By dropping it those 10 degrees to 15F you're adding ~20% to the size of the system, for no good reason.

This is the trick employed locally when faced with a well-heeled customer and a large home. If the salesperson thinks they can pull it off, you will see them drop that design temp so that the energy analysis indicates a heat load just barely larger than a single unit can handle and before you know it there are two furnaces/heat pumps/whatever on the proposal.

Posted By ICFHybrid on 17 May 2011 08:36 AM

By dropping it those 10 degrees to 15F you're adding ~20% to the size of the system, for no good reason.

This is the trick employed locally when faced with a well-heeled customer and a large home. If the salesperson thinks they can pull it off, you will see them drop that design temp so that the energy analysis indicates a heat load just barely larger than a single unit can handle and before you know it there are two furnaces/heat pumps/whatever on the proposal.

Designing to the record low temp or something rather than the 99% or 97.5% ASHRAE numbers wasn't invented yesterday, nor is erring to the high side on every assumption for the heat loss calc. 

I recently consulted on a situation local to me where 3 separate heating contractors had submitted bids to replace an oil boiler, with heat load estimates between 90K  to 117K BTU/hr.  Using oil consumption data even the 90K number would have implied fuel consumption in excess of 2x the source fuel energy used by the inefficient antique with the embedded coil that the new system was to replace.  Using the Brookhaven Nat'l Labs boiler model and the fuel consumption to measure the heat load, at the 97.5% design condition temp came in at around 38K, at the 99% condition heat load was about 42K.  At the time the homeowner was in the middle of insulating & air sealing his basement and putting down R5 XPS over the slab under a finish subfloor, which would probably drop the 99% condition heat load to under 35K. (We didn't model it with any precision.) 

Thus, the quoted systems were all ~3x more boiler than he actually needed to serve the load!

Mind you, this was a 2500' 1950s single story ranch house with much more exterior surface are per square foot of living space than a 4000' 2-story, and his 99% outdoor design temp is -5F, fully 20F cooler than the design temp used by the contractors in this SW WA example, and 30F cooler than the 97.5% design temp that would be more appropriate here.  He had replaced all the windows & air sealed to about 3.5ACH/50, and had upgraded both the wall and attic insulation with higher density fiber (at a higher R in the attic), but this was not a  "deep energy retrofit" gut job by any means, but more than the lowest of the low-hanging fruit.

The homeowner passed an all three boiler swap quotes, and is instead opting to pay the gas company to extend service to his house and is installing an HTP Versa modulating-condensing hot-water/heat combi, since even the smallest oil boilers would be nearly 2x oversized for his load (and many times oversized for any individual heating zone) and could only  operate well under it's rated efficiency even in a best-case system design.  With the gas fired system he'll hit the low 90s for efficiency rather than ~75% with a best-case oil scenario, and with a fuel approximately half the cost per BTU.

Between extending the gas service and boiler it came in at a bit more than 8 grand more money,  but between the difference in fuel energy cost and difference in as-operated efficiency it'll be made up in under 5 years (under 3 if oil stays at $4/gallon).  To do geo in his situation would have been on the order of 20 grand more than a simple boiler-swap, and the operating costs would have been only slightly lower than with the gas fired combi.  He's planning on spending some of the difference on (highly subsidized) solar PV and solar thermal, and is ultimately expected to end up with lower overall operating costs than geo for somewhat less upfront cash. YMMV.

Posted By Dana1 on 16 May 2011 11:12 AM
How are you currently heating the house (equipment type, and approximate AFUE efficiency), and what is your energy use like? It's possible to calculate a very close approximation of the whole-house load at any given temp by correlating heating energy use at the efficiency of the heating equipment against heating degree-day weather data.

Dana, the home is currently heated via cable ceiling (radiant) and wall heaters. There is no cooling...although we would probably be putting in a few window air conditioner units this summer (we moved in last Oct). The KWH used over the past several months is as follows:

4/11: 2940
3/11: 3300
2/11: 3420
1/11: 3300
12/10: 3960
11/10: 4140
10/10: 1890

We have a guest room (288 sq ft) and a large family room (920 sq ft) that were pretty much unheated; thermostats were kept at 50 deg or lower except on rare occasions when they were being used. Both rooms will need to be heated going forward. We kept used rooms no higher than 68 deg and would set them to 62 deg (or lower) at night or when not in use.

The windows are older aluminum frame and have storms on them (a few do not though). Window orientation is as follows: 7 South, 9 West (2 are very large), 9 North & 8 East.

The attic insulation is around R-30, perhaps a bit less. The house is 2x4 construction and there is wall and crawl space insulation (not sure of R-value, but sure it was typical for 1968).

Our energy use is drastically lower than the people who lived here before. Their bills were 2x as much. Before moving in I used spray foam insulation and weather stripping around all exterior doors, around plumbing fixtures on outside walls and put in foam gaskets over all electrical outlets and switches.

Forgive my ignorance as I am slowly educating myself on all of this, but you are of the opinion that our home should need substantially less than 6 tons?

Forgot to mention: there is no basement. The 920 sq ft family room area is on a slab, the rest of the first floor (1,387 sq ft) is crawl space.

For realistic heating degree-day weather data, what's your ZIP code, and the exact meter-reading dates?

And for a handle on how much baseline power used for other purposes to subtract out. You might be able to use your May & June billing to figure out a baseline to offset.

If the attic isn't used for storage, and the R30 is batts-between 10" joists, with the joist-tops exposed, blowing cellulose until the combined batt + cellulose depth is ~16-18" is worth it, after fully air-sealing the attic floor. Low density fiberglass batts in horizontal applications suffer significant performance losses at the peaks of winter & summer. (In winter from convection, in summer from IR translucency.) As little as 3" of overblow with a higher-density more air-retardent material like cellulose restores the R-value performance of the batts, but pushing it to R50-R60 is cheap & easy. And by putting insulation over the tops of the joists thermally breaking that thermal-bridge the net performance increases dramatically more than the increase implied by center-bay R-value. Going from R30 batts to a thermally-broken total R50 by doing a cellulose overblow will be at least a 50% reduction in heat gain/loss through the ceiling.

As a rough handle on it,assuming you're close to Longview WA, the mean temp for Longview in January 2011 was about 40-42F, or 23-25 heating degree days per day. http://weatherspark.com/#!graphs;a=USA/WA/Longview (Select a monthly view, zoom in to where it's giving you 4 gray blocks per month , and eyeball the average temp the grays using the cursor.)

Assuming your billing period was 30 days, and you used 500kwh for other things you then used 2800kwh for heating, averaging 93kwh/day, or which works out to an average load of 3900 watts for heating. 3900 watts is (3900 x 3.412=) 13,307 BTU/hr, when the average temps is 23F below 65F (42F), or 13,307/23= 579BTU/hr for every degree below 65F.

At 20F outdoors the heat load would then be (65-20) x 579= 26,000 BTU/hr.

At the ridiculously low 99.9% 15F outdoors the heat load would be (65-15) x 579=28,950 BTU/hr.

Judging by your November & December numbers either you weren't home during part of that period or the billing periods were longer than 30 days some months, shorter than 30 days other months, or ran a wood-stove some days. But even if you DOUBLED that number to factor out anomalies like that you'd still be under 60K, at a design temp delta from that's 20% lower than makes sense for sizing the geo.

If you take the November 2010 consumptioin of 4140 and presumed ALL of it to heating for 30 days, the WeatherSpark data looks like it was averaging about 40F, maybe a little less, for 138kwh/day, or an average of 5.75kw (x 3412= 19619 BTU/hr) at of 25F less than the base-65F. That's 785BTU/hr for every degree below 65, so at 15F you'd still be looking at only (65-15) x 785=39.3 KBTU/hr. Clearly a good chunk of that went to heating hot water and keeping the lights on (at least 10%), so call it 35K (less than 3 tons). At a more realistic 99% design temp of say 20F you'd be looking at 35.3K. Factoring out your other power use it comes in ~32K.

Whether your 20F heat load is closer to 26K or closer to 32K, there's absolutely IMPOSSIBLE to be anywhere near the 73K heat loss calc performed by your contractor. Your actual heat load simply can't be more than the amount implied by the energy inputs that kept you warm. You might factor in a small amount for solar heating if you model the weather data correctly, but in foggy-dew WA I'd be shocked if it was as much as 10% of the total energy input.

Get us the zip & meter-reading dates and we can zoom in on it with far greater accuracy, but first-blush it seems like 3 tons heating would be adequate, 4 tons would be oversized, and 6 tons would be RIDICULOUSLY oversized. And that's based on the actual performance, and BEFORE doing another round of the cheapest/easiest upgrades to the building envelope. It's not unreasonable to expect that spending the difference between 2 ton geo and a 6 ton geo on building upgrades that you'd end up under 2 tons for a total heating load. (It might even get there on the difference between 2 ton geo and 3 ton geo, since you may already be closer than you think.)

On the crawlspace area it's well-worth sealing off any venting to the outdoors and putting down a heavy (10mil) poly vapor retarder mastic sealed to the foundation a foot or so above the floor plus 2-4 inches of EPS (or 2" of XPS) glued to the foundation walls, held permanently in place with furring through-screwed to the concrete. Top seal the EPS to the foundation sill & insulate/seal the band joist with 2" of closed cell foam. It may also still be cost-effective to put a couple inches of EPS on the floor of the crawlspace, but that's of lesser priority. See:

http://www.buildingscience.com/documents/information-sheets/crawlspace-insulation/files/bscinfo_512_crawlspace_edit.pdf

and

http://www.buildingscience.com/documents/reports/rr-1003-building-america-high-r-foundations-case-study-analysis

If the slab edge is uninsulated (likely), there is likely to be some worthwhile retrofitting to be had there too, but the ease of execution in a retrofit would vary considerably with construction details and soil conditioned, etc. See:

http://www.buildingscience.com/documents/information-sheets/slab-edge-insulation/files/bscinfo_513_slab_edge_insulation.pdf

Better to concentrate on the cheap & easy stuff first (like air sealing & attic insulation).

Operable shades on your exterior S & W windows can probably shave literally tons off your potential peak cooling loads too. If you replace the west windows a heat-rejecting type might be appropriate, but you probably still want to use higher-gain S windows. The angle of the sun at the solstice is pretty high there at ~46 north latitude so it doesn't take much of an awning or shelf-type shade to rid you of half of the unwanted summer gain, but an exterior pull-down type may be necessary to kill gain on the W side.

He could get it all with a ductless mini air source heat pump, including the cooling. That would cost a third of what he was looking at previously. He would be in COP 3+ territory and could still retain the resistance heat as a backup or to hit cold spots.

Here are the Heating Degree Days for the same period for the Vancouver Airport station.

4/11: 511
3/11: 570
2/11: 689
1/11: 721
12/10: 679
11/10: 596
10/10: 332

Dana, thanks. My zip is 98642 and meter reading is done on the 4th - 6th of each month (so April's usage was based on a 5/5 reading). Days per cycle are as follows:

4/11: 2940 Reading 5/5 (29 days)
3/11: 3300 Reading 4/6 (33 days)
2/11: 3420 Reading 3/4 (28 days)
1/11: 3300 Reading 2/4 (29 days)
12/10: 3960 Reading 1/6 (31 days)
11/10: 4140 Reading 12/6 (32 days)
10/10: 1890 Reading 11/4 (29 days)

Please note that the home is 4,307 sq ft and 1,208 sq ft of it was pretty much unheated. Plus we were pretty stingy with the temps in the heated areas. I am not trying to argue with your numbers, but a 2- 3 ton system seems ridiculously small for this size of house.

I agree with Dana - 3 tons (about double your coldest month average rate) should be in the ball park - so I'd review a Manual J that calls for 6 tons. You want the electric heaters (edit: existing or new) to kick in on the coldest days.

Mini splits tend to perform much better than window units.

Jonr, ductless is out and we do not want to keep our existing heaters - they're old, ugly wall heaters or cable ceiling (we want to put in recessed lights).

I have two manual J's and am waiting on the numbers from a third. The first company (does geo & air source) came up with 73,342 heating at 23 deg and 46,373 cooling at 89 deg. That heating load is close to the second bid (geo only) of 75,595.

Posted By Tyler23 on 21 May 2011 10:44 AM
Jonr, ductless is out and we do not want to keep our existing heaters - they're old, ugly wall heaters or cable ceiling (we want to put in recessed lights).

I have two manual J's and am waiting on the numbers from a third. The first company (does geo & air source) came up with 73,342 heating at 23 deg and 46,373 cooling at 89 deg. That heating load is close to the second bid (geo only) of 75,595.

He refers to the auxiliary heater built in the output of the heatpump, which keeps upfront costs down and actually safes you operational costs, since a smaller unit running continuously is more economical than a larger unit which cycles on and off. If a system is sized correctly the aux heat should come on on the coldest days. Based on the data you have submitted, 3-4 tons (more 3 tons) are the correct size, 6 tons is way too much. Even 75000 btus in the manual J is rather 5 than 6 tons. It is usually a rookie mistake to oversize that much, I would question their experience in geosystems. Or they know about it and purposely try to sell you a higher priced system. Either way, for me that disqualifies a contractor. The company sizing you with a 4 ton seems to be more on target.

Thanks Doc; I misread.

The 4 ton listed by company #1 was only for part of the house (the upstairs and family room) with an additional 2 ton for the downstairs. So both companies are proposing 6 tons whether geo/air or single/dual systems. I will see what company #3 comes back with. If they are about the same, then I guess I have chosen three dishonest or clueless contractors or I have left out a big piece of the puzzle.

If they are about the same, then I guess I have chosen three dishonest or clueless contractors or I have left out a big piece of the puzzle.

I've seen five out of five bids that way, recently.

Maybe you neglected to tell them that cost and/or the cost/value ratio was important to you, the consumer.

If you posted the particulars from one or both of the studies, maybe someone could pinpoint where the discrepancies are.

What are the (uncombined) loads for the 2 zones? Sometimes the lack of 1/2 ton increments in 2 stage units makes me oversize when working with zones.
J

ICF: I have a hard time believing that both companies doubled the amount of tonnage needed because I didn't tell them I was dollar-conscious. I just ran some numbers here: http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm and I am coming up with 75k too. (finally got the attachments to take but not sure if you'll be able to read). I am taking a swag at some figures, but I can't imagine I am that far off. Please take a look and let me know if anything jumps out at you.

Joe, the air source HP proposal was for one 4 ton and one 2 ton unit (no halves). The 4 ton would have two zones and be a two stage.

Posted By ICFHybrid on 22 May 2011 09:32 AM

I've seen five out of five bids that way, recently.

Maybe you neglected to tell them that cost and/or the cost/value ratio was important to you, the consumer.

Bids for what?  What time frame constitutes "recently"?

ICFHybrid, did you also neglect to tell the contractor that the cost and/or the cost/value ratio was important to you?

I asked for the zone 1 load calc and zone 2 load calc, not proposed equipment.
75MBH does not indicate 6 tons to a geo pro unless it is a cooling load. If it is a heating load, depending on local electric rates we would use auxiliary for the to p 10 or 15 MBH possibly making 5 tons a good fit for you.
Rookies tend to try to meet 100% of a heating requirement with geo.
j

Thanks Joe. For geo, I don't have a break down by zone from either company. I'll give you what figures I can here for the air source HP (not 100% sure what I am looking at).

Zone 1 (upstairs) - 1,964 sq ft: 31,912 heating; 17,861 cooling
Zone 2 (fam/bonus room on slab)- 762 sq ft: 19,570 heating; 9,744 cooling
Zone 3 (downstairs) - 1,595 sq ft: 18,835 heating; 8,693 cooling
Totals = 73,342 heating & 46,373 cool (37,838 + 8,535 latent gain)

Zone 1 & 2 proposed 4 ton; zone 3 = 2 ton. It looks like the temps used for this air source was 15 deg heating and 100 deg cool (both "Outdoor Dry Bulb").

The second company doing geo only has a much higher cooling load of 81,533.

Does this give you the info you were looking for?

3-4 ton for first 2 zones around here and 1 to 1.5 down.
Itd be nice if we could divide zones differently.
Perhaps air source for 3rd zone would still make sense.
j

Yes, we have some ducting challenges so that's probably why the zones are set up the way they are. So your opinion is that the tonnage is not that far off (although closer to 5 rather than 6) provided the heating and cooling loads are accurate?

I think I will ask both companies to rerun their load calcs with less extreme temps and bump up the attic insulation and see how that changes things.