heat loss calc question

heatboy

We have all of these computer programs that can completely disect the construction of a structure to give very accurate component losses, but it still comes down to the ACTUAL construction practices that makes us look like the hero or the goat. Simply put, air exchange is the variable that cause the most flucuation in loads, but is the factor that is the most difficult to predict. You could always add this statement to your proposal, "These heat loss calculations were based on a .75 air exchange rate. Any deviation from this could seriously compromise the performance of your heating system." Even with all of the available information, it's still pretty much a SWAG and the reason we should not rely on anyone else, but ourselves, to do these calculations.

Warm Regards,

heatboy

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pat_3

heat loss calc question

what is the difference between a heat loss calc done for a baseboard heating system and a heat loss calc done for a radiant heating system....is it simply a different infiltration rate used to determine load requirements?

DanHolohan

The air temperature at the ceiling

is lower with a radiant system. There's less heat loss through the ceiling as a result.

Mike Kraft

Elaborate more please Dan.

> is lower with a radiant system. There's less

> heat loss through the ceiling as a result.

heatboy

Yes and No.

Most of the programs do use lower infiltration values when applying radiant over any other mean of heating, but it begs the question, "Do you feel lucky?" Just because it is radiant doesn't assure low infiltration rates. Another reason loads are somewhat smaller is the narrower Delta T near the ceiling. Since there is less (none?) convective heating, the ceilings don't get as warm with radiant so less BTUs leave the structure. One thing though, don't buy into that stuff about being as comfortable at 65° with radiant as opposed to 70° with covective heating. Somado and somadon't!

Warm Regards,

hb

"Expert in Silent Warmth"

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Mike Kraft

Elaborate more please Dan.

The ceiling loss is the same either or in calculating.If there is 100sq' of unheated ceiling that surface is the same in regardless is'nt it?The infiltration rate is what I thought was the difference also.

cheese

Mike Kraft

OK

With that said I understand.So do most design or compensate for the lower delta t with a lower infiltration rate?I am under the impression that convective currents from FHA or HWBB increase the ACH compared to radiant.Radiant as we know heats objects not air.....so no convective currents so lower ACH so lower infiltration rate?

cheese

DanHolohan

Suppose

the temperature in the ventilated attic space is 0 F. If the air temperature at the ceiling is, say, 80 degrees because of stacking, then the heat loss through the ceiling will be based on an 80-degree temperature difference from hot side to cold.

If the radiant system lowers the air temperature at the ceiling to, say, 65 degrees then there's less heat transfer through the ceiling, all other things being equal.

Mike Kraft

A little more help please

So we're saying that ceiling loss has a 15* DT comparing RF to FHA and HWBB.So in calculating this or interpreting this in a heat loss how is it done?

There are so many "rules of thumbs"I begin to see no fingers on my hands:)In IBR we address heat loss with building scenrios to fit the envelope.So what I see is the tighter a home becomes construction wise then the lower the factoring numbers become.

So if a home has average heights of 16' we have now doubled the typical ceiling height.This would also effect the DT even more.In a convective heat ie FHA would the temperature at the ridge be 95* and in a RF application would it be 45*?

I have a home that I installed RFH in 4 years ago.I was there this Fall.It is a post and beam home.The walls and ceiling are stres skin panels.......so infiltration is minimal.This home was a barn........some 250-300 years old.It was disassembld from its site and reconstructed.I installed Slant Fins Terra Therma in a suspended application.It is reset and constant circ.I asked the HO how the system was for him?He said"flawless".IMO thats a good word.The second floor is roughed for CIBB.The main living area also needed some supplemental.The HO's vetoed both on the final and wanted to live with the RF alone.

The second floor is barn loft and somewhat open to the main floor.They have four kids that sleep upstairs and bathe.There is no heat other than the RF.There is a IAQ air exchanger which someone else installed.So air is moving.But the loft areas are comfortable........no complaints.

Curious though,here are living areas that are heated with (yes moving air)RF from the first floor.

So rules of thumb.......

Lower infiltration rates T or F? It depends?

Lower your BTU load after calcs by 20% T or F?

RFH is cheaper to run because of lower water temps T or F?:)

For evey 3 degrees we lower the water temp. we save 1% of our fuel consumption T or F?(see above)

There has to be more!

cheese

Chris Maderia

Infiltration/Air Changes

This is by far the factor that is the difference between the two heat losses. Every other calculation used stays the same. We can't changes window or wall R-Value it is was it is. The lack of convection in a radiant application alone justifies this. Another factor is that we use either a 65 or 68 degree design temp vs a 70 or in some cases a 72.

larry

mixed systems

This discussion brings up an interesting question on how to approach heat loss calcs when you use multiple heating methods. Lets say it's RF on the first floor and either hydro-air or panel radiators on the second. It seems like a manual J program could get fooled into underestimating the second floor as first floor ceiling temps would be lower then normally expected, and the radiant heat loss programs (e.g. Wirsbo ADS) don't really speak hydro-air sizing anyways.

On another note, even though baseboard and hot air both create air temp stratification, I would think the induced air infiltration would be different between the two. In the case of forced hot air I'd imagine one of the primary issues are localized positive and negative pressure differentials due to typical imperfect supply vs. return balancing. Baseboard wouldn't really suffer from this problem.

tp tunstall

heat requirements

i think what we have here is two seperate questions. heat required and heat furnished. if you use standard IBR procedures you should be able to calculate the heat required (infiltration, volume, walls,windows,doors, etc.).that should be your starting point. next you would determine the type of system you want to use and using the tables and charts select the capacity required according to your calculations. In The ASHRAE guide they have a section on radiant heat which will give details on the percentage you may deduct using this type of heat. Although i read this many years ago, we use 15% deduction due to the added value of radiant heat. You can also find info along these sames lines with people who manufacture infra red radiant heat. they have charts and table that sy the same thing.another interesting note is that most if not all the baseboard manufacturers ratings have a 20% built in "heating effect" on their ratings. read the fine print on their tables!! tp

Bill Wright_2

Heat Loss Calculations ...

are the most precise Art in the industry. There are very detailed equations that very carefully, and precisely generate a number. But in the end they are all based on a series of estimates (which is engineer-speak for "guesses").

Consider that we commonly assume that walls have R-13 or R-19 insulation ... but do they really? They don't at the stud (every 16 inches), they don't where the insulation has been compromised, etc.

The ASHRAE Fundamentals says that "Typical infiltration values in housing in North America vary by a factor of about ten ...", or in other words, ACH are all over the place.

Another thing to consider -- because the ceiling spends all day "staring at the floor", it is not uncommon for ceiling temps to equal floor temps in radiant floor systems. I didn't believe this until I started shooting ceiling temps w/ my Raytec infared pyrometer. I guess it makes sense, because the ceiling is recieving all that energy via direct radiation (but it wasn't what I expected). So, yes the air temp is lower at the ceiling, but the actual surface temp could be higher.

All this to say that typically heat loss software/calcs do not do anything different for radiantly heated systems than for convector systems. Usually, the difference in heat loss is due to using lower ACH ... which is more a function of construction (radiant tends to go into new, tight, high end construction).

Are most systems oversized -- probably, but you have to remember that ALL heat loss calculations are just an estimate to begin with.

larry

My understanding of manual J

is that it incorporates some fudge factor for typical construction practices and accounts for the studs not being insulated, etc. Once had a contractor do a manual J on our house and came out with a very high design heat loss compared to the calc I had done. It turns out he was second guessing the program and downrated everything in the house (e.g. where we have R19, he would input R13, ...). After seeing the result, I consulted the author of the program I was using and learned that manual J already accounts for real world construction practices and doesn't assume a theoretically perfect building envelope.

The ACH also seems to be an educated guess, even if you do a blower door test. The conversion from a blower door ACH to a natural infiltration ACH is loaded with all sorts of assumptions.

heatboy

IBR & the magical 15%

I, like most everyone, started out with the IBR hand written heat loss forms (H-22?) It was a great experience because you really got a sense of what encompasses a building and all of the variables involved. The loads always seemed a little high as I look back now. I think they must've figured 20 mile per hour from all four sides simultaneously! Learning to do loads this way, did make switching to computer programs easier, for this dim bulb anyway.

When sizing baseboard, I always deduct that magical 15% that all manufacturers use to increase the output. It means more baseboard, but I never design for more than 160° anyway. If the project is so price sensitive that a few feet more baseboard will break the job, well.........

Warm Regards,

hb

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kf_2

Radiant Heat loss

ASHRAE states that you can reduce the room volume by 15% and increase the ceiling R-Value by 1.5% for a radiantly heated room. Other than that the heat loss is identical.


kf

Bill Wright_2

Could you please ...

tell me where I can find that information? I'm intrigued!!

I just re-read the ASHRAE Fundamentals (both 1997 & 2001) chapter 27 on Residential Cooling & Heating Load Calculations and ASHRAE HVAC Systems & Equipment (2000) Chapter 6 on Panel Heating & Cooling. I must be missing the section you are talking about -- or maybe I'm looking in the wrong places!

Thanks for pointing me in the right direction,

Bill W.