Hot Water Baseboard Heating

Mike Trzcinski

If a hot water baseboard heating system is designed (at design conditions) for a water "delta T" of 20F, what can I expect the "delta T" to be at when the outside temperature is warmer than design?

For example, if it is -7F outside, and the supply water is 180F and return is 160F, then the system is on-design. If the outdoor temp. rises to 35F, the boiler controller will increase the water supply temp. to say 160F, if the water is returned at 150F (delta T of 10), is this normal?

Also, say we double the gpm of the system. On paper, this will half the "delta T". In actuality, will the same amount of heat be given up by the baseboard?

Mike T., Swampeast MO

Output of fin b/b is determined by supply temperature, rate of flow and temperature of incoming air (at the element).

If the boiler is set to supply X temperature regardless of outside temperature, delta-t will remain nearly constant with changing outdoor temperature unless you change the flow rate or temp of incoming air.

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Some simple formuulas to help:

To determine rate of flow when you know output and delta-t:

gpm = Btuh / (500 * TD)

Btuh can be determined from output charts (given supply temp) for your baseboard model. TD is delta-t.

To determine delta-t when you know output and flow:

TD = Btuh / (500 * gpm)

To determine output when you know flow and delta-t:

Btuh = 500 * gpm * TD

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To answer your question about doubling the gpm:

"Typical" b/b ratings (¾" element, 180° average supply, 65° incoming air) show:

550 btuh/lineal foot @ 1gpm

580 btuh/lineal foot @ 4gpm

If we quintuple the flow rate and use above formula to calculate delta-t we get:

1.1° delta-t = 550 / (500 * 1)

0.29° delta-t = 580 / (500 * 4)

1.1 / 4 = 0.275 so delta-t is about quartered, but not exactly. Why not exactly? Because heat output increased slightly. (Sorry, but I don't have tables that include outputs for a 2x flow rate.)

Generally output is calculated at the 1gpm flow rate unless you are ABSOLUTELY certain that flow is greater than or equal to 4gpm, so it IS generally assumed that 2x flow corresponds with ½ delta-t.

Higher flow allows a greater length of baseboard to be served but with a series loop you MUST recalculate the individual elements with the new delta-t and incoming/outgoing temperatures to achieve reasonable balance. Since the added baseboard increases head loss you have to recalculate to ensure that the circulator will actually be delivering flow at your assumed rate. There is also a limit to the velocity through pipes in a heating system, so you can't just continue to increase flow to allow longer loops. 4 gpm is generally regarded as the maximum flow for ¾" copper; 1½ gpm for ½ copper.

hr

Actually & a correction

the faster the flow through the board the higher the output. Of course their is a point where the system will get noisey if the flow is to high, and potential wear to the tube and fittings at higher FPS velocities. Generally baseboard is rated with a 1 or 4 GPM flow. About 30 btu/ foot more at higher number.

The formula Mike indicated will show this. If a fluid were supplied to a string of baseboard at 180 and left the end at 160° the average temperature would be 170° Look up the bb output at that temperature.

Now if you increase flow and the supply is 180 but the return is 170, the average temperature is 175°. So a bit higher output at higher flow rates. You will see this in baseboard output tables.

hot rod

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R. Kalia

> If a hot water baseboard heating system is

> designed (at design conditions) for a water

> "delta T" of 20F, what can I expect the "delta T"

> to be at when the outside temperature is warmer

> than design?

>

> For example, if it is -7F

> outside, and the supply water is 180F and return

> is 160F, then the system is on-design. If the

> outdoor temp. rises to 35F, the boiler controller

> will increase the water supply temp. to say 160F,

> if the water is returned at 150F (delta T of 10),

> is this normal?

>

> Also, say we double the gpm of

> the system. On paper, this will half the "delta

> T". In actuality, will the same amount of heat be

> given up by the baseboard?

Empire_2

Loop Hot water heating

You can find all the answers to you Q: in this web sight. At a Supply of 180* and a return of 160* your average water temp is 170* at your -7 design temp. This will give you:Exmp: 540 BTU/linear ft at 3/4 " baseboard. This is at 4 gpm.If you are using 3/4" pipe. By decreasing the boiler temp to 160 on a 35* day and a return of 150* your avg.temp will be 155* this gives you roughly 140/btu/linear ft. Since the heat loss is less on the room or structor, your baseboard will give up less heat due to the lower avg. water temp and you can have a Del "T" somewhat lower.
By increasing the GPM,Doubling it, you will get the same amount of heat, (BUT) for exmpl: On a 3/4" line you will be increasing the Volocity big time. Sounds like you are having a problem with a particular zone or circuit... You have to consider volocity noise depending on the pipe size you have. I hope this helped and I'm sur that the guy's will correct me if I am incorrect in my calculations. I NEED A BEER.......................felt like I was back in school..Good Luck
Mike T/ E-Mail me m2112@adelphia.net My name is also Mike T. Boiler Question........

Mike T., Swampeast MO

Thanks Hot Rod

Forgot to mention that the average temp increases (thus increasing output) with higher flow/lower delta-t. Duh!!!

Thanks!

Don Walsh

° sign - how to do it

This is NOT a criticisim! I just want to pass on the method of creating the "degree" symbol (°) when it is desired.
Press and hold down the ALT key, while holding the ALT key down, type in the numbers 0176 on the keypad. The result is ° nice to see next to a number being related in degrees. For example 180°F supply, Delta T of 20°'s equals return temp of 160°F
Use it if you wish, but I intend this only as a helpful interjection.... Grumpy

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Ken_8

You're blowing my mind!

°

I'll be a monkey's Uncle!

Lookadat woodja!

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Weezbo

212

21
212 hmmm... Degree @212 212
the alt 0176 thing aint getting it:)))

Weezbo

Oki i try it again 212¡ã Hurray! wrong number keypad:)

-76¡ãF I kinda got the hang of it now:)

Ken_8

You must

use the number keypad - NOT the row below the "F" keys!

As in 212°F or 100°C

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doug_16

And here is the delta ∆

You can go to Programs/Accessories/Chacter Map. Pick the font (not all fonts work), then select the character and double click it. That moves it to the Characters to copy box. Click Copy, and then paste it in here. Don't have to remember numbers. °°°

ps If you are doing lot's of degree or delta signs, you can go to the character map and copy both signs. Then when typing here, just do a right mouse click and paste the signs where needed. Since they both will paste, just delete the one you don't want. ∆

Mike Kraft_2

like this?

°..............hmmmmmmmm ° hehehehe ² ž € ±  › ® ­­¬ Ÿ

cheese°

Uni R

Alt[0176]???

You mean my Alt[248] ° is obsolete and I have to use a 4 digit number now? L

Robert O'Connor_7

oi76°

Cool beans! I knew I'd learn something today, thats great..Robert O°Connor/NJ...°°°°°

Steve_48

Character Map

While trying out your tip about Programs/Accessories/System Tools/Character Map, I learned that you can drag and drop the symbol from Character Map as well as copy it. I never knew it could be so easy to insert symbols! Thank you!

Floyd_5

> Cool beans! I knew I'd learn something today,

> thats great..Robert O°Connor/NJ...°°°°°

Jeff Lawrence_24

Here

Here's a link the one of the ASCII tables I use. This is where I found the code for the degree symbol.

http://www.asciitable.com/

I also found out it won't work with the laptop because it has no number pad on the left like the desktops do.



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Mike T., Swampeast MO

A nice little program

You'll need an "unzip" utility to read.

Really handy, but alas I've never figured out how to get it to do the delta symbol as such is a Unicode symbol "above" the available range in this program.

tombig

Great!!

I can hardly type and now you guys are throwin this at me. What's an F key? Just kidding. I guess I'll save this post and practice what you've just taught me when I've got an hour or two to kill. Happy Mothers day to all you muthas' out there.

Tom

R. Kalia

spl chars

So now we have to type ΔT (or δT) rather than delta-T?

Kal Row

yeah - like knowbody here could figure out

what dT or dP meant , or that maybe 180f is the price of tea in china

ps the fastest way to bring up “charmap” is to hit the key with the window on it to bring up the start menu – then hit “r” to junp to run, the type “charmap” and enter

Tom Simensen

° symbol on Mac

To get the degree symbol on a Macintosh you have to hold down the shift and option key together and type an 8

Tom

\"Maine\" doug

Actually just creating a

shortcut on the desktop will do. Same as the desktop shortcut for The Wall. Double click and presto, The Wall and Character Map. Perhaps we could say the Character Map is the list of Wallers??