Radiant slab Heating Cooling

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Good Morning everyone,

I have been reading through the archives for modeling radiant slab heating
and cooling.

Most popular and the closest to real world approach I found was using a 4
pipe fan coil unit and making the fan energy zero.

I still have following questions:

? Did anyone think of using a factor, which when multiplied to the
energy usage by fan coil system, give us the actual energy usage by radiant
system. (I would guess it would be <1)

? As radiant heating and cooling forms a stratification layer in
the space. And we care about first 6-8 feet to be within desired
temperature range. So to take that into account while modeling ?it? as fan
coil unit, can we lower the ceiling height. Does that mean the conduction
load from ceiling/roof or the wall above 6-8 feet doesn?t count ?

? The temperature range for heating water (95F-105F) and cooling
water (55F-65F) pumped in the radiant system is different from the typical
chilled water (44F-45F) and heating water (170F-180F). How do we make sure
our model reflects this while using fan coil unit.

I am sure there are more questions you would have come across but these
were some of my major concerns and I would appreciate any inputs/
suggestions.

PS: This is a aspiring LEED platinum building.

--
Thanks and Best Regards,

Sincerely,

Varun Kulkarni

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Varun, your perception is nearly true, but it depends on what you are modeling? If you are modeling the energy performance of a building a four pipe fan coil works just fine as it maintains the required energy balance. It is true that the actual performance of a radiant floor is different than a fan coil, but you are looking at energy and not optimal modeling.
Your thoughts on stratification are somewhat correct. I would tend to think that stratification is caused by space heat load and air movement, especially displacement. Yes, there is a convective flow from the floor but this is quite small as the temperature difference between the floor surface temperature and the space operating temperature.
Regarding temperature; this depends on what program you are using and why you want to segregate the different temperatures?

Peter Simmonds, Ph.D.

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Thanks Peter,

I am using Carrier HAP and when I model a fan coil unit, I am asked to
input the design supply temperatures for heating and cooling (which is
usually 95F and 55F). Now, i think these are going to drive my energy
consumed at the boiler and chiller. The whole point (not the only point but
a major though) of using radiant slab heating and cooling is lower temps
for heating, higher temps for cooling and also the fact that water has
higher energy carrying capacity then air. So if you are not able to take
credit for that in modeling then obviously, the energy consumption number
is not right. Thats my reason to segregate the different temperatures.

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All

Is no one worried about the time characteristics of in-floor cooling (and heating) systems, i.e. the time lag between heat gains (or losses) and terminal/plant loads?, some kind of fudge factor would likely be required for software like DOE but what might one do?

Les Jones

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Varun, you might want to graduate from the HAP program. Unless you have two chillers one for 42F water and another for 55F, then most systems produce 42F water and then blend to 55F, so the potential savings are difficult to obtain. By the way what is the ratio of your radiant floor to conditioned air output?

Peter Simmonds, Ph.D.

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Please refer to work done by Bjarne Olesen and Michele di Carli and others that report on the time lag of radiant floor, which is very small. Refer to ASHRAE papers on this subject.

Peter Simmonds, Ph.D.

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Hi Varun

As a work around in DOE2. You may model the actual design temperatures and
increase/decrease the indoor set points for cooling/heating by some margin.

Regards
Hisham Ahmad

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All,
Radiant heating/cooling does not address the latent load. So if you have a
space/building served by radiant heating/cooling, you have to have a
ventilation unit (which might supply neutral air). This considerably gives
a huge amount of saving when coupled with DCV.
Another point point is that a chiller with 55F LWT and boiler with 100F LWT
would use less energy than conventional chiler/boiler LWT range. So, as
Amit said, a de-coupler is needed (may be a heat exchanger) for chilled
water and heating water to tie into other equipments in the building such
as snowmelt, air handlers..
By the ratio of radiant floor and conditioned floor output, do you mean the
ratio of energy consumed if the building was conditioned with either of the
two approach?

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So if using a DOE based tool for instance are you suggesting you use a "lightweight" classification

Here in Canada the EE4 Modeling Guide recommends using "one weight heavier than the actual construction" which intuitive I have always had a problem with.

Of course I would expect the time load characteristics to vary between a floor receiving direct radiation and one in shade but this is probably too much detail for the average modeling exercise.

I don't believe the Olesen paper (ASHRAE Sep 20008) addresses the time lag issue, is there another paper? I don't recall seeing anything as I have kept an eye open for such discussions>

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Varun,

You can use the workaround you mentioned for modeling radiant cooling and heating, and at the same time model the effects of your supply water temperatures for the chiller and boiler plants. There is no conflict in HAP for doing both. However, as Peter and others point out, you may need the chillers to operate with a cold LCHWT like 42 F anyway to serve your DOAS which is handling the full dehumidification load for your building and other chilled water consumers like AHUs if they exist.

As part of your software license you have free access to HAP software support at software.systems at carrier.utc.com. The support desk is staffed by engineers with decades of experience in HVAC design and application engineering, who are also experts in the software. They can help you sort out these kinds of issues to determine which features of the software are best to use for your application.

Best Regards,

Jim Pegues

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