[BLDG-SIM] suggestions other than eQuest

Renee J. Azerbegi renee at ambient-e.com
Wed Apr 6 09:35:54 PDT 2005


I live in a passive (and active) solar home with rock bed storage in my
sunroom in Golden, Colorado. We did not design this house ourselves....
There is about 4 feet of rocks underneath wooden boards with about 1/2" of
space between the boards and the rocks start about 1 foot below the boards.
There is also brick on both walls. It gets really hot in this room in the
winter (and in the summer since it was not designed with proper overhangs so
we need to add some exterior overhangs as well). There used to be a duct and
fan to blow heat to the floor above, but it was removed by the previous
owners probably due to aesthetics since it was right in the middle of the
sunroom. Due to the airspace, there are a lot of bugs and dust that collect
on the rock bed. Frankly, I think the sunspace itself is quite sufficient
with the brick and glazing alone, and due to indoor air quality concerns, we
will likely take out the rock bed or seal it up. So if you do this rock bed,
make sure it is well sealed so bugs and dust cannot collect on it,
especially if you are going to blow air through it. And if it is already
going to be located in a sun room, it may not be necessary if you have brick
or another high mass feature in the space. It sounds like yours is going
over tile. You'd have to design it to make sure the rocks below the tile
heat up.

Renee Azerbegi

-----Original Message-----
From: postman at gard.com [mailto:postman at gard.com] On Behalf Of Graham & Megan
Sent: Tuesday, April 05, 2005 3:19 PM
To: bldg-sim at gard.com
Subject: [bldg-sim] suggestions other than eQuest

Lee,
I tend to agree with many of Steve's observations.  When I was at Berkeley
we visited the Village Homes site in Davis, California.  While this is a
more temperate climate, the feedback we got at the time was that the houses
performed quite well but the fan coupled rock stores in most cases were
providing very marginal benefit in winter as the houses already had a number
of other passive features.  If my memory servese me correctly, the controls
were two complicated for the simple home owner to manage to get the most out
of the system.  The summer benefits were likely greater.  I believe there is
some case study material on the Vital Signs website.  Cris Benton at
Berekeley I believe still runs the Vital Signs program if you can't find the
website.  A web search on Village Homes will turn up additional background
information.
Regards,
Graham
----- Original Message ----- 
From: "Steven Gates" <steve.gates at doe2.com>
To: <bldg-sim at gard.com>
Sent: Tuesday, April 05, 2005 12:41 PM
Subject: [bldg-sim] suggestions other than eQuest


> Lee,
>
> OK, the rockbed is being charged with warm air from inside the house; not
> from a solar air heater which could produce air at a considerably warmer
> temperature.  If the maximum space temperature you are comfortable with is
> 80F, and the rock bed must be heated to at least 75F to later produce 70F
in
> the space, then we are talking about a 5F MAXIMUM temperature differential
> between the rock bed and the space.
>
> While I make my living by writing energy simulation software (DOE-2), I
> believe there are many times when a full-blown computer simulation is not
> justified.  So, before getting bogged down in trying to model a rockbed
> system in a detailed energy simulation program, it might be useful to
first
> look at some rough numbers, and make a decision as to whether the rough
> numbers justify a more detailed analysis.
>
> So, for what it is worth, here is my 15 minutes of thinking:
>
> To move 1000 cfm at 1" static will require about 0.25 kW of fan energy.
But
> the air has to first be used to charge the rockbed, and then again to
> discharge it.  So double the fan energy to 0.5 kW/1000cfm of heated air
> delivered to the space.  At $0.10/kWh, this amounts to $0.05/1000cfm per
> hour.
>
> The amount of heat that can be delivered per 1000 cfm is:  1.08 * 1000cfm
*
> 5dT = 5400 Btuh.  Adding in the fan heat, the net heating effect is 5770
> Btuh.  At an equivalent 80% furnace efficiency and $1.00 per therm of
> natural gas, the gross savings in natural gas are $0.072/1000cfm of heated
> air delivered to the space.  Assuming the system can provide 100% of the
> annual heating loads, the savings on the heating bill are 1 - 0.05/0.072 =
> ~30%
>
> The location described gets a lot of winter sun, which even in a
> conventional design will act to significantly reduce heating loads.  What
> would the house cost to heat if intelligently designed (well insulated,
> reasonable south glass, some thermal mass), but without the rockbed? $1500
> per year?  What does the rockbed cost, $20,000?  Does a potential ~$500
per
> year savings justify the cost of the rockbed?
>
> Of course, these numbers are very approximate, and the actual savings may
> vary due to a variety of factors:
> 1)  If natural gas is not available, then the system may be more
attractive.
> 2)  The 5F delta T may be too large.  80F space temperature in a cold,
dry,
> 5000 ft desert climate may result in extremely low humidity in the space.
> This may tend to desiccate the occupant's skin and sinuses, and they may
not
> find it comfortable.
> 3)  Many hours, the fan will be charging the rockbed with the house being
> cooler than 80F.  So effectiveness during those hours is reduced.
> 4)  Rockbeds tend to be dusty, and may become moldy.  You may need to use
a
> high efficiency filter, which may raise the required fan static to do the
> job.
> 5)  This quickie analysis assumes that the solar system can provide 100%
of
> the annual heating; some auxiliary heat will almost certainly be required.
> 6)  All losses from the rockbed to the ground are neglected.
> 7)  The bright sun will act to fade the furniture more rapidly than in a
> conventional house.  What is impact on the furniture replacement budget?
> The sun may also degrade the finish on cabinetry more rapidly, especially
if
> varnished.  Deduct maintenance costs from the energy savings.
>
> For many experienced people using this listing, the above may come across
as
> too preachy.  To those people, I apologize.  But I understand that a lot
of
> graduate students also subscribe to this listing, and I want to emphasize
> that considering too many details when first approaching a problem can get
> in the way of the answer.  It is often useful to first look at the forest
> before focusing on the trees........
>
> Lee, I would be interested in knowing what your ultimate conclusions are.
>
> Steve
>
> -----Original Message-----
> From: postman at gard.com [mailto:postman at gard.com] On Behalf Of Lee Elson
> Sent: Monday, April 04, 2005 4:18 PM
> To: bldg-sim at gard.com
> Cc: bldg-sim at gard.com
> Subject: [bldg-sim] suggestions other than eQuest
>
> First, let me say that I've been a bit overwhelmed with the quality (and
> quantity) of responses. I appreciate all the discourse and am struggling
> to check out all the suggestions.
>
> It is a good idea for me to address some of the issues I described
> originally. "ICF" stands for insulated concrete forms which consist of a
> foam form (about 2" thick) into which concrete (9-12" thick) is poured.
> These will be used for exterior walls for the single story structure.
> Although the insulating (sound and heat) properties are great, and the
> costs are reasonable, there is little heat storage by the concrete
> because of the interior foam.
>
> Some details on the rock floor. It will be a layer of 2" rocks, about 4
> feet deep. There will be insulation underneath and a concrete and tile
> floor above. Part (about 1/3) of the tile floor will be in direct sun
> when conditions allow. There will be a couple of plenums (consisting of
> stacked concrete blocks with spacers near the bottom) running the length
> of the rock floor and there will be an air barrier of some sort that
> will channel warm air from the plenum to the outer portion of the rock
> floor. Other plenums near the ceiling will allow warm air to be forced
> (with fans) down to the rock floor where it will circulate back into the
> house through floor registers (after having given up heat to the rocks).
> This rock floor will be used over the entire structure (about 3500 sq
> ft). The builder/designer says that he's used this approach and it works
> pretty well. There is sufficient heat transferred to the rocks to make
> them a reasonable thermal mass. This system also redistributes air from
> the warm (south/high) side of the structure to the cold (north/low) side.
>
> I realize that this type of thermal mass may be difficult to model in
> detail and that it is important to look at the energy cost of the fan(s)
> in all of this.
>
> I hope this description helps.
>
> Regards,
> Lee Elson
>
> Steven Gates wrote:
>
> >Is there a more fundamental issue that is being missed here?  Lee Elson's
> >description includes the phrase "insulated rock floor with active air
> >recirculation".  It also includes "ICF", which I am not familiar with.
> >
> >The description of the system appears to be a floor/rock-bed that uses a
> fan
> >to recirculate air between the rockbed and the space.  So solar falls
> >directly on the floor surface, but the floor also captures/releases heat
> >convectively using a fan.
> >
> >If that is the case, it is critical that the convective leg be simulated.
> >Assuming the space will not be allowed to be uncomfortably warm, and that
> >the rockbed temperature must be at least 70F to have any heating
> >effectiveness at all, this implies a very low delta-T between the rockbed
> >and the space when convectively charging or discharging the rockbed -
> >corresponding to a large, potentially drafty, airflow.  It also suggests
> >that the convective thermal capacity of the rockbed may be quite limited,
> >corresponding to a temperature swing on the order of 5F or less.
> >
> >It is essential that the fan energy of this system be modeled; with such
a
> >small delta T between the rockbed and the space, the cost to run the fan
> may
> >exceed the cost of the heating energy saved by the convective component
of
> >this system.
> >
> >It would be helpful if Lee described the system in more detail.
> >eQUEST/DOE-2 does not simulate either convective rockbeds or the combined
> >radiant/convective rockbed floor that this building appears to use.  Do
any
> >programs have this capability?
> >
> >
> >-----Original Message-----
> >From: postman at gard.com [mailto:postman at gard.com] On Behalf Of Blake, Jeff
> >Sent: Sunday, April 03, 2005 8:03 AM
> >To: bldg-sim at gard.com
> >Subject: [bldg-sim] suggestions other than eQuest
> >
> >Lee,
> >
> >Doe2 (2.1 or 2.2) is not the right program for looking at heavy thermal
> mass
> >buildings.  The limitation is a function of the thermal response factors
> >used to represent time-delayed heat transfer through opaque surfaces.
This
> >method is a legacy of the original DOE2 design requirements which were
> >somewhat hindered by slow computers.
> >
> >You should use a simulation program that uses a more fundamental heat
> >balance approach.  There are several but I believe that ESP-r (ESRU,
> >University of Strathclyde) is one of the best.
> >
> >Jeff Blake
> >
> >-----Original Message-----
> >From: postman at gard.com [mailto:postman at gard.com]On Behalf Of Lee Elson
> >Sent: Friday, April 01, 2005 3:55 PM
> >To: bldg-sim at gard.com
> >Subject: [bldg-sim] suggestions other than eQuest
> >
> >
> >I'm designing a 3500 sq foot residence at a 5000' desert-like elevation
> >(Nevada).
> >The site has good sun exposure and the climate is generally dry. Typical
> >temperatures in Feb are 20-40 F. The building is mostly oriented
east-west
> >and has
> >a 2' deep insulated rock floor (for thermal mass) with active air
> >recirculation.
> >ICF's will be used as well as tile over concrete (above rock floor) in
> south
> >facing rooms.
> >
> >I'm trying to get a realistic estimate of temperature swings and
auxiliary
> >heating
> >requirements. I've used eQuest, which seems pretty impressive and easy to
> >use.
> >I've entered the floor plan layout, specified the glazing (clear glass on
> >the
> >south side) and the building materials. The calculations are a bit
> >disappointing:
> >with 12% of the total floor space in south facing glass, the aux heating
> >required
> >is not too different (~20%) from a house with standard insulation and no
> >added
> >south facing glass. Another oddity: when I change the south facing glass
> >area from
> >5% of the available south wall to 90% of the south wall, I get an
> *increase*
> >in
> >aux heating requirements. The eQuest developer thinks this is due to
> >inadequate
> >thermal mass/heat recirculation modeling.
> >
> >I suspect that eQuest is not doing accurate modeling since other houses
> with
> >a
> >similar design in this area get much better thermal performance. Can
anyone
> >suggest either software or a service provider that might be able to do a
> >more
> >accurate calculation?
> >
> >TIA
> >
> >Lee Elson
> >
> >
> >
> >
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