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How does one model active heating windows like the ones listed below.

Namrata Vora

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Namrata,

I would model the heating as electric baseboards and select a glazing
type based on the U-value and SHGC.

Regards,

William Bishop, PE, BEMP, LEED(r) AP

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Namrata,

I would model the heat gain into the baseboard system. I had a conversation
about heating with our engineer and you just need to make sure that the heat
from the item and its efficiency are the same. The space does not really
care where the heat comes from. It would be just like putting a radiant
heater along a wall except you placed it in a window. Make sure to model the
specific heat gains accordingly.

If the window turns on and off you will need to figure out what the u-value
is when its off. It looks like they are compensating heat loss by
electricity versus heat loss by u-value of material. A radiant heater on a
CMU wall does the same thing. (Again architect and I'm making assumptions,
please feel free to correct.)you just heating the glass with electricity.

To me it looks more like a defroster you have on the rear window of your
car. If I were a betting man I know my car window in the back doesn't heat
up my car that much so I would be a little weary of this product.

Thanks,

PETER HILLERMANN

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If only I hit the send button a few minutes sooner.

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I'm a little curious - is 0.92 W/m2k a good U-value? Just poking around on online converters - it looks like the IP version of that is around 0.56 or something in that region - doesn't seem spectacular (you get 0.25 with a lot of double pane low-e IGUs). Unless the 0.922 is for the entire glazing assembly - in which case its around the same as other glass types.

I might have got the conversion factor wrong.

Vikram Sami, LEED AP

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Vikram,

The conversion from W/sq.m.-K to Btu/hr-sq.ft.-F is 5.68. This gives a
U-value of 0.162 Btu/hr-sq.ft.-F.

I would like to see a study of the energy consumption of this window
compared to a building with normal low-e IGU and actual radiators.

Sam Mason

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I stand corrected - it is a good U-Value :)

Vikram Sami, LEED AP

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On further thought, I think the equipment referenced would be
over-simplified if you just modeled an electric baseboard (that was my
intuitive reaction also - a few beat me to the punch)...

The "efficiency" of these units, meaning the btu/h delivered to the
space for every input kW, is going to vary hourly (probably
dramatically) based on the outside-to-interior temperature difference.
If it's -10F outside and 70F inside, these are going to lose a ton of
the input heat to the exterior for the amount desired heat flowing
inside - heat will flow proportionately to where it's coldest.

Time allowing, I'd propose achieving a much higher degree of modeled
accuracy by defining a series of equipment loads, each with a custom
multiplier schedule (even if the units in reality are under some kind of
thermostatic control) to account for when the units would be turned on.
This would be more work, but then you're able to define a distinct
sensible HG ratio to account for the fraction of heat actually entering
the space (instead of lost to the exterior). As you can define up to 5
separate equipment load/schedule combinations for one space, you could
come up with a weighted efficiency to apply to the SHG ratio for up to 5
different blocks of time. Further explanation: This would involve
creating hourly reports to compare the hourly exterior temps to the
target heating temp for the interior, for the times the units would be
operating, and to use that info to figure out an hourly efficiency for
the system based on how much heat would actually find it's way into the
interior zone. Those hourly results could be used to come up with
weighted SHG ratio values for 5 blocks of time.

On a related note, I've got a question for everyone: Does anyone know
of any workaround (aside from extensive excel number-crunching) to apply
an ON/OFF/TEMP schedule to an equipment or internal energy load?

On another tangent, for my own education, am I wrong to consider heated
glass as an intrinsically inefficient approach to space heating? It
seems most appropriately applied as a "band-aid" or "patch" solution to
a decision to put too much glazing where drafts will cause discomfort.
One of the previous links has a claim: "When installed as part of a zone
heating strategy, heated windows can actually reduce the overall energy
consumption of a building by decreasing dependence on an inefficient
HVAC system." Wouldn't this need to be a terribly inefficient heating
system to compare unfavorably to a heating element losing >50% of its
energy to the exterior? I'm concerned I might be missing something
fundamental for HVAC design here... =/

NICK CATON, E.I.T.

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Nick,

"The "efficiency" of these units ... is going to vary hourly (probably
dramatically) based on the outside-to-interior temperature difference."
Don't think of it as less heat to the space, but more heat to the
exterior. The window U-factor should take care of this.
"apply an ON/OFF/TEMP schedule to an equipment or internal energy load?"
You can apply an equipment power curve as a function of space
temperature (EQUIP-PWR-FT).
"I'm concerned I might be missing something" For the same glazing
properties, the heat loss to the exterior should be the same, EXCEPT
that the interior surface is now warmer, so radiative and convective
losses should be higher. Hopefully the manufacturers account for that in
their rated U-factor. I'm not convinced yet that there is any energy
benefit to this system.

Regards,
Bill

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Hi Christian

The miscellaneous section for the heat pump has values for the evaporator HX
and condenser HX. I assumed that these referred to the chiller itself,
however, there are actually two additional heat exchangers assumed in the
model (see attached schematic). Once I read through the schematic, I was
able to supply values to these fields that were more appropriate, and this
error went away.

Regards,

MaryAnne Mason

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All,

Thanks for your prompt responses

The mfg of this technology claim that since the temp difference b/n
surface 3 & 4 is zero or minimal, there is no heat transfer out through
the windows. In addition the radiant nature of the heat allows the room
temperature to be set lower.

This particular company claims a u-value of 0.14 in non-heated state. I
tried creating such a low u-value dual pane IGU using WINDOW5 and was
unable to get that. So I created it directly in eQuest using simplified
glass type method. Now will try adding the radiant heating as electric
baseboard as suggested.

http://www.iqglass.com/products_iqglass/products_specs.html

I agree w/ Nick's comment. In addition I wouldn't think the window would
be in heating mode all the time. I would need to figure out when to
trigger the heating.

Namrata

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Namrata,

I have patched Bill Bishop's latest response in below in case you missed it. He's a proven smart cookie, and if he's telling me I've got my head wrapped around the problem the wrong way then I do believe him...

...but with that caveat, the manufacturer implying zero heat transfer through the entire window assembly (because two of at least 4 surfaces have equal temperature), sounds like flat-out misinformation to me. If two surfaces of a material have equal temperature (say, 100 ?F), then yeah, the conduction between them would be zero, but that won't prevent convection/radiation from sweeping a majority of heat away when one side is subject to sub-freezing temperatures! My car for example has a heated rear window which helps me dislodge nightly glacier-formations in the wintertime. On a cold day, the heat generated in that glass may help heat the interior air somewhat, but surely a majority is going straight to the literally frozen exterior, especially once the interior air has risen to a temperature in which I can breathe regularly.

I remain skeptical of the concept of using heated windows as a good idea for a space heating source (as opposed to a draft or condensation mitigation strategy - perhaps to melt snow in a skylight as well?). Still, if these units do put a significant fraction of heat into the space, I think modeling electric baseboard heating will at the least be a good start to emulate the space temperature behavior the actual system may help achieve - though I'm still stuck wondering how you might account for the amount of energy that is lost to the exterior as the units operate... hear me out as I think my ducks are in a row here =)!

I'd be cautious that your manufacturer may be, intentionally or not, dumbing down the discussion by framing their "effective U-value" as a material property like "conductivity." Conductivity is an arbitrary material property that does not change with temperature - (i.e. steel has a higher conductivity than air). U-values (and R-values) on the other hand have that tricky little ?F (or ?C) in their units, because they are tied directly to a heat transfer rate, which requires having a delta-T - in other words, for a given set of material properties, the rate of heat transfer is dependent on the difference in temperature the assembly is subject to. That's one reason we come across "summer" and "winter" U-values for window assemblies.

It's my intuition that eQuest (among other variables) uses the hourly exterior temperature from the weather file alongside the hourly calculated interior temperature for the parent zone to determine what temperature difference to use when figuring the hourly heat loss through a window assembly. A heated window however (when on) should be using a fundamentally different figure for the interior surface temperature than, say, 70?F when determining a temperature difference to find the hourly heat loss for the zone.

I think I'm understanding now what we're all getting at when we speak of "effective U-values:" If a baseboard heating element in eQuest (or other software) uniformly distributes 100% of its heat to the zone, then we might account for the reality of a significant (?) fraction being lost straight out the window by increasing the window's effective U-value, in turn modeling a realistically higher heat loss through the assembly on account of the higher-than-normal heat temperature difference between the innermost and outermost surfaces of the window, right?

I could certainly be wrong, but an approach defining an electric internal equipment load with an custom equipment power curve (see Bill, below) *might* be easier than an approach defining dynamic window properties (which I do not think is feasible in eQuest).

Again, I am not writing with full confidence that I completely understand exactly how heated windows truly function, but critically thinking this through is helping me get there! I look forward to anyone stepping up to the plate to affirm or refute my current understanding =).

NICK CATON, E.I.T.

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Nick you understand perfectly and Bill is the one you should be
listening to. This is what happens when marketing gets hold of
something without talking to engineering. It is all BS. I looked at
both sites, basically they are taking a low E thermopane and adding a
third pane of their heating glass. A real good 2 pane IGU will be about
R-4.2, add another pane and you could be in the R-7 range or a U value
of .14, all centre of glass. There is no way 2 pieces of glass in this
3 pane unit are the same temperature. If the middle piece is almost the
same temperature as the warm inside piece that actually proves you have
a huge amount of heat moving to outside not to the inside. There will
also be radiation, and in this instance it will be significant as well.
The reflective properties of the low E glass will help, reflecting a
certain percentage back into the room as the glass should be throwing
long wavelength radiation but a significant portion will still flow
through unless the low E coat is thick enough, ie a mirror, which
doesn't do much for looking outside. Like you said earlier you have a
radiator with a 70F room one side and a winter night at -13F on the
other side, it is not rocket science to know in which direction most of
the heat will flow.
Getting to the modelling using baseboards I don't think you can. A
baseboard is going to have a R-20 opaque surface behind it and the
heated glass is going to have a R-4 transparent surface behind it.
As far as I see it the only real benefit of this glass is you will
be able to run your humidity higher in the winter without having
condensation on the edges of the glass but at what I would estimate to
be a significant energy penalty. You would get most of these benefits
just by having the triple low E IGU with low conductivity spacers or by
having curtains. You will not heat a room with it and be able to pay
the hydro bill. Note all the conformances are for glass. I don't see
any insulating window certifications. Until these are tested no
jurisdiction in NA will allow you to use them with the numbers they are
quoting.
Bruce Easterbrook P.Eng.

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Hello All,

I'm starting to model a building with many "changes in direction" in the
exterior walls creating the need for a custom building footprint. The
building is 2 story and the second story is only partially as large as
the ground floor and the "footprint" of the 2nd floor differs from the
first.

So far I'm only using schematic design wizard. I can't seem to find a
way to create a different 2nd floor shape in the schematic wizard. Am I
missing something? Can this be done in the DD wizard, or will I have to
go straight to detail data entry?

All help appreciated.

Chris D. Mullinax, P.E., LEED AP, CEM

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Chris:

You cannot make different floor shapes in the Schematic Wizard. You can in the
DD Wizard. That is the major difference between the Wizards.

John R. Aulbach, PE, CEM

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My only source of heating in a building is electric baseboard heat.
However, when I have the control set to thermostatic, I have usage year
round, granted much less in the summer. How do I shut off the baseboard
heat during the summer? Alternatively, if I did try to use the reset,
where do I create the reset number? Do I use a schedule in the "Heating
Tstat Sch"? In the reset case, wouldn't I overshoot my set point
sometimes? Thanks for any help!

David Gajewski, CEM

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My only source of heating in a building is electric baseboard heat.
However, when I have the control set to thermostatic, I have usage year
round, granted much less in the summer. How do I shut off the baseboard
heat during the summer? Alternatively, if I did try to use the reset,
where do I create the reset number? Do I use a schedule in the "Heating
Tstat Sch"? In the reset case, wouldn't I overshoot my set point
sometimes? Thanks for any help!

David Gajewski, CEM

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David,

Just a quick answer but see if you can manipulate the thermostat schedules
for your days weeks and months. You could in the summer time put the
temperature of the thermostat so high that the baseboard would never turn on
because it would never get that cold. Remember that turning a thermostat off
just means the temperature not getting to the point where it needs to turn
on. The other is use the tempON-Off schedule.

Thanks,

PETER HILLERMANN

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