Guys, gals,

Does anyone out there pad the design temps while doing load calcs?

Why?

 

The reason for the question, our design temp for winter is about 0 in mid MI.

Yet I keep running across bids where the estimator changed it to -10 over riding the software.

 

What is the point of doing a load calc if you intentionally input misinformation?

J

Joe,

I have seen this done many times with air-to-air system but with less precision.  As a general rule, in my area, the local installers will use rules of thumb for sizing and then add a fudge factor just so the homeowner can rest assured that the unit is large enough.  Ridiculous, but that is the easy and less time consuming way to do it.

Like any other engineering software or calculations there is always unknowns,variables and alterations to the original design of a building. A fudge factor is often used to protect the designer and contractor if in case something was missed or climate changes out the norm do occur. Experience tells me to trust the numbers to some degree but be flexible enough to look past my nose and into the real world of construction.A happy customer is far more important than squeezing every last btu of energy.

It seems with the granularity of a 1/2 ton or 1 ton, you either bump the first cost as a result of moving the design temps or accept a new balance point.  I have done this when I'm close to the maximum output on a product selection to see what happens to cost vs. efficiency.

To answer your question though, No, I don't change low temps on bids, but do while I'm am looking at bid options.
The customer always gets local design low temps on heating here in the NorthEast, but the cooling side gets bumped to 100 instead of 87. It doesn't affect the equipment choice for residential.

It would be interesting to know what the range of inaccuracies is on a load calc. From what I've seen, they give a value but not a standard deviation.

When my system was being spec'ed, I received 2 different quotes. 1 for for a 4-ton system, the other for a 3 1/2 ton. Both contractors took extensive measurements of windows, doors, square footage, etc. The contractor offering the 3 1/2 ton WF didn't give me any kind of load calc, and his system was $32,000. The 4-ton contractor gave me a load calc sheet, priced his GeoMax system at $25,000. I went with the 4 ton. I figured a 1/2 ton difference wouldn't make that much difference, and I'd rather err on the side of caution (plus it was cheaper!). Even though the system is working well right now, I'm glad I went with 4 ton. I keep discovering how poorly insulated this house is, and I'm sure the extra 1/2 ton helps a bit while we get all that sorted out.

(Little did I know what terrible follow-up I got from that 4-ton contractor, but that's been covered already on this board!)

Posted By acwizard on 31 Mar 2011 06:30 AM
Like any other engineering software or calculations there is always unknowns,variables and alterations to the original design of a building. A fudge factor is often used to protect the designer and contractor if in case something was missed or climate changes out the norm do occur. Experience tells me to trust the numbers to some degree but be flexible enough to look past my nose and into the real world of construction.A happy customer is far more important than squeezing every last btu of energy.

I get all that, but why intentionally gigo the load calc?
Why not simply use a larger unit than the load might indicate? That's a "fudge factor". Artificially elevating the requirement is valueless.
j

>I get all that, but why intentionally gigo the load calc?
>Why not simply use a larger unit than the load might indicate? That's a "fudge factor". Artificially elevating the requirement is valueless.

Aren't the design temps for an area based on a average over a few years?  Why is it gigo if you are taking a peak temperature into consideration instead of the average?

-10 wouldn't be the peak (or trough), it is simply 10* less. we've seen closer to -20F.
Maybe I'm obsessing about nothing, but why do a calc at all if you want to pick arbitrary numbers?

It would simply seem easier to me to say "here's your load, however since we know it gets colder than 0F around here I'm gonna give you the next size up in case we need a little extra Ummmph."

Since we started monitoring our systems, I continue to be surprised how different retrofit in older houses behave, and how different the outside climate is. 30 Miles south of us, in the souther tier, peak low temps were -26F this year, 15 north of us on the Niagara escarpment, lowest temp this year was 10F, but is exposed to much higher winds. Quite a difference. Weather data is from Buffalo or Niagara Falls, which one can use as a ballpark, but nothing more. Without manual "adjustment" we would have unhappy customers, since the design programs do not account for certain scenarios. Things are different when trees are around the house, since it protects you from the wind, but also affects the solar gain.....Just a few examples.

I'm with DJ - I apply a little Kentucky windage to account for micro climate (and micro client) differences. A concrete example is the case of a beachfront house in a town with a lower-than-typical design temp for the area, but in this case a state highway runs 30 feet from the house on the west side - pavement soaks up and re-radiates heat

I also assess client's tolerance for a bit of drift off setpoint during torrid afternoons. If they are amenable to dropping the setpoint a couple degrees mid-morning on forecast torrid days in exchange for best dehumidification from a truly right-sized system, then I go with the smaller choice. If not, I round up.

Unlike most of the rest of you, I'm screwed in that there is no such animal as an "auxiliary strip cooler" to bail me and the system out for a buck or two per day during severe weather.

My typical system squares off against both high latent load and multiple small zones, so it is in my interest to hold system size to the bare minimum so as to maximize run times and avoid blowing the registers off small zone supply runouts.

I'm really really looking forward to true variable speed compressors capable of 4:1 or more turndown. Daikin and Mitsu have figured this out; why can't the rest?

Curt,

Would the Veri-Dry system http://www.veri-dry.com/ be an add-on that could increase the cooling ability of a heat pump by removing more moisture from the air?

Does anyone have any experience with this particular brand name unit?

Posted By engineer on 31 Mar 2011 10:24 PM
Unlike most of the rest of you, I'm screwed in that there is no such animal as an "auxiliary strip cooler" to bail me and the system out for a buck or two per day during severe weather.

I've often thought that one of these ducted free standing ACs from big box store would make a fine aux. cooler.

Again guys I'm not suggesting that their aren't reasons to pad a design, but why create "the requirement" on paper. Just do it.
j

Joe - if you mean the portable ACs not needing installation in a window, my understanding is that they test well below advertised capacity. To my knowledge they are not energy rated like conventional window airconditioners, nor are they subject to minimum efficiency mandate. The sky's the limit for efficiency and capacity claims.

Alton - I browsed Veri-dry site. I found it very sketchy - long on superlatives and hackneyed word choices ("super flow control"? - puhleese!), no technical info. However, I infer from several hints that VD may be an automatic runaround bypass damper controlled by RH. That would have the net effect of reducing net CFM per ton, increasing temperature split across the unit, and swapping sensible for latent capacity, though NOT on a 1:1 basis.

Depressing evaporator temperature via actually or artificially (via bypassing) reducing CFM per ton increases refrigeration lift, reduces total capacity and reduces efficiency.

VD "guarantees" no coil icing, but action to reduce coil temperature always increases risk of icing as filters or coils foul, ambient temperature drops, etc. That guarantee likely is stacked with wiggle room and disclaimers.

In short, cooling capacity would drop, not rise.

We have used the "Veri-dry" equipment.  Really great - it does what it says!

when they did my calcs. they only had information on the averae temp 35 miles south or north and there was a 10 degree difference. they used the north one which i agree with. came to 4 tons which was ever so slightly short of needed btu but the cost of the 5 ton vs using the aux for maybe 10.00 of heat was a no brainer.

If you're using the ASHRAE 99% binned hourly number for the heat loss calc, oversizing the pump by using a 10F lower number is a ridiculous thing to do, and leads to inefficiencies. Using 70F as an indoor temp and 0F for a design temp, increasing the delta-T by 10F yields a 14% upsizing that has no effect on comfort, and the decrease in the annual aux heat duty cycle for that 1% of binned hourly outdoor temp is more than offset in lowered system efficiency and upfront cost for the extra half-ton of pump & well.

Most heat loss calculation tools overestimate measured reality by 20%+ already- if anything you'd be better off starting off UNDERSIZING it by a half-ton from the numbers the tools deliver from a total cost point of view.

The data we collect on the houses have taught me to do a manual J and then "adjust" since the software does not allow me for adjustment. Every program tell me that the deep ground temp is 49 degrees, however, the water we get from open systems is between 53-54 degrees all year long. For closed systems, we know we are at a good target if we have the EWT at 30 degrees at the end of the heating season, and the aux kicked in between 30-50 hours.

Yes, BIN hours and HDD are great numbers, but no program adjusts the solar gain correctly, together with the balance point of the house. We have much more clouds being closer to the great lakes, meaning much less solar gain and computed but the software or the algorithm. On a cloudy day my balance point t home is 60F (keeping the house at 69F), on a sunny day the balance point is 35F. Weather data does not fully account for that. Things change when trees are around.

I emphasize that similar houses at different locations, just a few miles apart, can behave very differently, so I adjust, because the software does not allow me to do so. The difference can be larger than 1-2 tons.

I also adjusted to the usage, for shops I use larger heat pumps and smaller fields, since they need more peak capacity, and for residences, I use smaller pumps but larger loop fields, since they pull the heat out of the ground continuously.

We should not be too fixated by the heatloss calculations, since we need to admit that they can be off, and I have seen to many being off by more than 100% compared to real usage data, including my own, although admittingly more in older houses. Changes in temperature and moisture content can change the leakyness of old windows, doors and other things, and your 2 month old energy audit and blower test goes out of the window.

Just a few examples....In my opinion software guides, experience decides.