Re: [EnergyPlus_Support] Can Radiant system save energy?

Consider "Time Setpoint Not Met" in Rad1Table.html : during cooling it is 3198.50 hours. You cannot compare the energy spent for cooling of the two models if a model doesn't meet the setpoint temperature.

As far the heating demand, I think your result is strange. I see in the envelope summary in the .html file that the U-value of RAD_EXT_FLOOR is 0.519 Btu/h-ft2-F, i.e. 2.95 W/(m2*K). It is a very high U-value. Is that a heated floor?

You should check the amount of energy the boiler provides the water and the amount of energy the boiler needs: Boiler Gas Rate [W] and Boiler Heating Rate [W]. Then you shall compare those values with the energy demand of the heating coils of the air system. You should understand where the model with the radiant system is inefficient (envelope, radiant panels, boiler?).

Have you checked the performance curves?

I see that the radiant systems you modeled are generally in the ceilings. Could you try with radiant systems in the floors?
The advantages of the radiant system were explained well by other members of the mailing list. A disadvantage is that to meet the setpoint temperatures during the occupation hours the radiant system shall work also before the occupation hours (the reason is its thermal inertia).

Since I have often to compare radiant systems with air systems (because I model for LEED submissions, and the proposed models have very often radiant systems, while the baseline models have always air systems), I would appreciate reading a publication about this subject.

Best Regards

Francesco Passerini

--In EnergyPlus_Support@xxxxxxxxxxxxxxx, <wanghaojie630@...> wrote :

Thanks all for your comment. I have attached my comparison and the results. (This is purely for comparison, its not a actual system since I deleted all the ventilation load (no outdoor air) to make the comparison more consistent. The first one is a CAV system (base1.idf), the second one is a radiant system (variable flow). Cooling is a air cooled chiller, heating is boiler. All the rest parameters are the same. The HVAC system is only used to meet the building load (no ventilation load) so it should be comparable. As you can see, the heating energy for radiant system is 3 times higher than all air system, which is quite surprising. And radiant system has trouble to meet the room setpoint. I checked the detailed hourly report, and I see higher envelope loss during heating season due to higher envelope temperature (radiant system heat up the surface). Also, I see the heat loss through the window also increased, which is strange to me because radiant system only radiate long wave radiation, window should reflect that back instead of transmit it.

Please let me know your comment. Thanks

HJ

On 7/22/2014 6:08 AM, Julien Marrec julien.marrec@... [EnergyPlus_Support] wrote:

It slipped my mind: you can look into the european norm EN 1264

--
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
T: +33 6 95 14 42 13
www.julienmarrec.com

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2014-07-22 12:01 GMT+02:00 Julien Marrec <julien.marrec@...>:

HJ,

I think first you need to look at the system as a whole and not just the terminal, and also consider that what matters is the operative temperature and not just the air temperature (at low airflow (<0.2m/s) it's fairly well approximated as the average of air and mean radiant temperature). More than half of a floor heating energy is in the form of radiant heat.

Among advantages:
- it's a more efficient way to transmit energy than air systems, and as Annie said you separate OA loads from zone loads.

- in heating mode, the main advantage of radiant heating is that you can operate at lower temperatures, and therefore maximize the efficiency of your heating system if it is a condensing boiler or a heat pump for example.

(- In cooling mode, if you have a ground heat exchanger, you can cool it without using the chiller/heat pump in cooling mode since the ground temp is low enough.)

- Better use of thermal mass for load shifting

- It's more comfortable

- For large volumes, it's more efficient than trying to heat the entire volume of air (you're close to the floor...)

One thing to note is that for floor cooling, it's tougher to actually get a system running properly or efficiently, and with enough capacity. The convective heat exchange coefficient for floor cooling is lower than for heating, and you need to take into account many comfort factors (dew-point in particular, acceptable floor temp, vertical air temp difference, etc.)

I don't really see why you would necessarily see significantly more envelope losses. Because of more radiation? Or do you have a slab that is not very well insulated (or worse, that extends to the outside or even a balcony...)?

To see whether it makes sense, you need to ask yourself what's your design exactly. What system is providing your hot/cold water? What's your terminal unit: Floor, Active or Passive Chilled Beams? etc.

Robert,

Have you by any chance modeled your radiant heating system and building in E+ (or other software) and output PMV/PPD? I'd be curious about what it says about thermal comfort of people.

As far as ASHRAE guidelines, I don't know of any. There's the technical committee 6.5 (http://tc65.ashraetcs.org/) but they haven't published anything aside from articles. Bjarne Olesen has published some very interesting oned about radiant systems, that you can find pretty easily online.

As far as guidelines, you might have a better chance looking in Europe. If you read french I can give you some, but otherwise maybe CIBSE?

Best,
Julien

--
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
T: +33 6 95 14 42 13

LinkedIn (en) : www.linkedin.com/in/julienmarrec

2014-07-22 3:36 GMT+02:00 annie anniemarston@... [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx>:

I think one of the main advantages of using radiant systems is that you separate the conditioning of the space with the conditioning of the ambient air. With radiant systems you can deliver the air into the room at ambient temperature and use the ceilings to heat up the rest of the room. This will help avoid all the reheat energy that comes from using a VAV. I believe an NREL study showed that by separating the ventilation from the zone conditioning it was possible to save up to 30% of the energy in the building. I do not have the study to hand so I apologise if this isn't the case.

I have found that in Europe radiant systems are very common this is because in general the building envelope is better, reducing the heating load.

The second advantage particularly with heated ceilings/ floor is the thermal comfort of the occupants.

None if this helps you size your systems but I thought it might add to the discussion.

Annie

Sent from my iPhone

On Jul 21, 2014, at 6:54 PM, "RobertWichert robert@... [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

Interesting question. To me anyway. I have a project in San Francisco where the loads using ASHRAE Handbook methods are well above the radiant capacities in the apartments, but there has not been a huge outcry of being cold all the time. Granted, San Francisco is pretty mild (very mild), but the loads being twice or three times the capacity, well, I have to guess that it's because the loads using ASHRAE methods are met by lower radiant heating capacity than air system capacity. This seems pretty intuitive, really, but I have not found any standard guidance yet (still looking).

So at the risk of being shown to be wholly ignorant, can anyone point me towards any design documentation regarding radiant heating capacity and how to meet heating loads calculated using "air system" methods? ASHRAE guidance, perhaps?

Robert Wichert

On 7/21/2014 12:26 PM, HJ wanghaojie630@... [EnergyPlus_Support] wrote:

I use E+ to calculate radiant system compared to traditional VAV system.
The results showed that radiant system uses more heating energy but less
cooling and energy. (of course less fan energy and more pump energy). In
total I could see some energy saving but I would assume this will
diminish in a heating dominate climate. I wonder what is the energy
benefits of a radiant system? I could understand the heating energy
increases because we have more envelope loss. But why then the cooling
energy decrease? This looks like a little inconsistant to me.

HJ