Degree days/ system sizing

Mike T., Swampeast MO

Outdoor design temp is rather confusing.

Record low temps are readily available but designing for that temperature is likely excessive in most cases. Modern outdoor design temps are [supposed] to come from <I>hourly</I> temperature data over a 30-year (or more) period. With that data a percentile rating of any temperature can be obtained. e.g. 67° at the 50th percentile would mean that half of the hours in the period were higher than 67° and half were lower.

Where hourly data for such a long period is not available (frequent), various statistical methods are used to convert daily data into hourly. Each method has stengths and shortcomings.

Two percentiles seem common. 99 and 97.5 The 99th percentile number [appears] to be the one used by most heat loss programs and most commonly referenced in other sources. Unless very recently change however, Manual J recommends the 97.5 percentile. From what I could find, ASHRAE no longer even publishes the 97.5 percentile ratings.

Objective studies of actual heating systems in actual homes frequently question this difference. A frequent conclusion is that the difference helps mask some of the duct leakage in forced air systems as Manual J assumes lower duct leakage than is found in the vast majority of systems (even new and "good"). There are other such overstatements of assumption in Manual-J that would [seem] to make up for that "little" problem of understated duct leakage.

Barring unusual local conditions, it would [seem] that the 97.5 percentile rating would be sufficient for hydronic systems.

As to sizing a steam system, Constantin gave a perfect answer. Since most steam work is on existing systems however, you're generally only concerned with the radiation and piping, not the actual heat loss. Since you can assume that residential steam is about 212° with extreme certainty, your supply temperature is fixed. Thus, unless you want to compute the heat loss and re-size radiation where necessary, your boiler is sized by the ability of the system to condense steam, not the actual heat loss of the structure.

You must however be very careful when adding radiation. You need to assess the current system carefully. Conduct a heat loss calculation and determine how the current radiation meets the loss. In many instances you will find that it is quite oversized. In order to achieve reasonable balance, the new radiation should be oversized by similar degree.

thfurnitureguy_2

System sizing

can anybody go through the steps to size a steam system? I'm ok with the boiler matching the radiation and pickup factor. My questions are mostly with degree days and design days etc. Where can you get the info for a specific area. I have the heat a heatloss program and would like to do some calculations. Do you design to the coldest possible day or average? This is somthing I have a small understanding of and would love to know the steps to do it right. What do you start with. thanks T.

Alan R. Mercurio_3

Here's a site that will help you with degree days.

Degreeday.com

Your friend in the industry,
Alan R. Mercurio

www.oiltechtalk.com

Constantin

Here is how I think it's supposed to work

My understanding of how steam systems are supposed to work is that you first determine the heat loss of each room. Then you size radiators to match the maximum anticipated heat loss. Then you size the pipes, boiler, etc. to cover the load of the system under those circumstances.

Whether to use the ASHRAE 1% design day conditions or the lowest reported numbers in your area for the purposes of your calculation is up to you. Apparently at least some of the fudge in the heat loss calculators seems to be accounted for by the balmy design-day conditions that ASHRAE publishes. For example, our area is supposed to have a design day condition of 9°F when last winter we had a week of -15°F.

Steamhead (in transit)

Around Baltimore

the standard is to calc for 70 degrees inside when it's 0 degrees outside. Once in a great while we get colder than that but it's rare.

Edward A. (Ed) Carery

DD Information through NOAA

For the DD information go to the site below then go to your specific area. Great research tool.

Are you designing and installing a complete new steam system from the ground up, or are you doing a boiler and/or system replacement?


http://www.erh.noaa.gov/box/dailystns.shtml


Regards,

Ed

thfurnitureguy_2

Ed and all This is a new system in a vacation home. Thank you for all of the input. This home has a ton of good windows and I would like to install as many CI baseboard rads. under the windows as possible, to minimise the lost space in the rooms. I need to do the calcs. to find the sq.ft of radiation needed for this. My main dislike for the slantfin baseboard radiant that is now installed (and old) is the wall space is almost all covered with baseboard radiators. this makes furniture placement look slipshod at best. My hope for steam is a smaller setup in the rooms.

Steamhead (in transit)

Sounds like whoever installed that Slant/Fin BB

did what we call a "Long Island Heat Loss Calculation". Briefly, this means putting baseboard on every available inch of outside walls.

thfurnitureguy_4

I have often wondered why there was so much of it. I'll do the calc and see.