radiant and vaulted ceilings

benny

what is the best way to handle heat loss calcs in room with high vaulted ceilings and using radiant?Do you add floor to highest point of ceiling when figuring wall loss or do you just figure total cu. ft of room and figure for infiltration based on those numbers.

Mark Eatherton1

Heat leaves building 3 ways...

Through the doors, through the walls and thru infiltration. Ceilings, due to their high insulation values, loses very little comparitively, but it IS a part of the equation.

That said, in areas with hig heat loss (lots of tall glass walls) radiant kicks butt in the comfort zone. And even when it gets real cold outside, it still kicks butt in the area of human comfort.

Heat loss ain't what it used to be...

I'll let that last statment hang out there to dry for a while.

Remember my house at design condition? It was using a LOT less energy than it was supposed to be. I think this is true for most semi mass intensive buildings. THe more massive, the less energy per square foot it will demand at design condition. If it ever goes completely cold, it will take forever and a month to get its momentum back up and running, but once topped off, its a simple matter of "maintenance" energy. And thats not NEARLY the energy required for fast pick up.

The problem with all of our heat loss calculations is that they assume a state or condition that rarely occurs, and when it does, it's not for very long. Building for the Anarctic is a different animal. What a great place to do building research eh... But I digress.

None of our loss calcs take anyting into consideration for alternate gain (solar, internal, wood induced, physical plant standby, etc...)that we KNOW is going to in fact occur. I think there is one DOE program that takes "some" internal energy consumption and credits the heating demand with its availability, and it only allows for one or two refirgerators/freezers. Even guessing at their demand is a risky proposition at best.

There was an article ran in P&M Magazine a long time back about a guy who used the consumers gas bills to size his boilers, and he was doing a grand job of keeping people comfy, AND he was reducing their utility heating bill by like 30 to 40 percent or something like that. In most cases, the boiler was reduced by around 50% from existing situations. I know, time changes and so do peoples living habits, but by an avergae of 35%???

If you look at a similar buildings gas bills, you can see the effect of thermal mass and alternate gain. Real time energy consumption. Now hopefully, the statement about heat loss not being what it used to makes sense... or not.

ME

Boilerpro_3

I have used fuel bills for sizing boilers

and it often points to much smaller equipment than what Ashrae load calcs will tell you. It is particularly useful when sizing step fired modular or multiple boiler heating plants. It gives you a real good idea of what your typical day heating load is so you can size your baseload equipment very tightly. I typically see fuel usage reductions of 40% and up to 60% when using this method along with upgraded controls for the church structures I work with (my specialty in commericial).

Boilerpro

Mark Eatherton1

BP...

care to share your method of madness regarding fuel bill analysis? Are you taking the therms consumed and dividing by the hours of operation to come up with an input, or what?

TTIA, and Merry Christmas!

ME

Boilerpro_3

Method of madness

Funny how the word madness is often used in the same breadth as my name! Hmmm, wonder if that means something?

Anyway, our local natural gas supplier prints out much of the necessary data on the fuel bill. I start with the degree day data for the month being analyzed. Typically, this is the degree days with a base of 65F. In other words, the degree days don't start adding up until the average daily temperature is below 65F. For older single family, detached homes with a interior comfort temperature of 70F, heating is not needed until the temperature drops below 65F average daily temp. This is also termed the building balance point. Internal gains from people, lighting, sunlight, etc. generate enough heat to keep the building warm above 65F outdoors. You take the total degree days for the month and divide them by the number of days included on the bill. This gives you the average daily temperature delta tee for the structure, assuming a building balance point of 65F.

1200 degree days 65 / 30 day billing period = 40F

Around here, this represents a pretty typical mid winter month. The average outdoor daily temp for this month would be 65F - 40F = 25F.

As an aside, the design temp for us is -5F or 70F delta tee. If the daily temp is typically 25F and design is -5F your typical load is:

40F / 70F = 57% of you peak load. If you add an IBR pickup factor of 15% to you heating load, you are now at only 50% of the boilers peak design output. Hence, the long standing support of modulating or step fired heating plant. For 90% of the heating season you are need at most 50 to 60 % of your design day capacity. Back to the subject.

Next I take the therms used and divide them by the number of days in the billing period. Then this number, representing the average therms used per day, is divided by 24 and converted to btu's per hour input.

400 therms monthly usage / 30 days = 13.3 therms per day

13.3 therms per day/ 24 hours = .556 therms per hour or 55,600 btu/hr input

Now you have the average btu/hr input for the month (55, 600) and the average delta tee (40F). Dividing 55,600 by 40 will give you the number of btu's needed per 1F delta tee

55,600/ 40F = 1,390 btu/hr input/ degree F

This gives a good starting point to fine tune the heat load.

You have to watch out for out loads running through the meter, like water heating, etc. If you look at summer monthly bills, and building usage remains the same throughout the year, this should give you an idea how many therms per month go to these other uses.

Now, some experience and intuition comes into play. First off, is the balance point for the structure you are analyzing really have a balance point of 65F? Most newer structures do not. They often have balance points between 55 and 60. Lets say its 60F instead of 65F. For every day of the month you have to substract 5 degree days. So for the month in question:

1200DD65 -(5F x 30) = 1050 DD60 or 35F delta T

Something else that effects this is if the temperature in the structure is also set back for long periods of time. So lets say its one of the churches I work with and they set the temp at 50F and only warm it up on Sundays. The average temp of the space is only roughly 55F instead of 70F, so the delta tee just dropped by 15 F.

35F delta T - 15F = 20F delta T

So let's take the 60F balance point and low thermostat setting into account. We now get:

55,600 btu/hr / (40F -5F - 15F) = 2780 btu/hr/F input

Now for a couple of tough ones, what is the actual efficiency of the boiler during the month in question and what is the air leakage rate of the structure? Without getting a handle on these, it's pretty hard to get a good number for the actual heat load.

A great big firetube boiler idling a 180F all month, even when no heat is needed, is going to have a huge stand by loss. A newer compact cast iron boiler or even more so a copper tube boiler, only firng on demand is certainly going to be more efficient. I'd expext the firetube boiler would see an efficiency of probably 55% in this application so....

2780 btu/hr input/ F x 55% = 1529 btu/hr /F actual heat load

Then you also need to look at infiltration. This 1529 btu/hr is the combined load due to all types of loss. Typical conductive losses are basically linear through the temperature ranges we design for. However, infiltration accelerates nonlinearly as the temperature difference increases between indoors and out. This 764btu/hr/F needs to be corrected to reflect this. In very leaky structures this can be very significant and much less so for tighter structures. For typical structures, when the raw load is calculated at a delta tee of about 25 to 30, I usually add about 15% to compensate for the added heat loss of increased infiltration. In our case the delta tee is a little low so we need to correct a little more for infiltration. So.....

1529btu/hr x 1.20 = 1834btu/hr

This 1834 btu/hr would be the same as the peak UA calculated from a detailed heat loss.


While this all may sound complicated and iffy, with the kind of experience and understanding you have, it can be quite accurate. Also, going through this really helps give you a "feel" for the structure and, as stated prevously, is a great starting point for sizing baseload boilers. In my opinion, this baseload is where the payback is for the investment in expensive, high efficiency equipment. No sense spending money on high efficiency equipment that almost never runs.

Hope this gives you something to "chew on"

Boilerpro

mph

Vaulted ceilings

The short answer is that the heat loss of any surface is directly related to its exposed area. To find the surface area of a wall with a vaulted ceiling just take the average height (mid-way along the pitched ceiling) times the width. If you're using a canned heat loss program, it will subtract the areas of any doors and windows you place on this wall when it does its calculations. It handles those surfaces separately.

Jeff

Mark Eatherton1

Consider it

printed! Thanks for the info Dave.

And trust me when I say I understand what you mean about madness being associated with your name...

The Colorado Madman

(ME)