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# Emitter Surface Area unexpectedly tight

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Member Posts: 83
I'm trying to understand my system, as my modulating boiler is nearing EOL and I want to know what my options are when replacement time comes.

I have a 2700 sq foot 1895 house that has insulation in the attic roof. I live in climate zone 4, and online calculators estimate I need a 133k BTU furnace (current one is 140k). (I know it's not a manual j, but I'm just spit-balling here for now, trying to get a sense of things).

I want to know if I can drop the temps on my supply. My estimate is that I've got 645 sq ft of Cast Iron radiator surface area (which is basically EDR? I don't understand the difference).

I was expecting to have excess EDR (due to original design for poor insulation and somewhat recent improvements in insulation), but, if I'm understanding correctly, I need to get 206 BTUH/SQ Foot, which would put my source temp at 190 (or, if I'm understanding this post correctly, that actually needs to be my average temp).

Is my math right here? If so, I guess the idea of getting a heat pump is out until a bucket load of new emitters are put somewhere in this old house.

• Member Posts: 2,831
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Hot water of steam system ? I am confused , we calculate EDR for sizing a steam boiler and figure a heat loss calculation for sizing a hot water system .

There was an error rendering this rich post.

• Member Posts: 22,476
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2700 sq ft X 30 btu/ sq ft would be 81,000 btu/hr required.

Maybe try this calculator

https://slantfin.com/slantfin-heat-loss-calculator/l

Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 8,167
edited January 2023
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Big Ed_4 said:

Hot water of steam system ? I am confused , we calculate EDR for sizing a steam boiler and figure a heat loss calculation for sizing a hot water system .

You are absolutely correct Ed. However I believe @North_by_Northwest or whatever the OP's Moniker is, Is trying to see if he can operate at a lower temperature by taking the heat loss calculation and working backwards to the room BTU requirement and compare the EDR to the equivalent BTU output at said lower temperature. Their math is coming up short because 190° water temp is much higher than they expected.

Edward Young Retired

After you make that expensive repair and you still have the same problem, What will you check next?

• Member Posts: 8,167
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I'm trying to understand my system, as my modulating boiler is nearing EOL and I want to know what my options are when replacement time comes...

...Is my math right here? If so, I guess the idea of getting a heat pump is out until a bucket load of new emitters are put somewhere in this old house.

A heat pump is not out of the question. You should be able to get 140° water from your air to water heat pump or even higher geothermal water to water heat pump. if there comes a time when you need higher than 140° or 150° water in the radiators, you could have an inexpensive gas operated boiler. Nothing fancy, just a standard cast iron or steel boiler with an atmospheric burner operating at a little over 80% AFUE will operate less than 5% of the winter.

Think about that! Geothermal may even get you to 160° water temp.

Edward Young Retired

After you make that expensive repair and you still have the same problem, What will you check next?

• Member Posts: 83
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ok, so that’s my misunderstanding of EDR. It’s hot water.

@EdTheHeaterMan is correct; I’m guesstimating load and seeing how my radiators size up at different temps. And temps are a bit higher than I expected.

I’m urban, so I don’t think geothermal is going to happen.

@hot_rod those are how I calculated radiator size; the load calculator link seems to be Busted. I can’t find the app on the iOS store and the link for the web based version seems to have a broken link.

Given the responses here, I think part of my issue is that I looked up “how big of a furnace do I need” and assumed that was the same as my heat loss.

Sorry for my newbie-ness and resulting confusion.
• Member Posts: 9,959
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you can calculate output of an emitter from edr and water temp.

i think whatever heat loss calculator you used is defective, i think you need to find a better calculator or some of your assumptions were wrong.

your current boiler isn't running constantly on the coldest day, is it?
• Member Posts: 22,476
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if you want to go "old school". All the various calculators are based on Manual J. I think ACCA, the keeper of manual J has a hand calc sheet download also. Although it may cost \$

The load calc number needs to be accurate, else all the other questions are moot.

Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 8,167
edited January 2023
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I'm not sure this link will work but let me know. I took the old I=B=R heat loss form and put on a sperad sheet on my Google Docs account

Sheet 2 at the bottom has the design temperature list.

Edward Young Retired

After you make that expensive repair and you still have the same problem, What will you check next?

• Member Posts: 83
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@mattmia2 yes, I just used a “how big of a furnace do I need” calculator and assumed that was my heat loss.

No, furnace isn’t running all day. We were in the single digits last week and I setback from 65 to 57 during the day. By 4:30, I was still at 60 (might’ve even been 62). So that was the loss without the furnace running for 10 hrs.

So yeah, some assumptions are wrong somewhere. I pulled out my wife’s old broken android and got the slant fin app and will proceed from there.
• Member Posts: 9,959
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If it was able to recover from a setback in single digits then your heat loss is significantly less than the size of the boiler, assuming your design temp is around 0.
• Member Posts: 83
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@mattmia2 just learning about this; I’m in Philadephia, Pa. Haven’t had time to dig in yet, but the first thing I doing online said design temp was 14. But that was just a comment on a forum somewhere (not here)
• Member Posts: 8,167
edited January 2023
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I grew up in Philadelphia. I drove an oil truck thru Olney, Logan, Germantown and Roxborough. It gets colder than 14 degrees. But that is what the load calculations say you should use for that area. If you think about it, there are so many fudge factors in the sizing of boilers, there is the 15% piping and pick up factor. And you never do a load calculation with exact numbers like 14 feet 5 inches. That always rounds up to 15 feet. do that with 10 rooms and you have a few extra BTUs, And boilers never fit exactly. If you come up with a 72,483 BTUh heat loss, nobody ever makes a heater that size. You always get the size higher than that like the AHRI Net rating of 84,000 BTUh boiler. So there is usually enough extra to account for those 3 or 4 hours in the middle of the night where the temperature might drop to 3°F every 4 years or so.

Just so you know, I used ∆T of 70 degrees for Philadelphia and South Jersey and still came up with really small furnaces and boilers compared to what I was replacing. So if my load calculation was 83,374 BTUh then I was comfortable installing that 84,000 BTUh NET rated boiler.

Edward Young Retired

After you make that expensive repair and you still have the same problem, What will you check next?

• Member Posts: 9,959
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Philly is that much warmer than detroit? on dec 23rd it never got above 0.(although our design temp is 6 or 7f) so it was below 0 for 20 hours or so.
• Member Posts: 22,476
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There is a bit of a fudge factor in those load calcs also. Even if you miss by a small amount it would mean on a few days a year your indoor temperature may drop a few degrees. Maybe you maintain 68 instead of 70. You could supplement, or wear a sweater those days, IF that even happened.

If you hit design exactly, build. A system to that load, what happens if you get a week of below design temperature?

Load calcs have some assumptions and the load on a building can change, even second by second. A door opening, strong wind gust, internal gains, etc

Don’t make yourself crazy trying to get the load to an exact number. You may never see that condition. And you may see conditions below the lowest temperature you plug in
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 83
edited January 2023
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@EdTheHeaterMan I just finished a load calc with the slantfin app that puts me at 85550. (and my furnace is 140k...). I've got 670 sq feet of radiators (well, one I need to replace; previous owner took it out and never replaced it. But I've calculated for the biggest I can fit in there, which will be necessary to heat the space).

So, If I'm understanding correctly, to maintain 68 degrees in the house:
85550/670 = 130 BTUs per sq ft.

And, if I'm understanding correctly from this post, that means my avg. temp needs to be 160.

Although I'm a bit confused, because the graph that @hot_rod linked makes it look like I need 190 degree water (maybe that graph lists not average but supply-side temp).

Assuming 160 average will work and I don't have a gauge for my return side temp; I crank the heat up and hit the return pipe with an IR thermometer to get my return temp, then work out my average and set my furnace to max out at (160x2) - return temp. Correct?

If that is the case, I assume my return will eventually get closer to 160 as the rads, etc heat up. How long should I wait before checking my return temp?

(I also have an outdoor reset being shipped right now).

I've got some long cast iron runs in the basement; the supply is insulated (although it looks due for replacing) but the return is not; would it worthwhile to insulate my returns as well? Figuring higher return temps mean I can lower the output temp to increase the average.

Please let me know if I'm getting the cart ahead of the horse here.

And, for future reference, my 140k furnace seems like overkill, does it not? Is there something I'm missing?

EDIT: Does Manual J (assuming that's what the Slant/Fin app is doing) account for heat rising? Looks like all my third-floor rads, except the bathroom, are undersized vs the rest of the house.
• Member Posts: 8,167
edited January 2023
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EDIT AT BOTTOM
First I need to clarify that a furnace is connected to ductwork and a boiler is connected to a radiator. Just one of my per peeves. Many make that mistake and it causes confusion sometimes.
Your 140,000 BTUh boiler has three different BTU ratings.
If 140,000 is the input, then the DOE output and the AHRI Net are both lower than 140,000.
Where are you getting he 140,000 number from? Input, Output or Net
Input is for selecting the gas pipe sizing (or the nozzle if oil heat)
NET is what you want to use for matching the Heat Loss Calculation.

Your calculation for 160° average water temperature sounds correct. However that is based on design conditions (what I used to call "the coldest day of the year"). You don't get Design Conditions all winter so your water temperature in the radiators may be cooler, as long as you maintain 140° return temperature on "Non Condensing" type boilers. Your near boiler piping will determine if you can run the radiator temperature lower than 140° and still maintain 140° at the boiler return. Usually there is a bypass pipe (from the supply to the return) of some type involved.

As far as your boiler being overkill... If the 140,000 is the input, then your NET may be as low as 90,000. Not too far off from 85,550.

EDIT
As I remember, you are interested in generating 140° water temperature from an air to water heat pump. So you never need to worry about condensation of flue gas at temps lower than 140°if you go that route. Once the outdoor temperature drops enough that requires a higher radiator temperature, the inexpensive (non modulating) conventional boiler will be able to take that 130° return water and increase it as needed to cover the rest of the load for that "less than 20% of the winter". You just need a good control logic design.

Edward Young Retired

After you make that expensive repair and you still have the same problem, What will you check next?

• Member Posts: 22,476
edited January 2023
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You can find weather data going back 20 years or more to make some predictions. Put it into a graph form like this, to get a good visual.

In this part of NY, using a design day of -5, notice how may hours you are at or below design. As @EdTheHeaterMan mentioned, 80% of the heating season you will supply lower temperature to the radiators to cover the lower load.

Nice to have a modulating boiler and ODR control, knowing this info.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 83
edited January 2023
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Ah, great! Thanks for clarifying furnace vs. . Boiler. 140k btu max input, per the sticker on the side.

It’s a modulating boiler, so my understanding is that, ideally, I want my return temp below 130 for max efficiency. And lower is better, if I’m not mistaken, correct?

I think this is the bypass your taking about, correct?  (Valve in center)
I was told in another thread that that may
be useful to leave partially open for furnace longevity,
as this furnace would prefer a primary/secondary piping (which I don’t know anything about). My basic understanding is that it’s there to ensure flow, but I don’t really know.

As far as undershooting 160 avg temp to save a bit because it’s infrequently needed; outdoor reset I have ordered will manage that automatically, correct?

(This is building does have a rental unit on the same boiler, so I’m hesitant to have it set to struggle on cold days).

Still not sure how to figure out how to figure out where to set my output temps to get an avg temp of 160 at 14 degrees outside temp. Is it just a guessing game?

Or, I guess I should just wait till my ODR arrives and set the curve so that 14 degrees outside is 160 degrees.

• Member Posts: 22,476
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I'd put it at 87% so 140,000 X 87%= 121,000 output. On a good day.
The more it runs at condensing temperature the better the efficiency, into the 90% perhaps.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 83
edited January 2023
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Please pardon my ignorance; Are you saying I should set my temps so the rads are kicking out 121,000 BTUs?

I'm not sure how to do that without knowing what the average temp of water in my system is. The example in the idronics article assumes an avg. temp of 105; with that calculation, I'd need water temps of about 185 to get 120000 BTUs.

Should I just run with that and adjust until it's comfortable?

(assuming the average temp of 105), I thought I'd be able to run at 160 degrees to get the BTUs I need.

Also, where can I get these outdoor air temperature bins for my area?
• Member Posts: 22,476
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Three things should ideally line up. The boiler should be is sized
To heatload, and the radiators sized to the boiler output at the highest temperature you want to run the boiler

Good news is you have a modulating condensing boiler, although not piped properly😳

If you are comfortable with the heatload calc, then add up the radiator area, see what supply temperature you need to run to get the output from the radiators to match the heatload.

If all works out, the boiler runs up to that temperature on the coldest days.
With outdoor reset enabled, the boiler will modulate itself to meet the decreasing load as the weather warms

The temperature drop thru the radiators is based on the flow rate you circulator provides, 20 is typical.

So your load is 88,000, the boiler ca n provide 121,000, that if fine.
From the radiator charts, how hot do you need to run to provide 88,000 btu/ hr from the radiators.?

Set the boiler high temperature at that number. Set the ODR at 1 to start.
So every degree the temperature drops the boiler increases 1 degree
You may need to adjust that ratio to get the exact comfort you desire. Read the boiler manual to se3 how to set and adjust the ODR
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 83
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Thanks, @hot_rod

If you are comfortable with the heatload calc, then add up the radiator area, see what supply temperature you need to run to get the output from the radiators to match the heatload.

The part about this that confuses me is that the calculations I've used (one from the hydronics efficiency article, the other from the BTUs per avg. temp post) both require me to know what the low temp is

The temperature drop thru the radiators is based on the flow rate you circulator provides, 20 is typical.

Here I assume you mean that the average temperate drop is 20, so I can use that as a rough guess when running my calculations in above paragraph. I think I've seen some

I just want to ensure that you don't mean that the flow rate is typically 20. That idronics article is listing flow rates from 0-6 GPM, so I doubt that's what you mean.

From the radiator charts, how hot do you need to run to provide 88,000 btu/ hr from the radiators.?

From idronics article pg. 34, examples given for chart 3-31: (avg. temp - room temp) = X (and then refer to the chart for how many BTUs you get out for value X).

If I use 148 as the average temp, then I get 82000 bts.

So, if I'm understanding you and Delta-T of supply and return is typically 20, I need to run at 158 (let's say 165 to be safe). Correct?

Just watched a video on primary/secondary piping and want to go see how complicated it would be to switch it over. Cracks me up that the two previous owners basically ran this boiler for it's entire life cycle without taking advantage of any of it's efficiencies.
• Member Posts: 22,476
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Hydronic design software make this easy, or draw it out, maybe like this

Now you know if radiant floor is possible, or how the radiators match the load, and radiator output in each room.  I would do this room by room, not adding all radiators into one big number

it will show you where rads may be too small, or too large. Or if radiant floors are even feasible

when you predict heat output from any heat emitter you use AWT to be most accurate

if you supply 180, return at 160, running a 20 delta, use 170 to get the actual radiator output if radiators are in series, temperature to the second is now 160, AWT 150

the delta t that you chose is just for the sizing of the emitters, pumps, pipe etc. the system delta will move around as the space warms

10-15 is used for radiant floor
20 is common for fin tube

the wider the delta, the lower the return, the higher the boiler efficiency

But you need to make sure you still get adequate btu output at the wider delta, lower AWT

so you juggle numbers based on all this input. the software programs do it instantly as you change any number

as you shoot for lower temperature operation, the size of the heat emitter needs to be increased, possibly

this is why radiant walls, ceilings , floors can work at low AWT, your heat emitter surface area is very large
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 83
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Ok, cool. Yeah, I've done a room-by-room calculation. Looks like I should probably get my reset curve so at 14 degrees (my design temp) AWT is 170.

At 170 all radiators except the entrance hall (-25%) and the third floor front (-5%) are putting out enough btus. There's excess heat coming out of all the other radiators on each floor, so I'm hoping it compensates.

At 160 AWT, all floors (except the third) have enough BTUs, but not all rooms.

It's an old victorian with the stairwell in the entrence hall; thermostat is on second floor of the stairwell. Probably about the worst place for it, except for the fact it'll be the last place to come up to temp (there isn't even a radiator in the hall on that floor, or any doors that stay open).

Then, next summer, I'll put in some TRVs to kick those overpowered radiators off.

I think my plan here is to assume a delta of 30 and set my high water temp at 180 (so AWT is 165). And maybe my delta shrinks as the system warms and I've got some extra BTUs.

• Member Posts: 22,476
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Use one of the new ECM variable speed circulators. Then you can fine tune by both temperature with boiler settings, and flow rate with the adjustable circulator
Get a delta P version for when you install TRVs. Grundfos has a new 15-58 Alpha out that has some nice features for fine tuning flows.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 8,167
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Ah, great! Thanks for clarifying furnace vs. . Boiler. 140k btu max input, per the sticker on the side.

the I=B=R number is the older NET rating. On a new boiler is will be called AHRI Net. Same number, different name. All these alphabet organizations keep changing their names.

Edward Young Retired

After you make that expensive repair and you still have the same problem, What will you check next?

• Member Posts: 83
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ok, a few questions again;

- Are dynamic TRVs available in the states yet, or are they only available overseas? sounds like the way forward.

- i’ve currently got a taco 007. so i assume a 007 ECM delta p circulator will be a drop-in replacement (if the 007 comes in delta p; i haven’t looked into it yet). seems like previous owner used a plumber for this work; i’m not sure when to call a plumber vs boiler tech (and, if it’s bolt-in and accessible for a moderate to highly skilled DIY-er, i could do it myself)

- i’m hoping that installing trvs next summer will solve the front/rear heat imbalance (which is currently controlled via what i think is called a ball valve. manual control). previous owner advised me that he was advised to add some circulator pumps somewhere to resolve that problem (he doesn’t remember where).

but. maybe this is all pointless because i’m hesitant to dump \$1000 of trvs in here if this dies in a few years because i’m guessing i’ll be replacing it with a high-velocity forced air system.

i’d love to keep the radiant heat and radiators, but a cursory search tells me that air-to-water&air heat pumps are still a ways off. (then again, i’m not sure how efficient a heat pump would be at the water & outdoor temps i need, meaning i’d need a backup NG boiler… but then i’d have two separate heat pumps if i ever wanted AC, unless air-to-water&air becomes viable).

maybe i’ve just been too strongly influenced by @hot_rod s radiant efficiency article posting.

• Member Posts: 9,959
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This system is all cast iron radiator, right? There isn't some fin tube or somethin mixed in? You should be able to balance it by closing the valves on the radiators that heat first some as long as the load on the house doesn't vary differently in different parts of the house.
• Member Posts: 104
edited January 2023
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but then i’d have two separate heat pumps if i ever wanted AC, unless air-to-water&air becomes viable

Why wouldn't it be viable?

You don't need two heat pumps. You just need someone who knows what they're doing. Air to water heat pumps and hydronic fancoils are common enough to entertain your ideas.

• Member Posts: 8,167
edited January 2023
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oops

Edward Young Retired

After you make that expensive repair and you still have the same problem, What will you check next?