If a load calc comes up with 30,000 btu/h loss, what other factors play into sizing the boiler? Are there reasons to size it more than 30K? How much excess capacity is a good thing?
Listeners write in about shower panels and cordless tools and ask questions about old wiring, air leaks, and gutter covers.
"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.
Dig into cutting-edge approaches and decades of proven solutions with total access to our experts and tradespeople.
Start Free Trial NowGet instant access to the latest developments in green building, research, and reports from the field.
Start Free Trial Now
Replies
Don't have a direct answer for you, but I can tell you that oversizing equipment too much will result in shortened lifespans for equipment. The boiler and/or AC units will be kicking on and off for short periods of time, and never really reach their full operating efficiencies.
Bob
Remember your load calcs are base on heating degree or cooling degree days for your area. Those are averages. Any particular year may fall above or below the numbers you use. My HVAC guy says 10 to 15% oversize is safe. When you get to the 20-25% oversize range you stand a chance of equipment short cycling.
Not sure about boilers. The only heating boilers I have ever worked on were old mega-monsters from the 50s, all gas fired/ forced draft tube types. The buildings they were in were so leaky that being 80% oversize wasn't enough.
Dave
First. I'm not a AC man. So double check before taking this as the whole answer. Just my impressions:
Some of the most uncomfortable buildings I have been in had oversized AC units. They would short cycle constantly but never run long enough to remove the humidity. The general feeling was clammy, musty, and sometimes mouldy. Attempting to make it feel better the HO turns the thermostat down so then it is cold and clammy.
A friendly AC tech clued me in when he was called to the house because the HO was sure the unit was undersized or not working properly. The AC tech, from the same company that had installed the system, told me that the HO had demanded a larger unit be installed than what the heat load calculation had recommended and had been warned, thankfully in writing, that it would not operate well.
I was on a unrelated service call and left when the AC companies owner arrived and got into a shouting match with the HO. Threats were made, from both sides, about bringing in the lawyers. About this time I was feeling a bit late for the door and exited stage left.
I am also not a not a home HVAC expert but, if you size your components to provide just the heatload calc values you will not have any extra capacity to change the temperature of the space when at or near design outside air temps. or to deal with extraordinary conditions. Our design cold temps is 32 degrees but we often see temps below zero. This 32 degree differential will be realized inside your house as a 32 degree drop from it's design condition, say 70 - 32 = 38 degrees. Try explaining that to your wife as she makes your breakfast with her parker and gloves on!
If you plan on using a programable thermostat to lower house temps during the night and want to raise the house temp say 7-10 degrees when you get up, a 30k heater will take forever to make that change. You must consider how much capacity you need to provide for a reasonable rate of change of space temps as well as the ability to replace heat losses.
With a design loss of 30k a 90k heater and a 3 ton ( 36k ) a/c unit would seem to be about right. Just MHO. Our house had a 29k heat loss calc and this is what I installed and everything is quite comfy year-round. Lastly, install some room fans, they are worth a ton of a/c and are infinatly cheaper to operate.
>Our design cold temps is 32 degrees but we often see temps below zero.
Huh? A design temp is something you should be above 97 or 98 or 99% of the time. If you're missing the mark by >32° "often" then something ain't right. Where do you live, if I may ask?
I live in far western N.C. and per the IRC, chapter 3, our winter design temperature is 32 degrees for 97.5% of the time. I suppose that you could interpolate between the isolines and come up with something like 23-24 degrees, but even these are still a far way from zero. The code does not require this interpolation.
The point that I was trying to make was that it is important to build in extra capacity to allow your system to control your space temperature where you desire it and not to have the space temperature vary according to outside temperatures near, at, or in my example quite a bit below the design temperature.
Using a simple disign heat loss calculation as the major guide in sizing heater and a/c equipment may be misguided. For one thing, the design delta T during the cooling season is different than that of the heating season, and during the cooling season a significant portion of the a/c capacity is used to remove latent heat ( moisture ) from the air. Other responders to your post addressed the need to size your equipment so that this water is removed from the air rather than just flash cooling it which would increase the relative humidity of the air ( clammy ).
If you want a good guide to proper equipment size check out new homes being built in your area by a good builder. They don't spend any more than they have to to get a reasonably comfortable product. Leave the "red line" designing to the truss designers.
Lou, I'm also in WNC, and the design temp for my system by my HVAC contractor per CPL (Carolina Power and Light) and Wirsbo (and IPEX, too) was 14° or so. We're likely using a 99% confidence interval. IRC puts the 32° isoline down by Gainesville, FL.
>check out new homes being built in your area by a good builder
It is typical to oversize systems to avoid callbacks. Happens all the time, but doesn't make it the best practice. I'm trying to figure this out a bit more precisely, be/c our energy requirements can be figured out a bit more precisely. When the load is 30K--and I have every confidence that is accurate at that client's design temp of 5°--and the proposals are all for installing 75K to 120K btu/h capacity, something isn't adding up.
> during the cooling season is different than that of the heating season
I'm only asking about boilers and only about heating. The house in question will not have a cooling system.
Edited 9/24/2003 2:33:39 PM ET by Cloud Hidden
Florida power and light has some thoughts on oversizing - its bad in summer as humidity control suffers
in winter you are running on heat strips below 32 if you have a heat pump - just get the larger KW heat strips
Do yourself and the HVAC world a favor. Go post this question with pertinant details on "The Wall" at http://www.heatinghelp.com
I would love to see it discussed there. Some of the very brightest minds in Radiant Heating hang out there. These guys dream in radiant terms.
A lot of these guys are very knowledgeable on over/under sizing boilers.
Give it a catchy title. Let me know if you do and I will pop in to welcome you.
Then put a link to that discussion in this thread so everyone here can learn from the pros.
Deal?
Deal. http://forums.invision.net/Thread.cfm?CFApp=2&Thread_ID=9445&mc=1
Jim,
Do those making the proposals know about dome efficiencies? (I'm assuming it's a dome??)
They may just be shooting off-the-cuff BTU/sqft estimates back at you based upon standard stick construction.
Just a guess...
Mongo
It's not unusual to have contractors new to the insulated thin-shells not be willing to accept the things we've observed about them and the things the engineers have calculated, if ya know what I mean. :) I can somewhat understand that, be/c if they accept these things and end up being low, it's their neck on the line. Or their warranty. No one likes an "oops." But the usual answer is we keep searching for one who understands, or is willing to go on faith.
I'm pushing for a registry of these homes and what gets installed and how it works. I don't know what the numbers will show...willing to let the chips fall where they may...but at least there'd be an institutional memory that people could refer to. I'm using 8k btu per 1000 sf to heat. Another at 6000' in Montana is using 6750 btu total for 1700 sf! Damn, that puts me to shame. Put these with hundreds of others, and it'll be hard for someone to justify 125,000 btu for 2150 sf.
Wet Head Warrior made a great introduction of a contractor to me, and I've connected him with the client. We'll see how it goes.
If the load is calculated accurately, a 10-15% system regain allowance is usually adequate. With large internal masses this may result in slow recovery but that should be necessary once a year or so. By system regain I mean capacity necessary to bring the system water content, equipment mass and building mass up to temperature after being shut down, or when using temperature setbacks. With your domes and their internal mass inside the insulation I would never recommend setbacks. Setback is also very questionable practice with radiant floor slabs. It just takes to much heat to recover the mass temperature in any reasonable time.
So, did you vere get a good answer to this question?
"...what other factors play into sizing the boiler?"
Pick-up or heat-up if you operate with a set-back, confidence in the heat loss calculation, sizes of equipment that are actually available, potential for expansion, operating efficiencies, and non-heating loads (such as indirect water heating) are but a few.
For your own personal use, you can take the chance that all of your assumptions are correct or at least conservative, and if you are wrong on the one -20 degF cold snap you didn't account for, you get cold or run some supplemental heat.
Can you heat 4200 sf with a 125k condensing furnace? Many say no. My calcs say yes. This winter I'll know for sure. If it wasn't my house, I'd say get more heat.
Typically, I find that calculated loads usually fall between available unit capacities and the next larger size provides adequate margin. Then, I am fairly conservative on heat loss calculations. Cooling and dehumidification handled differently.
>So, did you vere get a good answer to this question?
Got some good explanations and a referral. If the client can't make that work, well, I can't do a whole lot more. The people at The Wall gave some really great in depth info on derating for altitude, etc.
Thanks, all.