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# Help me understand heat loss and how much heat is needed to replace it

#1
02-18-11, 12:36 PM
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Help me understand heat loss and how much heat is needed to replace it

I'm trying to figure out what size baseboard heaters to install for a basement renovation and I'm having a little trouble wrapping my mind around how much heat is required.

I've done a few heat loss calculations and I come up with about 4,000 BTU/hr. First off, does that sound about right for a 1,000 sqft basement in Minneapolis with R18 insulated walls and not much window area?

Assuming that heat loss is right, how many BTU/hr should my heating system be capable of? Obviously if there's less than 4k BTU/hr the heating system won't be able to keep up but if it provides exactly 4k BTU/hr it seems to me it would have to run 24/7 just to keep from losing any more heat but would not be capable of actually raising the temperature in the basement. Is that right?

It seems like I would need more than 4k and the higher the BTU/hr the faster the space can heat up. For instance, if the temp gets down to 50° F it might take several hours to get up to 70° if I put 4,500 BTU/hr into the space. But if I were able to push 10,000 BTU/hr it would take less time to heat up and the boiler would run less often. Is that right?

Is any of that right or am I totally missing the point?

Thanks

#2
02-18-11, 01:11 PM
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With all that insulation and if your basement is mostly below grade, then 4,000 btu/hr might be right on. The heat loss you figure out is for the coldest day of the year and it is meant so your system runs near constantly to maintain the temperature. Keep in mind that in these forums, it has been found that manual J calculations add about 20% to what you actual loss would be. It calculates for a worse case scenario.

You are right that matching your radiation to your heat loss would mean it will take longer for the space to heat up. But why do you want to go from 50 degrees to 70? That sounds fairly extreme.

Adding more baseboard will give you the ability to heat up faster. It will also give you the ability to run lower temperatures. That will also depend on the setup of your boiler and the setup of the reast of your system on the other floors. Just be mindful that baseboard loops can only be so long. You don't want to lose all your heat into the room at the beginning of your loop and then have cold radiators at the end. If this will be an issue, have more than one loop.

#3
02-18-11, 01:36 PM
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The r-value sounds great, but we don't know what the air leakage is and where the house rests on the foundation the leakage can be high. Now, where that leakage goes we don't know, but if it finds its way into the basement, it will head for the floor. Even cold air leakage on the main floor will follow plumbing and electrical openings and head to the basement.

Next, what did you use for a design temperature to calculate the heat loss? Did you use a heat loss program or just use the math?

Last, 4,000 btus is tiny. That's a dozen 100 watt light bulbs. Nice and bright, but on the coldest day in a Minnesota winter, sounds like it would fall behind.

Give us the area of exposed foundation, windows, and what is between the basement ceiling and the floor above. Is there a walk out? Is there a closed door at the top of the stairs? And, where is the boiler located?

Bud

#4
02-18-11, 02:04 PM
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Thanks Drooplug.

You are right that matching your radiation to your heat loss would mean it will take longer for the space to heat up. But why do you want to go from 50 degrees to 70? That sounds fairly extreme.
Yeah, that's wouldn't be normal! I was using it as an example because currently we have two little 1000W electric in-wall heaters for the entire basement and they cost so much to run that we just leave them off most of the time and don't go down there. When we do need to use the space we have to turn the heaters on in the morning and let them run all day before it's up to a comfortable temp.

Adding more baseboard will give you the ability to heat up faster. It will also give you the ability to run lower temperatures. That will also depend on the setup of your boiler and the setup of the rest of your system on the other floors.
If I get you more info can you help me figure it out?

Just be mindful that baseboard loops can only be so long. You don't want to lose all your heat into the room at the beginning of your loop and then have cold radiators at the end. If this will be an issue, have more than one loop.
I didn't know that, what's a good length to stop at? Here's a really rough idea of how I can lay out the heaters. Should I split it into two loops or use a manifold for more than two?

Thanks again for the help.

#5
02-18-11, 02:29 PM
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Here's some quick info for you: 2000 watts is equal to 580 BTUs.

Check out: btu_pipe baseboard The middle column is BTU output per foot. You will find that the max amount of baseboard output you can have is equal to the capacity of the pipe used. http://www.comfort-calc.net/pictures/Piping_chart.JPG

Double check the capacity of the baseboard you buy and use those specs to make sure your calcs are accurate.

As far as water temp, you will need to know your heat loss for the rest of the house and the output of the radiators you have. Which I think you figured in your other thread. Take your heat loss in the rest of the house and divide that by the total sq ft of EDR for your CI radiators. That will give you BTU's need per sq ft of EDR. Look at this chart: Steam Information emmisions for radiators That will tell you what water temperature you will need. You should be able to work out how many feet of baseboard you will need based on that water temperature and your basement heat loss. At the moment I don't have the time to work that out. Need to make pizza for the family. Sorry. I'll be back later and hopefully can get it together. I do beleieve there is a minimum temperature that you can run through the copper fin baseboard though.

BTW: I'm not an expert, just a diy guy learning his way through as well.

#6
02-18-11, 02:51 PM
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If you have a conventional boiler, you will need to install a boiler bypass to run temperatures lower than 130 degrees.

#7
02-18-11, 03:04 PM
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2000 watts is equal to 580 BTUs.
Ummmm... I think it's more like 6,800 ... 1KW = 3410 BTU

What droo is talking about with the length of the loop is similar to what we were talking about in the other thread.

If you can move 40K BTU through 3/4" pipe, then you shouldn't really install more than about 65' of 3/4" fin-tube element per loop. (based on 600 BTU/FT emission from standard fin tube) This is NOT the total length of the loop, just the amount of fin-tube ELEMENT that's installed... you wouldn't really have to count the connecting pipes.

Last edited by NJT; 02-18-11 at 04:02 PM.
#8
02-18-11, 03:06 PM
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Thanks Bud, I had used an air exchange rate of 0.93 but that was just a guess really. I did the calculations with math and excel. If you can make sense of it, here's a copy.

I plan to have the rim joists spray foamed so they should be fairly air tight and insulated. I tried to include the rim joists in my calculations but adding an extra half foot to the above grade walls calculation.

The exposed foundation is 400 sqft below grade (4 feet) and 350 sqft above (including rim). 4 sqft of single pane wood window and 8 sqft of double pane aluminum. 100 sqft of partition between unconditioned space with R13 insulation. It's not a walk out but the stairs goes up to a landing with a back door and stairs to the first floor (no door to first floor). The back door goes to a 3 season porch so it's not AS cold as outside but pretty cold. I hadn't factored the stairs in... cold air can freely come down the stairs and there's not really space for a door anywhere. The ceiling is just drywall with a subfloor and hardwood on the first floor. The floor is slab with vinyl on top but no insulation. The boiler is in the unconditioned space - see my response to drooplug for a floor plan.

Thanks.

#9
02-18-11, 03:28 PM
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If you can move 40K BTU through 3/4" pipe, then you shouldn't really install more than about 65' of 3/4" fin-tube element per loop. This is NOT the total length of the loop, just the amount of fin-tube ELEMENT that's installed... you wouldn't really have to count the connecting pipes.
Good to know. So far it sounds like I won't need anywhere near 65'.

#10
02-18-11, 03:34 PM
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Originally Posted by NJ Trooper
Ummmm... I think it's more like 6,800 ... 1KW = 3410 BTU
You are right. I went the wrong way with my math.

#11
02-18-11, 03:43 PM
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Ok. So we know what 6,800 BTU's does in your basement. It would be helpful to know what your starting temp was and what a "comfortable" temp is as well. Keep in mind that if you maintained a certain temperature over time, let's say 70F, it would feel more comfortable than if you brought the room up from 65F until the air was 70F. That's because the walls, floor, ceiling, and everything else in the room is colder than 70F. Also, having baseboard in each room will deliver the heat to those rooms more efficiently than those 2 electric heaters.

#12
02-18-11, 04:12 PM
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There must be something wrong with the calculations... you are talking about 4 BTU / SQ FT, and that's durn near an impossible number.

I know how much we all love those 'rules of thumb', but let's talk about those as a 'reality check' here...

25 BTU / SQ FT is pretty good for a normal wood frame constructed home. A leaky old Victorian with minimal insulation might have as high as 40 BTU/SQFT ... (or higher!). A very well constructed home might possibly be down around 15 BTU/SQFT, but that is really VERY low... and would require great care in construction.

Going back to what you stated as heat loss previously, IIRC you had said you came up with 51K ? and if the area that relates to is the same 1000 SQ FT as the basement footprint, then there's something wrong with that cipherin' also... I think... that's 51 BTU/SQFT... mighty high!

As a point of reference, my home is a two story converted Cape... about 1900 SQFT total. Heat loss calc came to about 65K with the program, but ACTUAL MEASURED heat loss (clocking oil usage, converting to BTU, comparing to Degree Days) was more like 45K, thus droo's statement about the program overstating the heat loss.

I wouldn't be at all surprised if your home was closer to 30K...

I'm gonna take a WAG and say that your basement might be around 12-15K ...

#13
02-18-11, 04:19 PM
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I would seriously consider doing a perimeter series loop for the baseboard in the basement. You don't have to stick with the two pipe that you have running the upstair rads. Much less material... just run the pipe around the perimeter and add the elements where needed. You can use empty 'dummy' cabinet if you want a clean look all the way around the wall, to hide the connecting pipe.

[thinking about this a bit more though, all that empty cabinet could add up to a few bucks... let's say your HL in the basement is 12K... that's only about 20 some odd feet of element... which by the way could be piped in HALF INCH which is good for about 15K BTU ( 1.5 GPM X 10K per Gallon ) ... ]

#14
02-18-11, 05:34 PM
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Hi HA, I reviewed your calculations and although I can disagree with some of the numbers, the bottom line is, your 4,000 btus isn't far off. Try simplifying the whole process and calculate the heat flow through 6' of wall, for a 140 foot perimeter, using R=15, and a delta T of 78º. Virtually all of the heat loss will be through the exposed foundation, plus the rim joist, plus one foot below grade. The heat that escapes the bottom area of the wall and the floor just accumulates in the soil and stops flowing.

Since this is a basement and the cold air will flow down hill, stairs included, you will need to follow your logic of being somewhat oversized as the calculations do not include that aspect, essentially it becomes an educated guess. But looking at a heat loss of 4,000 btus at least gives you a starting point and some confidence that it isn't going to cost a fortune to heat this space. I'll let the others help you with the guess .

You will probably need to add an outside air source for your boiler and domestic hot water if not electric once everything is sealed up. Some burners have a kit for just that purpose. The boiler boys can help you better in that area, Hi NJ.

I'll add one more caution for which the jury is still out. The new codes and new practices of fully insulating basement walls, floor to ceiling, fail to take into account what is happening on the outside in our very cold climates. Basically, the ground is freezing solid. Will it push on the foundation and cause damage over time? That is yet to be seen and the final results will vary from house to house based upon many variables, like frost susceptible soils and drainage. It is something, IMO, worth reading up on, but probably not something to justify a change in plans.

enjoy
Bud

#15
02-18-11, 05:41 PM
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Hi Bud!

I dunno... FOUR BTU per SQFT ? ya think? even for a basement I think that's pretty optimistic. And it seems that two 1KW electrics won't cut it... and that's almost 7K right there.

accumulates in the soil and stops flowing.
Heat never sleeps... keeps on moving... wherever there is cold, heat will travel to meet it.

#16
02-18-11, 05:48 PM
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fail to take into account what is happening on the outside in our very cold climates. Basically, the ground is freezing solid. Will it push on the foundation and cause damage over time?
I've thought about this as well... I think a good approach would be to insulate the OUTSIDE of the foundation with XPS or similar. This would allow the foundation walls to stay warmer, and add some thermal mass to the structure. The foam would probably add enough resiliency to counter the effect of the frozen ground mass pushing at the foundation.

This of course would be a major PITA on an existing structure, requiring excavation to at least 4' depth around the foundation, but for NEW construction... it's the way to go... or use the insulated precast stuff... forget the name...

But we digress.

#17
02-18-11, 06:32 PM
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Originally Posted by NJ Trooper
Hi Bud!

I dunno... FOUR BTU per SQFT ? ya think? even for a basement I think that's pretty optimistic. And it seems that two 1KW electrics won't cut it... and that's almost 7K right there.
But his electric heaters do cut it. It's the way he is using them that doesn't. He doesn't maintain comfortable temps down there because the electric costs too much \$\$. So anytime they plane on using the space, they turn on the heaters and it takes a long time to heat up. I assume at least a 10 degree temperature rise.

#18
02-18-11, 10:18 PM
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Oh, lots of posts while I was out. I had done an online heat loss calculation for the whole house and come up with 51k which makes 20.5/sqft. Most of that was from the main level's uninsulated walls and 18 windows. Not much was coming from the roof and basement. 4k does sound low (from the little I know), especially when I convert it to feet of baseboard... could 10' really heat the whole basement? There really aren't many windows though, and it makes sense to me that a well insulated wall against warmer-than-air soil wouldn't lose a ton of heat.

NJ- thanks for the tip about plumbing the baseboards in a loop at 1/2", that will save a lot of pipe and \$\$.

Bud - thanks for looking over my number. If I knew I was going to show it to someone I might have laid it out more logically The work room is already vented to the outside, that's part of the reason I'm going to leave it unconditioned and just insulate the partition walls around it.

Did someone say I need to lower the temp for baseboard heaters? I've seen some that mention the output BTU/hr is rated at 200°...

As for the length and therefore total output of the heaters. I was thinking 3' in the laundry, 2' in the bath, 4' in the bedroom and 6' + 3' in the movie/game area. That's 18 feet and roughly 7200 btu/hr depending on the water temp. Sound good??

#19
02-19-11, 01:37 AM
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"The work room is already vented to the outside," A vent to the outside and a combustion air supply are noy always the same. It requires a considerable negative pressure on the inside to pull in the needed air for the boiler. That negative pressure then competes with the exhaust pressure and you can end up with combustion products being pulled back into the house.

There is a CAZ (combustion air zone) test which puts the house in a worst case condition, all exhaust appliances running and other details, and then measures the CAZ to be sure the exhaust will function safely.

A dedicated air supply needs to be an approved air source and something that can't be inadvertently closed by someone wanting to eliminate that darn cold draft, I've heard that more than once. A bit of a pain. but an important consideration. Is your hot water electric or a combustion appliance also?

Bud

#20
02-19-11, 04:30 AM
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Originally Posted by HomeAlterations
Did someone say I need to lower the temp for baseboard heaters? I've seen some that mention the output BTU/hr is rated at 200°...
I mentioned that if you added more radiation than you need, you could run lower water temperatures. You don't have to do that. I looked on Slant Fin's website and they have information on their baseboard that tells you the output per foot at different water temperatures.

#21
02-19-11, 06:43 AM
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Originally Posted by drooplug
I mentioned that if you added more radiation than you need, you could run lower water temperatures. You don't have to do that. I looked on Slant Fin's website and they have information on their baseboard that tells you the output per foot at different water temperatures.
Cool. I was looking at Slant Fin's anyway!

#22
02-19-11, 07:09 AM
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The hot water heater is gas too. The vent is a 6" tube that runs in through the window and down almost to the floor. I know it was installed for the boiler but I don't have a way of knowing if it's sufficient. How do you measure CAZ? I definitely won't want CO entering the basement.

#23
02-19-11, 11:24 AM
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It requires a very sensitive manometer to measure the zone pressure under worst case conditions. You can Google it and there are many descriptions of how to, but I don't know of an easy way aroung the expensive meter. the testing also involves checking the draft capabilities of your heating system.

If memory serves me, years ago I estimated the natural draft opening needed to deliver air without creating a problem with air pressure and it was large. I know my commercial building with a 280Kbtu boiler needed a 3' by 3' weather protected vent. If the calculation is linear, then a 90Kbtu boiler would need 3 sq ft., but I'm out of my specialty and you should get those numbers from a HVAC or local energy auditor. Most energy auditors could quote you a price for doing a CAZ check. The advantage of an outside air kit is that it draws the air directly into the burner and doesn't dump it into the room. So, when it is off, there isn't a lot of cold air leaking in.

Bud

#24
02-20-11, 05:06 AM
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Just some Info.
I just completed a heat loss for a couple that lives outside of Lancaster, PA. The home was built in the 40's. The basement is 25 x 28 with 3 foot exposed above grade, 6 windows and 1 exterior wooden door.
The heat loss is 8,111 btu's @ 8f OD design temp and 72f inside temperature.

#25
02-20-11, 07:59 AM
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That sounds close to my guess... 8111 / 700 = ~12 BTU/SF ... 12 BTU/SF X 1000 = 12000

BTW, one thing I like to do when sizing baseboard is to count on the fact that the NEW CLEAN units may put out 550-600 BTU per lineal foot, but very quickly they do get dusty... and the output goes down. I would much rather account for this during design and use a more conservative 500 BTU / LF ... so that would look like about 24 feet of BB.

I have not used this manufacturer, but it is a higher output unit... it's worth a look...

Smith Environmental Products Hydronic Baseboards. The Heating Edge Hydronic Baseboard. Hydronic Heating Systems - Radiant Heating System, Solar Heating, Geothermal Heating

#26
02-20-11, 08:01 AM
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rbeck, my guesstimate for the one posted above with r-15 walls was about 7,000 btu's, so the 4k is a bit low, but not like 4 vs 20. Your 8k is in the same ball park. Since this is the coldest day calculation, I feel his proposal will work fine.

Bud