BTU Estimation (using App) seems high?

[clock]

Hello all,

I am not very experienced, so please forgive me if this sounds naive. It will not bother me if you tell me I'm completely off on my calculations or being taken for a ride on the estimates. Regardless, I certainly can't afford to do all these. This is for the northeast.

I used an application to estimate the heating and cooling loads at 4 properties I rent out (the app uses Google maps and other information coupled with user input for its estimates). I am interested in ductless heat pumps (I'll describe why for each below). Here is what I calculated (seems high?) and the quote I was given from a vendor (estimated savings for each: non-commital).

==Home 1:==
~400 square feet, 1 floor, 1 bedroom, 1 bath, 2 other rooms and hallway
Cape cod roof
Newer oil boiler (hot water radiators)
Goal: A/C, possibly remove bulky oil tank

This App's Estimates:

• 38,988 S Cooling Load (40,693)
• 4,908 L Cooling Load
• 94,026 Heat Load (87,518)

Vendor quote:
2 Mitsubishi Hyper Heat Multi-Zone Heat Pump Condenser, MXZ-4C36NAHZ
4 Mitsubishi M Series Indoor MSZ-FH06NA
$12,600

==Home 2:==
~2,000 square feet, 2 floors, 5 bedroom, 2 bath, 4 other rooms
Cape cod roof/knee walls on 2nd story
oil boiler (hot water radiators) on last legs (goal: replace system)
insulation added last year, double-pane windows, no insulation between basement and 1st floor

The App's Estimates:
Total:

• 47,894 S. Cooling Load
• 12,668 L. Cooling Load
• 123,062 Heating Load

1st Floor:

• 31,922 S. Cooling Load
• 71,861 Heat Load

2nd Floor:

• 14,483 S. Cooling Load
• 52,201 Heat Load

Vendor's quote:
2 Mitsubishi Hyper Heat Multi-Zone Outdoor Unit, MXZ-5C42NAHZ
2 Mitsubishi M Series Indoor MSZ-FH12NA
2 Mitsubishi M Series Indoor MSZ-FH06NA
4 Mitsubishi M Series Indoor MSZ-FH09NA
(Happy to provide more details on other supplies - drains, brackets, etc)
$26,750

==Home 3:==
~3,000 square feet, 2 floors, 6 bedroom, 3.5 bath, 1 great room with pantry
Barn style roof
22" fiberglass wool insulation in attic, blow-in walls, leaky wood beadboard, batt insulation under floor
oil boiler (hydro air) high-efficiency but over $600 in oil a month; still, I'd be happy just doing 2nd floor for AC and some savings, but I believe this quote is for 2 floors.
Goal: A/C (at least on 2nd floor), reduce energy costs; minor goal to help distribute temperature a bit between between bedrooms

The App's Estimates:
Total:

• 43,990 S Cooling Load
• 14,757 L Cooling Load
• 150,626 Heating Load

First Floor:

• 23,735 S. Cooling Load
• 83,533 Heat Load

Second Floor:

• 19,200 S Cooling Load
• 67,094 Heat Load

Vendor's quote:
2 Trane XR14 2 Ton Heat Pump Condensers R-410A 13 SEER Model 4TWR4024G1000A
2 Trane 2 Ton Variable Speed Air Handler R-410A Model TEM6A0B24H21SA
2 Trane 8 Kw back up electric strip heater BAYHTR1508BRKC (seems silly because we have existing oil system, but they propose replacing existing handlers - not sure if it makes sense)
$18,000

==Home 4:==
~5,000 square feet, 2 floors + "finished"-ish basement, 5 bedroom, 3.5 bath, 4 other rooms not including basement
Cape cod roof/knee walls on 2nd story
newer gas boiler (hot water radiators) on 1 zone
Goal: A/C, particularly on 2nd floor; due to 1 zone, warm up 2nd floor bedrooms without grilling those on 1st
no insulation, double-pane windows

The App's Estimates:

• 277,820 S Cooling Load
• 20,139 L. Cooling Load
• 520,627 Heating Load

2nd Floor:

• 97,582 S Cooling Load
• 221,098 Heat Load

1st Floor:

• 105,923 S Cooling Load
• 209.284 Heat Loss

Basement:

• 72,650 Sensible Cooling Load
• 90,246 Heat Load

Vendor Quote: (I believe this was limited to just doing the 2nd floor)
1 Trane XR16 2.5 Ton R-410A Condenser 16 SEER 4TTR6030J1000A
1 Trane 2.5 ton variable speed R-410A Air Handler TEM6A0B30H21SA
$13,750

Ok, I've spent a little too much time looking into this for one day! Time for bed, I eagerly await any responses you have

 

[Bob Beasley]

What App did you use? I'm at the same stage as you albeit in the South. I'd like to run mine through the App and see how it compares to the proposal I got from a local Mitsu dealer.

Thanks!

 

[Sponsor]

 

[clock]

www.coolcalc.com

It is free! Beta version, it looks like. What do you think of the output? If it is wrong, it could just be my ignorance in trying to use it

 

[Dana]

The northeast is a pretty big place, with 99% outside design temps ranging from -15F to +20F, with 1% outside design temps from 78F to 92F. Care to be a bit more specific? If you're outside design temps are under 17F it makes a real difference on how one would specify the heat pumps.

I've looked at Cool Calc before, and haven't been favorably impressed with anything other than the price (which it may or may not be worth.) Whether you used the app correctly or not, there's no way a 5000' house would have a heat load of 520,627 BTU/hr unless you left a lot of windows and maybe even some doors open.

A 400' house with a heat load of 94,026 BTU/hr would be a tent located at the south pole. Six tons of ductless compressor for a 400' house is absolutely insane. Even if that's actually a typo and it's 4000', not 400', those numbers are on the high side.

As much as I detest rules of thumb...

At 0F outdoor temps a reasonably tight insulated 2x4 framed houses come in with a heat load of about 15 BTU/ft-hr of conditioned space if it has an insulated foundation or very limited uninsulated foundation above grade, to about 20BTU if the foundation is uninsualted with more than a foot of average above grade exposure. If it's pretty air-leaky it can run 25 BTU/ft @ 0F. If uninsulated it could run 30-35BTU/ft @ 0F. Scale accordingly to your local 99% outside design temps. A 2x6 framed and code-min low-E windows typically comes in at about 11-12 BTU/ft with a less-lossy basement, 15 BTU/ft with a lossier basement.

In New England cooling loads of upper floors come in at roughly a ton per 1000-1200' of conditioned space, first floors with conditioned space above come in between a ton per 1500-1800'.

There are many exceptions to these rules, but it's nothing like a 2x error, let alone 10-20x.

If you have a heating fuel use history on any of these places you can nail down an upper bound on the heat loads using fuel use against heating degree-day data if you have exact fill-up dates & quantities on winter period fill ups (or meter reading dates & quantities if on the gas grid.) If you have that data, take the time to run the heat load numbers using this methodology. (No whining about how much time it takes- you'll be saving literally 10s of thousands of dollars in equipment cost.)

 

[clock]

Dana, wow! I won't complain, but I am in the middle of a both landlord and non-landlord work this week, so I can't go through the whole methodology now - but I am going to sit down and really study what you wrote and go through my records to see. You can see my naivete on the location item - so to be more specific, we are talking about the middle of CT.

What I should mention - because I didn't make it clear - was that I did the Cold Calc app after getting the estimates because I did not trust them. Partly because my partner was not happy with the quotes and I decided to examine them further. I have no idea what the vendor used for his load assessments - could have been even further off!

I should also mention - the reason some of these are not insulated is simply because I don't trust the approaches I've heard. For example, for home 1 and 4, there is drywall covering every inch of attic. They want to just blow fiberglass in there, no ventilation. I just can't do it. I'm a stubborn mule sometimes.

Definitely not a typo - 400 square feet. It is small.

More details:


==Home 1:==
(~400 square feet, 1 floor, 1 bedroom, 1 bath, 2 other rooms and hallway; Cape cod roof; Newer oil boiler (hot water radiators); Goal: A/C, possibly remove bulky oil tank)

This one is real brick, no insulation above, garage below, there's some old crappy insulation above garage ceiling. Built in 50s.
We put in new double-paned windows, total of about 6, sizes normal-to-small
Oil: 4 deliveries a year, newer furnace used for both heat and hot water for 1 person. However, I think furnace is oversized? The hot water oscillates between hot and cold.


==Home 2:==
(~2,000 square feet, 2 floors, 5 bedroom, 2 bath, 4 other rooms; Cape cod roof/knee walls on 2nd story; oil boiler (hot water radiators) on last legs (goal: replace system); insulation added last year, double-pane windows, no insulation between basement and 1st floor)

This one has vinyl siding, normal-sized double-pane windows. Built in 50s. We had 6 deliveries of oil one year, but last year FAR FAR more - despite the fact that we added an electric hot water heater to take hot water off the boiler and insulated the previous year! I mean, at times it would burn through a tank in 2 weeks last year. The boiler is about 50 yrs old.


On a different note, thinking this over now - if the boiler is really old, but (before it started going bad) was going through only two more tanks than the new boiler in Home 1 - and home one is 5 times smaller - what is going on with home 1? Doesn't that seem off? Could that be due to lack of insulation alone?


==Home 3:==
(~3,000 square feet, 2 floors, 6 bedroom, 3.5 bath, 1 great room with pantry; Barn style roof, 22" fiberglass wool insulation in attic, blow-in walls, leaky wood beadboard, batt insulation under floor; oil boiler (hydro air) high-efficiency but over $600 in oil a month; still, I'd be happy just doing 2nd floor for AC and some savings, but I believe this quote is for 2 floors; Goal: A/C (at least on 2nd floor), reduce energy costs; minor goal to help distribute temperature a bit between between bedrooms)

This one has wood siding, our energy audit definitely knocked the beadboard walls. The bedroom with a beadboard ceiling is always colder in winter. Built - who knows how long ago. Double-pane windows. 6 deliveries of oil one year. This has a fairly new Buderus boiler that does heat and hot water. To me, this is a bit of an oddity. Although it isn't perfectly tight, from an insulation and equipment perspective, I really wish we were spending a lot less of oil.


==Home 4:==
(~5,000 square feet, 2 floors + "finished"-ish basement, 5 bedroom, 3.5 bath, 4 other rooms not including basement; Cape cod roof/knee walls on 2nd story; newer gas boiler (hot water radiators) on 1 zone; Goal: A/C, particularly on 2nd floor; due to 1 zone, warm up 2nd floor bedrooms without grilling those on 1st; no insulation, double-pane windows)

This one is all brick, huge windows, which we replaced with double-pane. Built in 50s. Huge, newer commercial boiler for heat, we added a separate gas hot water heater as well. We spent 1,765 in natural gas over course of a calendar year, although I don't have exactly how much gas that represents with me.

 

[Dana]

On the oil deliveries, if they stamped a "K-factor" on the slips, what is the K-factor for mid and late winter deliveries? Simply knowing the number of deliveries without the dates & quantities, and the ZIP code is useless, but a K-factor contains the same information. Mid to late winter fill-ups would have less influence from cycling losses or hot water use, so those are the ones to use.

Taking it to the basics using Home 1 as the example...

The 99% outside design temps in central CT are about +5F, give or take 3F. The code minimum interior temp is +68F, and for load calculation purposes that is what should be used. So you're really talking about an interior to exterior temp delta of about 60-65F.

A double-wythe brick wall (~7-8" thick, no masonry cavity) with painted brick interior (no plaster, no wallboard) has a U-factor of about 0.40 BTU/hr per square foot per degree difference. If it has furring + plaster & lath or furring + wallboard on the interior that drops to about U0.30. So for load calculation purposes every square foot of wall adds between (U0.30 x 65F=) 20 BTU/hr and ( U0.40 x 65F =) 26 BTU/hr to the load. A code min window is U0.32, which isn't much different from your wall's U factor. An uninsulated vented attic has a U-factor somewhat lower than that, but for now why don't we just assume all surfaces have a U-factor of U0.35?

Let's also assume it has 11' walls (which includes the depth of the floor & ceiling joists), and 15 x 27' rectangular footprint.

Total attic area: 400 square feet

Total wall area: (2 x (11' x 15')) + (2x (11' x 27') = 924 square feet

Total above grade surface area: 924 + 400= 1324 square feet.

Conducted heat loss: U0.35 x 65F x 1324'= 30,121 BTU / hr.

Go ahead and double that as a gross fudge factor to cover for air leakage and losses out the floor and you're still at about 60K (reality will be way less than that), but they're no way to get it that 90K number without opening a window or two. It's well worth air sealing and insulating at least the attic, which would cut the conducted heat loss down to about 20K, and throwing in a 50% fudge factor (lower now that the attic is air sealed from the space) for infiltration losses it might hit 30K, but that's probably a stretch, and a ridiculously lossy number for a 400' house.

Home 2's load is probably around 40,000 BTU/hr if it has R11s in the walls, which would have a U-factor of about U0.10. The kneewalls and probably leak like a sieve, and you probably only have U0.10 for the cathedralized head-banger ceiling too (R11s beween 2x6 rafters?). Tightenened up you should be able to bring it down to the 30-35K range. If you air seal and insulate the basement walls (preferable to trying to air seal & insulate the basement ceiling, since the latter would put the boiler & distribution plumbing outside the thermal envelope) it would likely come in around 25K.

Home 3 may be at risk for moisture issues if the interior finish walls are air leaky. Milled bead-board plank allows a lot of convective moisture transport into the walls. Hopefully the siding is at least somewhat back-ventilated(?) such as cedar shingles to promote drying toward the exterior (?), with decent roof overhangs to limit the amount of direct wetting(?). With blown cellulose in full-dimension 2x4 walls, plank sheathing and clapboard or shingle siding you're looking at about U0.08 for the walls (on the order of 1/4 the heat loss per square foot of wall for house #1).

Home 4 is hard to guess- dig up the gas bills, but here's the order of magnitude. Larger houses and multi-story houses tend to have signficantly lower BTU/ft ratios than 1 and 1.5 story capes & ranchers.

At a buck a therm that would be about 1800 therms, in an 85% efficiency boiler delivering (0.85 x 1800= ) ~1500 therms net into the house (the rest went up the flue) and a ~5800 HDD heating season (roughly the 25 year average for Hartford) comes out to (1500/5800=) 0.2586 therms per degree-day. At 100,000 BTU/therm that's 25,860 BTU/degree-day

With 24 hours in a day that becomes (25,860/24=) 1078 BTU per degree-hour.

The heating degree base presumes a 65F heating/cooling balance point, so at +5F you have only (65F -5F= ) 60 heating degrees, for an implied heat load of 60F x 1078= 64,680 BTU/hr.

At about 13 BTU/hr per square foot hat would be somewhat load for a double-wythe brick 2 story with no wall insulation, sure it's not a brick-veneer with 2x4 framing?

We can plug in the real numbers using the actual wintertime fuel amounts and real weather data, but the heat load wouldn't even reach 520,627 BTU/hr even at an outdoor temp of absolute zero. Larger houses and multi-story houses tend to have signficantly lower BTU/ft ratios than 1 and 1.5 story capes & ranchers.

 

[clock]

Dana,

I just want you to know how much I appreciate this! It is incredible how knowledgeable you are, even down to the particulars of CT - I feel like I am talking to a home doctor. I won't have access to some of these things for about a week, but I will definitely get back to you by then. In the meantime, this is an excellent start!

A question for you - once we have BTUs, assuming that for at least House 2 that we want it to take care of 100% of the heat, what do you typically put down as the ratio of BTUs to tons? I have heard anything from 500 BTUS to 1500/ton. Or is it more about the BTUs for each space? In which case - how on earth do I find someone who knows what they're doing? Do you travel?

 

[clock]

Reading through this again - for house 3, I have been thinking about replacing the siding with cement board because I feel like painting it is useless. Maybe it is just because, by comparison, I never need to paint the others! It just looks so crappy. If so, I was thinking of putting a layer of extruded foam on before putting the siding up. Sounds like that could be a disaster from a moisture perspective. If you have any thoughts on this - I'm all ears.

 

[Dana]

Reading through this again - for house 3, I have been thinking about replacing the siding with cement board because I feel like painting it is useless. Maybe it is just because, by comparison, I never need to paint the others! It just looks so crappy. If so, I was thinking of putting a layer of extruded foam on before putting the siding up. Sounds like that could be a disaster from a moisture perspective. If you have any thoughts on this - I'm all ears.

Extruded foam (XPS= eXtruded expanded PolyStyrene) is labeled at R5/inch, but is manufactured using HFC blowing agents (which are also extremely powerful greenhouse gases) that leak out over time, eventually falling to about R4.2 inch. Even though manufacturers' warranty limits that degradation to not less than 90% of initial performane at 20, 50 or lifetime, those warrantys are little more than marketing fluff. EPS is the same polymer, blown with pentane (with about 1/200th the climate impact), and has a stable thermal performance pretty much forever (about R4.2 inch for 1.5lb per cubic foot density "Type-II" , which is the most commonly used density) and is 25% cheaper per R than XPS.

Rigid polyisocyanurate is typically labeled R6-6.5/inch, but takes a performance hit when the average temp through the foam, and in a CT location needs to be derated to R5/inch (independent of it's labeled performance) for this application.

To limit the moisture content of the sheathing from wintertime moisture drives requires sufficient R-value to the exterior foam to keep the average wintertime temperature at the sheathing no cooler than 38-40F or so. The amount of R thus varies by local climate. A prescriptive minimum amount has been drafted into the IRC codes (see TABLE R702.7.1 Class III VAPOR RETARDERS, about 1/4 of the way down) that will work with reasonably air tight latex-painted wallboard, but for air-leaky panel/plank interiors you'll need to fatten that up a bit. Connecticut is mostly on the warm edge of Climate zone 5:

The IRC prescriptive for 3.5" deep milled 2x4 construction is a minimum of R5, but for full dimension framing that should be bumped to at least R6. With an air-leaky interior that should really be more like R7.

EPS is rated R4.2/inch at a mean temp through the foam of 75F, but at a mean temp of 40F (appropriate for you location and application) it's rated at 4.5/inch, so you should be looking at 1.5" as an absolute minimum, and 2" (R9 @ 40F mean temp) would be better. For polyiso you'd get comparable or slightly better performance at 1.5", as long as it's foil-faced, with an air gap between the foil and the new siding, otherwise go for 2".

This is not a cheap or easy fix, but it IS a fix, and would lower the heating load of the house considerably. There are lots of details to attend to about bulk-water management & defining which layer is the drain-plane, lapping the window & door flashing acceptibely, etc. It's usually possible to use the exterior sheathing as the primary air-barrier, backed up by taping the seams of the foam. At 2" or less of exterior foam it's much more tractable than at 4"+ (the thickest wall- foam retrofit I've even been remotely involved with was 4.5".) You can get a better idea of what's involved here and here.

The definition of a "ton" of cooling is the ability to move 12,000BTU/hr from an 80F room at an outdoor temp of 95F. The amount of heating you get at an outdoor temp of +5F from a heat pump per rated ton cooling varies by quite a bit between manufacturer, but the Mitsubishi "Hyper Heating" and Fujitsu's xxRLS3H series ductless heat pumps are typically good for between 12,000-16,000 BTU/hr @ +5F per ton of rated cooling capacity.