We're going to try and move on the insulation part of the job ASAP, so we'll see what the load looks like afterward. Thanks again, all!

I was remiss in not thanking dana for putting the Science in my SWAG

No thanks necessary-  arithmetic seems work about the same everywhere, even in PA. 

The load in as-is condition still pretty hefty, (and clearly geo-worthy, if it were the post-insulation load).  I suspect the ~4-ton as-is design load can affordably be cut down to two  or two-an'a-half with insulation & air-sealing given where they're starting from, at which point you have to start looking at whether un-subsidized ductless air source can makes more sense than geo w/subsidy.  Ducted air-source is still less efficient, but that might also be a lower-cost option if it's 10 tiny rooms rather than a relatively open floor plan or something. 

I recently saw this movie played out on a barely-insulated 3-family apartment retrofit in central  MA, and much much higher than code insulation + 3 mini-splits was cheaper/better than all other options, despite decent subsidies.  Without  high-R walls it would have had to be bigger multi-splits or the room to room balance within the units would have gotten out of hand.  The calculated design-condition load on the worst-case unit was still under 20KBTU/hr.  A single geo systems might have made sense were it a single family primary residence, but there was a requirement for separating the utilities between the units.

We've had an insulation expert out to look at the place, and she said that we have R8 in the knee walls, and probably in the attic too since it's in the knee walls. She did a "quick and dirty" load calc while she was here, and said we were looking at a 2 ton even before improvements. (?)

I asked our potential contractors (we had a few last questions for them before making a decision; we'll choose someone within the week) about how much the quote goes down if we only need 2 tons-- noting that we're having a blower door test/ energy audit to confirm, and he said "I’d still recommend staying with the 3-ton system.  This is vital in keeping your heating costs at the lowest during the winter, because of the 2-stage system and the variable speed.  Your balance point for kicking on the auxiliary heating will start-up earlier in the winter with the 2-ton system and ultimately will cost you more to operate.  The 3-ton system will have the lower set-point and keep the geothermal running longer before kicking into auxiliary heating.  My recommendation is to stay with the 3-ton because of a cheaper operation and never compromising on your comfort."

I know that a 3 ton will cost less to operate, but it costs more to put in. Does his argument here make sense? I understand that a 2 ton would require more auxillary heat, but-- depending on how much-- it might take quite a while for that additional cost to pass the additional cost of the 3 ton unit/ drilling.

Posted By lgeldred on 30 Aug 2011 05:12 PM
I know that a 3 ton will cost less to operate, but it costs more to put in. Does his argument here make sense? I understand that a 2 ton would require more auxillary heat, but-- depending on how much-- it might take quite a while for that additional cost to pass the additional cost of the 3 ton unit/ drilling.

A 3ton will not necessarily cost less to operate, the mind set that you want to avoid second or third stage has uneducated contractors reinventing single stage equipment. Didn't we all decide that 2 stages were better?

Any geo guy worth his salt can model op costs for you. Have them show you based on real load calcs for improved envelope which works best and what the pay back is. The best fit for your job depends not only on loss calcs, but set point (where you set the temp), future set-points (in someones forever house higher winter temps might be necessary down the road), as well as cost/kwh.

The guy with the 6 ton offering isn't even close to actual load or usage and needs to justify his offering as well. His chance at your job should be an uphill climb at this point as his ignorance added thousands to your install if you followed his advice.

Joe

Based on the fuel usage analysis, the quick & dirty 2-ton as-is numbers (before improvements) is on the low side of reality, since that would require your furnace to re running well under 50% system efficiency. OTOH, if the heat is being distributed via uninsulated unsealed ducts in the attic, that may very well be the case. If it's a 4x oversized high-mass steam or hydronic boiler it could be running that low too, despite 80% measured combustion efficiency.

For second bite at the "measure the heat load with the existing system" apple, what IS the existing heating plant (type, model, & manufacturer), and what do the nameplate numbers say in BTUs per hour (in/out/IBR/DOE/etc.)??

Sorry for the delay responding. We've been having crazy weather-- lightning storms that knocked out our internet, phone, and cable, followed by record flooding...

I took a look at our furnace. We don't have ducts right now-- it's a radiator system. It's a Weil-McLain, series 2, boiler size AB565-- it looks QUITE old. The only other # I could read on it was 1.35 GPH. Here are some figures from our most recent inspection/maintenance last year; they don't make a whole lot of sense to me, but may to you:

D sm co2 temp
c/s= 0.2-0.6 - 0 - 9% -491.7 =81.3% efficiency

We've decided on a geo contractor and insulation/ air sealing person. The geo contractor is strongly leaning toward 3 tons but will confer with our insulation contractor to get blower door results, learn what we're going to do for insulation, and so forth. Our insulation contractor is going to try to put R49 in our attic and some R13 in the knee walls, assuming there's space. She's also talked with us about Quanta Panel storm windows, which should make our single pane windows much more energy efficient-- and are a fraction of the cost of real replacement windows (http://quantapanel.com/).

Assuming it hasn't been re-jetted for a lower rate, 1.35gpm is about 186KBTU input. Assuming the 80% combustion efficiency is correct (and it may be high if they didn't account for excess combustion air properly) that meant it's output is on the order of 0.8 x 186K=150KBTU/hr.

The napkin arithmetic calculation is about as crude a model as it gets, but it's a reliable upper bound. Using the NORA FSA calculator tool (based on boiler modeling done at Brookhaven National Labs - it's a freebie see: http://www.nora-oilheat.org/site20/index.mv?screen=home scroll down about halfway for the download link) selecting "old boiler with tankless" and bumping the output 150KBTU @ 80% efficiency, and using 2.1% for idle loss, at Harrisburg's heating design temp it comes up with a design heat load of

...33.8KBTU/hr , or about a 3 ton load (which is credible given the idle losses under hot water only loads)

...and an AFUE= 55%

If I bump the idle loss to 5% (probably unrealistically high, but maybe if it's in really rough shape with the exterior jacket rusted through, insulation falling out) it's still coming up with 28KBTU/hr for a heat load and an AFUE of 38%, and still more than a 2 ton load.

But that's the as-is condition. Tightening up the place could be enough to knock the whole-house load by 1/3, which would be enough to put it in the 2-ton range, but it wouldn't hurt to keep a few kilowatts of auxilliary heating strip in the design if you go with 2-ton geo. You'd probably want to go with 30-36K if doing it with a ductless air-source mini/multi-split.

Exterior storms really ARE cost effective compared to replacement windows, as long as the original windows are in good shape and reasonably tight.

I'd still wanna see the model comparing op cost between 2 and 3 ton

The below is from our energy audit/ insulation company.

I suspect, based on previous conversations, that our geothermal guy is still going to recommend a 3 ton; I think he believes that long term it will be cheaper, and that the unit will be more efficient.

I've asked him to run a cost comparison for us.

I also have realized that I have apparently been miscounting the amount of oil we've been using; though we generally fill up once a year (1000 g tank) and then add another 200 in the summer when we run dry, those fill-ups haven't been full fill-ups for some reason. It looks like (looking at my actual receipts) we're using more like 900-1000G per year total, so perhaps that puts us even closer to the 2 ton.

Energy audit company (did our audit on Monday):

"I reran the load cal of your home after gathering more detailed info and this is what came out…

The house as is with no changes would most likely need a 2.5 or 3 ton system. My number came to 2.8 so it is a flip of the coin as to which way to go. Personally I would down size a hair or 2.5.

However once improvements are made to your home I feel you will be good with a 2 ton. My number comes to 2.3, again I would recommend dropping down not going up. Once I get all the reports completed I will set an appointment with you to go over the details but thought you might want this info now for your Geothermal guy."

Assuming she's right, and we're at a 2.3, is it better to downsize?

yes - the number of hours during which you'll need the extra 0.3 ton is fairly small.

Upsizing may well cause you to spend an extra $5000 to save $50 per year in aux strip use. That's a poor way to invest $5000. The larger unit will short cycle, reducing its actual, boots-on-theground efficiency which will eat into the (apparent, not proven) cost savings supposedly occasioned by reducing strip heat use.

Downsizing in green grass heat-dominated climates (basically anywhere north of I-40 and east of the Rockies) results in better, more efficient cooling season performance - smaller units run longer, resulting in more dehumidification and greater comfort.

I think "down size" is an unfortunate choice of words. How 'bout "right-size" or how 'bout simply choosing not to "over-size".
Most of the pros around here do not favor 100% loading; that is trying to cover your entire heat loss with geo vs employing auxilliary heat.

j

Certainly, Joe-- perhaps that was a poor choice of words. I definitely want a right-sized unit, especially since I understand that both under and over sizing units have negative consequences for the unit's longevity and functionality (based on reading this forum!). However, I would guess that most people's exact load is probably in-between two numbers, so a decision needs to be made to go with the higher or lower tonnage. I've told my audit person and geo person to get together and come to a consensus; they're both rational and intelligent, so that should work, right?

My nightmare is that we end up with a system that costs us far above estimated operating costs. I've seen some stories online of people putting 20-40k into a system and then having very high electric bills that wipe out their supposed geo savings. It seems that these situations generally come from people working with inexperienced installers who make serious mistakes, including perhaps radically undersizing a system so that aux heat runs constantly. I don't want that to happen to us, and all the variations in load calculations have increased my anxiety on this front. But I do think we have a very competent installer who has a lot of experience with geo, good references, etc.

100% or even 150% of loading might be cost effective with variable speed compressor split air source systems, since they operate significantly more efficiently at low-to-mid speed than maxed out, and typically have at least a 3/1 ratio between max/min. A 2.5-3 ton mini/multi-split will be able to modulate to under 10K in cooling mode, below the capacity of larger window-shaker AC units. With oversized geo you'll have cycling loss & equipment wear issues.

Without doing a careful heat loss analysis than a Manual-J (which will almost always oversize by at least 15-20%) the "after" picture, even a 2 ton system might prove oversized from an optimal economic point of view. But being 10% oversized is a far lesser crime than being close to 100% oversized (the 3.4-ton proposal #5 ). Your insulation contractor says it calculates out to be 2.3 tons after upgrades, but if that's a Manual-J type analysis & overshooting by 20%, your true design condition heat could be more like ~22K. With a 2 ton unit would have ~110% of max needed capacity, but at 3.4 tons (proposal #5) you'd be looking at ~185% of the actual design condition load.

As engineer correctly points out, paying an extra $5K up front to deliver the full load, saving $50 on the power bill for auxilliary heat strip use is a lousy investment. At current subsidy levels the same $5K could buy you enough photovoltaic solar to return several times that amount. (A popular "NetZero" option on new construction these days is mini-split + PV solar, once the high-R passive solar balance points have been optimized in the economic modeling. But with anything, it's a moving target as subsidies come & go.)

elgeldred,

Curiously the experts working for you there are slowly matriculating towards the expert advice you are receiving here. It will be up to you to decide when they are "close enough".

You are correct in assuming many loads fall between heat pump sizes, but in heating apps it is the uneducated installer who suggests the higher than load tonnage (rarely is it the preferred choice).

j

What joe said about under vs. oversizing- it's true for most types of heating systems true x2 for geo. (It gets fuzzier with modulating burners or varialble speed compressors though.)