Stoopid question about geothermal slinkies?

[Plan-B]

Dear Rocket Scientists,

Please allow me to pick your brains before engaging a professional HVAC
consultant. Frankly, paying someone to tell me that what I have in mind
can't work would max out my pre-mortgage project budget--a Chicken vs
Egg, lose/lose dilemma...

I'm planning an apartment [for myself] as an addition to my detached
garage. All living space will be on the second floor, accessed by an
outdoor stairway and entry deck--400 sf above the garage plus 300 sf
above new storage/utility space. Affordable heat is the primary concern.

The new-construction footprint is 18'0" x 17'4" plus an attached solar
greenhouse, 12'0" x 9'4".

A possibly related consideration is that the greenhouse [SE exposure]
rises to a second-story sun porch [12' x 6'] located at the far end of
the deck [mesh flooring between levels]. Ideally, awning windows
connecting sun porch and living space will contribute significant backup
heat on sunny winter days.

Local codes [Philadelphia suburbs] require footings excavated below the
frostline, approx -3.5'. So here's my stoopid question...

What if the entire new-construction site were excavated, maybe as deep
as a basement, lined with slinkies and backfilled with earth or
something better [soil around here is hard and rocky]?

Since the space to be heated is small [704 sf total], would this plan
serve a GT heat pump supplying a dual-zone radiant-floor system? Would
the expense be justified?

Many, many thanks for your advice. --Anna

 

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I'm planning an apartment [for myself] as an addition to my detached
garage. All living space will be on the second floor, accessed by an
outdoor stairway and entry deck--400 sf above the garage plus 300 sf
above new storage/utility space. Affordable heat is the primary concern.

Upstairs is good.

The new-construction footprint is 18'0" x 17'4" plus an attached solar
greenhouse, 12'0" x 9'4".

Bigger would be better. Will the greenhouse contain plants, or can it
drop below freezing at night?

A possibly related consideration is that the greenhouse [SE exposure]
rises to a second-story sun porch [12' x 6'] located at the far end of
the deck [mesh flooring between levels]. Ideally, awning windows
connecting sun porch and living space will contribute significant backup
heat on sunny winter days.

You might have a passive one-way damper over an open window--lightweight
plastic film, hinged at the top. With a duct to bring down hot air from
the ceiling to enter the greenhouse at ground level.

What if the entire new-construction site were excavated, maybe as deep
as a basement, lined with slinkies and backfilled with earth or
something better [soil around here is hard and rocky]?

Bad idea. Solar heat is better, with a COP of 1000 or more, vs 3 for a
heat pump. Collect 0.9x1000x12x16 = 172.8K Btu of warm air passively in
your 2-story T (F) single layer polycarbonate R1 greenhouse with 90%
solar transmission on an average 30 F January day in Phila and lose
about 6h(T-36)12'x16'/R1 Btu through the glazing.

If your airtight 704 ft^2 square space has an Rv ceiling and walls and
few windows, its thermal conductance will be 704ft^2/Rvalue Btu/h-F for
the ceiling + 849/Rvalue for the walls. At a 65 F average temperature,
it will need Q = 24h(64-30)1553/Rvalue Btu/day of heat. Rv = 30 makes
Q = 42.2K Btu. If 172.8K = 6(T-36)12x16+Q, T = 149 F, approximately.

If 75 F ceiling mass can keep the apartment 70 F with a slow ceiling fan
and a room temp thermostat and an occupancy sensor, storing heat for 5
cloudy days in a row requires that (149-75)C = 5Q, ie C = 2854, eg
2854/704 = 4 pounds of water per square foot of ceiling.

Your body heat and indoor electrical usage will help.

Nick

 

[Plan-B]

Thank you, Nick and TG, for your kind attention!

I started to write separate replies, then figured that pictures are
worth 10,000 words. Some of your questions will be answered by these
preliminary drawings [a bit primitive--no CAD app]:

http://www.typeheritage.com/Plan-B/Site.htm
http://www.typeheritage.com/Plan-B/Level-One.htm
http://www.typeheritage.com/Plan-B/Level-Two.htm
http://www.typeheritage.com/Plan-B/Section.htm

Township codes are strict, so I can't do anything too "hippy" that might
be out of sync with the neighborhood. Besides, the other half of Plan B
is to rent the main house so my apartment must be non-intrusive on the
tenant. I already anticipate a zoning battle for a two-family lot, much
less the commercial aspect of renting.

BTW I plan concrete block walls for all new contruction with stucco
finish matching the stone and "Tudor Style" trim popular in this area.
The interior finish will be tinted plaster or paint only, so all
electric switches are located in [non-nearing] frame partitions.

All plumbing is in the addition to avoid the cost of penetrating the
stone garage walls or 6"-slab floor. All plumbed fixtures/applicances
are clustered at a Working Partition [Level Two], so this works for me.

As you will see from the site plan, the main house and/or oak tree may
cast winter shadows on the greenhouse, which already has one strike
against it by facing SE instead of S. The coming winter will answer
this question--frankly, I'm not convinced that solar alone will be adequate.

I'm in love with both radiant floor heat and fresh, humid air from a
solar greenhouse. Can I have it both ways?

Confession... The numbers you guys crunch make my head spin!

Thanks for bearing with me, Anna

 

[[email protected]]

Thank you, Nick and TG, for your kind attention!

You are welcome.

Township codes are strict, so I can't do anything too "hippy" that might
be out of sync with the neighborhood.

US codes are only concerned with health and safety. PA now uses the ICC,
which is less restrictive than our previous Skippack Township BOCA code,
which allowed my 32'x12'x16' tall $500 plastic film sunspace. Section
R403.3 "Frost protected shallow foundations" allows less than 3.5' of
foundation depth.

Confession... The numbers you guys crunch make my head spin!

It's just high-school algebra.

Nick

 

[Plan-B]

Dear Nick and TG,

I'm very grateful for your generous guidance. Still studying Nick's
math-intensive advice <LOL>!

It seems that neither of you approves of a GT heatpump for such a small
living space.

What do you recommend as a "mainstream" backup to the solar greenhouse?
My preference is a radiant floor system--would a conventional HP be
the next-best thing?

Thanks again for your input, Anna

P.S. I'll be unavailable between Fri and Mon, so have a good weekend!

 

[[email protected]]

http://www.typeheritage.com/Plan-B/Site.htm
http://www.typeheritage.com/Plan-B/Level-One.htm
http://www.typeheritage.com/Plan-B/Level-Two.htm
http://www.typeheritage.com/Plan-B/Section.htm

Very nice

It looks like the upper sunspace has no summer shading. You might unroll
some greenhouse shadecloth over it in summertime.

And consider making the roof flat and glazing the whole southeast side with
a single layer of R1 polycarbonate with 90% solar transmission (corrugated
Dynaglas greenhouse roofing in 4'x12' $60 sheets or 0.020" clear flat Lexan
in $300 4'x50' rolls with a 10 year guarantee) to make an 8' deep x 42' long
x 16' tall sunspace that would collect 0.9x1000x42'x16' = 604.8K Btu on
an average 30 F January day in Phila, if you trim the beloved oak a bit.

If the air near the sunspace glazing is 130 F during the day, the glazing
will lose about 6h(130-36)42'x16'/R1 = 379K Btu/day, leaving 604.8K-379K
= 225.8K Btu of useful solar heat, no?

A 42'x28' R40 130 F ceiling might lose 24h(130-30)42x28'/R40 = 70.6K Btu/day,
which leaves 225.8K-70.6K = 155.2K Btu/day for the rest of the 2nd story.
The ceiling needs a low-e surface (eg foil) to avoid overheating the room.
If it slopes up to the south, less foil is needed.

If you have 32 2'x4' windows on your 704 ft^2 space, that's 36% of the
floorspace--too many You only need 2-4% of the floorspace for light.
Windows are expensive, and their framing is expensive, and they and their
framing lose more heat than walls, and they can leak air and rain and
bugs and burglars and baseballs. If you change to 64 ft^2 of R4 windows
on 42x20 = 840 ft^2 of living space, they might lose 24h(65-30)64ft^2/R4
= 13.4K Btu/day, leaving 155.2K-13.4K = 141.8K for 928 ft^2 of non-window
R20 walls losing 24h(65-30)928/20 = 39K Btu/day, leaving 141.8K-39K
= 102.8K in the solar heat budget, so you might add a FEW more windows.

If not, on a cloudy 30 F day, the 100 F apartment ceiling might lose
24h(100-30)42x28/40 = 49.4K, and the windows and walls might lose 13.4K
+ 39K = 52.4K, totaling 49.4K + 39K = 91.4K Btu/day, so the apartment
would need 5x91.4K = 457K Btu for 5 cloudy days in a row. If 75 F ceiling
mass can keep the apartment 70 F with a slow ceiling fan and a room temp
thermostat and an occupancy sensor, storing heat for an unlikely 5 cloudy
days in a row requires that (130-75)C = 457K, ie C = 8309 eg 8309/840 ft^2
= 10 pounds of water (2") per square foot of ceiling.

As an alternative to water under the ceiling, lots of fin-tube pipe under
the ceiling could heat water in a $1200 1000 gallon 7' diam x 4' tall STSS
tank in the sunspace, with a copper pipe coil to heat water for showers.

Nick

 

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Dear Nick and TG,

I'm very grateful for your generous guidance. Still studying Nick's
math-intensive advice <LOL>!

I redid all that more simply, after looking at your nice drawings.

It seems that neither of you approves of a GT heatpump for such a small
living space.

Heatpump bad. Solar good.

What do you recommend as a "mainstream" backup to the solar greenhouse?

Cloudy days are like coinflips. If a house can store enough heat for
1 cloudy day, it can be 50% solar heated at best, 2 make 75% possible...
and 5 make 97% possible. At that point, the backup fuel cost is much
less important than the cost of the backup system, so you might buy
something cheap, eg a 5 kW 240 V portable space heater or some electric
baseboards or an air-source heat pump, if you want AC, which might be
rarely needed near Phila if you store coolth in ceiling mass and use
your ceiling fans--an $80 window AC would be cheaper.

I wonder which suburb you live in. I'm near Skippack.

Nick

 

[[email protected]]

No idea what you are talking about water in the ceiling...

I noticed

Nick