during those 60 degree days. I can't speak for your house but mine has no insulation in the outside walls, but it is brick, has the original single pane windows, no storms but the mains are insulated and I leave the tstat set the same throughout the winter and shoulder months.
We're not talking about undr-siing your boiler, we're talking about right-siing it. Matching the EDR to the Boiler output is the right thing to do and you still have the 33% oberhead to play with. Can you get by with an over sied boiler, sure. Is it the right thing to do from an effeciency perspective and from a cycling perspective, definitely not.

KC_Jones

You are getting into a topic that has been discussed on this site many many times. First question (based on your scenario) what makes you think the boiler is going to run a bunch of times on a 60 degree day? I have mine set to not run at all above 61 OAT and the inside stays at 69-70 all day long. My house is 100+ years old not insulated original windows etc. The warmer it is outside the slower the heat loss. Another thing to think about if your house is that drafty you will never get it from 60-66 in one cycle. What will most likely happen is it will run almost the same amount of time to raise it back up as it would have just to keep it at that temp all day. This is the part that gets controversial. What I have concluded after reading about this extensively is do what you want and every house is different. If you are seriously worried about cost set the heat to 50-55 all the time and leave it there. If you are worried about comfort set it to what makes you comfortable and do that. IMHO the heating system is for comfort so my primary concern is human comfort and whatever that costs is what it costs. I will try and make that cost as low as possible, but I honestly don't let it dictate the comfort level of my family. Again if I did that I would set it to 50-55 just to keep the pipes from freezing and hand out blankets. If your boiler sizing concerns are because of coming out of setback that is a none issue. Size the boiler to the rads, it doesn't matter how big you go on the boiler the rads have a fixed output on steam so the bigger boiler is just wasting fuel AND NOT giving any additional heat. This is why you size them to the radiators. Additionally if you are worried about economy, tighten up the envelope that is the best bang for the buck. Just my $0.02 worth.

Fizz

I'm with NBC on this. For yrs my wife would turn t-stat off when not home, or felt too warm in spring/fall. Very uneven not comfortable. Now tstat set at 72 in early fall and never moved. Comfort, comfort, comfort! I really believe you use less fuel. Just wondering, if you leave t-stat at 66, thinking if it's 60 outside, your not going to get many cycles during day. Also, if it's 60 and stat is off for 8 hrs on 60 degree day, wondering why indoor temp is = to outdoor temp?

Jamie Hall

You know, I thought that I would try to contribute... but... not much I can, since "your results may vary" applies to heating more than almost anything else.

A couple of thoughts. Just to mess things up,

First -- EDR is not a constant. The actual condensing capacity of a radiator is a function of the temperature of the space which it "sees", and can increase substantially if that space is cold -- such as recovering from a setback.

Second, keep in mind that if a boiler is significantly undersized, it won't be able to get steam to all of the radiation. I don't care how good your venting is; there are going to be radiators which get left out. Is this desirable?

Third, a boiler which is significantly oversized is going to be inefficient because of cycling once the radiation is satisfied. The inefficiency is not that large, but it's there. No question.

Fourth, as has been noted, the problems are worse in the "shoulder" seasons, but at least part of the reason is not, perhaps, as obvious: the time between starts is long enough that the radiation has cooled off. This reduces the perceived comfort level. This, incidentally, is one of the better arguments for forced air: the thermal inertia in the heating/cooling system is almost nil.

I have no answers. I don't even have good advice -- except that the individual who has to live with the system needs to do what he or she finds makes them comfortable!

ChrisJ

And you thought this was going to be easy. Didn't you?



But one thing is consistent between all of us.

An oversized boiler is BAD.

I say skip the 33%
Others say keep it but no one is saying go above it.

I should mention, with my way your radiator vents will last a lot longer because they will practically never even see steam.

ChrisJ

Hatterasguy said:

Jamie Hall said:

Second, keep in mind that if a boiler is significantly undersized, it won't be able to get steam to all of the radiation. I don't care how good your venting is; there are going to be radiators which get left out. Is this desirable?

Third, a boiler which is significantly oversized is going to be inefficient because of cycling once the radiation is satisfied. The inefficiency is not that large, but it's there. No question.

I do believe we need to narrow the definitions.

"Undersized" is a boiler that is sized below the actual EDR of the radiation without any pickup factor.

"Oversized" is a boiler that is sized above the actual EDR with included 1.33 pickup factor.

I think we can all agree that nobody wants an undersized or oversized boiler based upon the above definitions.

Now, we can all argue until the cows come home whether the boiler needs a 1.0, a 1.1, a 1.2, or a 1.33 pickup factor.

The smaller factors will probably be a benefit to those with proper piping arrangements that don't have 100' radiator out all by itself as they prevent any cycling in the cold ambients.

Not that simple.
I was just telling KC that if my boiler ever fails there's a good chance I'd go with an MST288 with a gas gun. That's 91K output and my radiation is 94K so it's 3K below my radiation but still 11K above my houses requirements.

The next step up is just way to stinking big.

Jack M

Fizz said:

... if it's 60 and stat is off for 8 hrs on 60 degree day, wondering why indoor temp is = to outdoor temp?

Fizz; I was just assuming that after 8 hours the heat loss of the house would be sufficient to bring the indoor temp down to that 60F. Just an assumption (maybe not a valid one).
There is a lot of useful information here that will help me get my ducks in order when this boiler decides to go. Tightening the envelope is going on that "to-do" list. From what I've learned here and comparing my existing boiler, I can see why the MST288 is the best fit (of the sizes offered). I did not know that Burnham had certified at a gas gun for the MST288 (that's a huge bonus for the future).
Jamie, thanks for keeping things interesting. I grabbed a copy of "Greening Steam" to see what else I can learn.
I've been studying Chris's system. He is truly "at one" with the Steam Gods. I'd be interested in fuel usage data should anything become available.

Fred

Just so you know, the Burnham MST has not been certified for natural gas. Chris said that's the way he'd go I guess because he won't let the loss of warranty stop him from doing that.

ChrisJ

Fred said:

Just so you know, the Burnham MST has not been certified for natural gas. Chris said that's the way he'd go I guess because he won't let the loss of warranty stop him from doing that.

Very true.
A few others have done it as well but my guess is Jack M intends on using this for oil.

Jack M

ChrisJ said:

my guess is Jack M intends on using this for oil.

Fred said:

Jt my guess is Jack M intends on using this for oil.

Would a direct vent installation provide better control over the fire? The draft on a chimney can be such a wildcard. I would think that a direct vent installation would tighten the performance of the burner. A modulating flame would seem to make sense as well, but I don't hear about those for steam. Too bad Bulderus does not get into the steam game. I think that natural gas is in my near future.

ChrisJ

I don't know of any direct vent steam boilers? If you're going to be switching to gas in the future the megasteam would be a bad choice as you will likely have issues finding someone willing to install a gas gun in it. I think Smith is commonly used with gas guns

RobG

Where are you located? The most important thing about any boiler installation is the contractor who is putting it in. It is good to use due diligence prior to the install but the right contractor is the key.

SWEI

ChrisJ said:

I think Smith is commonly used with gas guns

Smith is no longer Smith (for residential sizes.)

vaporvac

The Slantfin Intrepid is approved for a gas gun.
There are modulating burners as well as lo-hi-lo burners for gas power burners (Midco and Powerflame), but it depends on the size of your boiler. MarkS has the Midco Modulating. It's very expensive. The dual-stage burners aren't much more expensive than a single stage and would be the way to go if they come in your size.
I'm not familiar with options for oil burners. The MST is a superior boiler by DESIGN; that's why some folks have installed the gas gun without approval. I had a hard enough time getting my approved burner installed, so I wouldn't be inclined to go that way.

Over sizing costs more money to run and may not deliver the heat as well as you think; it's constantly cycling on and off as it builds pressure. If you raise the pressure it makes the heat delivery even slower and, costs significantly more since the water now needs more BTUs to make steam. I think you now get the picture, just adding additional thoughts. C

Jack M

I guess I was refering to "power vent" when I mentioned "direct vent" Some have said they are noisy. Looks like they would work for a Megasteam installation.

ChrisJ

I've heard a lot of guys say power vents for oil burners are a disgusting disaster. It's also noisy without a doubt.

If I was you, I'd stick to a chimney. Just remember to have it inspected and cleaned from time to time.

vaporvac

What is your objection to a chimney vent? Be aware that if you switch to gas you will need to line your chimney, if it isn't already.

Jack M

With a boiler approaching 86% will there be condensation issues. I'm guessing my current 500 degree F exhaust gasses are not doing a lot of condensating before they leave the flue (center chimney).
But with a Megasteam is that going to be an issue? The need for a SS liner? I've had the chimney inspected and while its a masonry chimney it's not a basket case. It is not currently lined.

ChrisJ

I don't believe 86% has any issues with condensation.
If / when you switch to gas though you will likely have a lot of nasty black stuff running down out of the chimney for a long time.

At least that's why my chimney guys told me before we ended up realizing my chimney was improperly lined and couldn't be lined correctly. If condensation is a concern of yours, installing a proper size SS liner, or even an insulated liner will help greatly.

EBEBRATT-Ed

225 EDR =54,000 btu/hr. Your heat loss shows 45,122so you have the correct radiation. I vote to keep the 1.33 pick up factor especially if you use night set back and to help cover your domestic hot water load.

ChrisJ

EBEBRATT-Ed said:

225 EDR =54,000 btu/hr. Your heat loss shows 45,122so you have the correct radiation. I vote to keep the 1.33 pick up factor especially if you use night set back and to help cover your domestic hot water load.

He's got 8878 btus extra and adding extra boiler doesn't make your radiation produce more heat. You'll get a little extra from the extra pressure, but not enough to justify it in my book.

I'd still go for no more than 10-15% extra.

EBEBRATT-Ed

If you can heat the domestic with 8878 btu/hr and come out of night setback at the same time then go for it.

This is a family that likes playing with the thermostat. If you leave the thermostat alone you most likely could get away with 0% pick-up with no DHW load. I usually would ignore the DHW load (as long as it is a "normal" load) if the PU factor was in there.

IMHO coming out of set back with a dhw load ---some allowance needs to be made. Guy gets up, turns up the stat and jumps in the shower