Re: [EnergyPlus_Support] Radiant with DOAS sizing error

Most (unlike Ned's floor) radiant heating pipes are imbedded in the floor, so the heat is conducted to the outer surface. Ned's pipes convect heat to air (to increase the convection rate he has added metal fins which one can clamp around the pipes) which then convects it to the underside of his floor surface (right?). Then the heat conducts through the wood floor to the outside surface.

HJ, e+ does not have a hanged radiant hydronic model. But there is a research project underway which will hopefully address this short-comming.

Essentially, you can use the embedded pipe model as a work around, but you have to live with the increased thermal innertia due to the limit on the thermal diffusivity. That means slow response times and running slightly more extreme water temperatures to achive the correct surface temp. Energy-wise, the dT of the water remains the same, allbeit more extream, so that impacts slightly on correct plant energy usage simulation.

I have experimented extensively with this model and can setup pretty accurate systems. I find autosizing slightly conservitive most times, but this depends largely on what pressure drops were thought of as good to use in the energyplus autosizing routine. Mostly, I do manual sizing, but I'm an engineer with experience in this area, which I don't even expect the others in my office have, let alone other energy modellers. My advise is to get a manufacturer to size these systems for you and then use those values for manual sizing.

As I said before (and as Ned also said), check your surface temps. This is the key to everything.

PS convection coeff calculated by e+ is as expected. For best results, use the ceiling diffuser model or the adaptive model.

Mit freundlichen Grüßen- Sent from my iPhone (excuse the brevity)

i. A.

Jean Marais

b.i.g. bechtold

Tel. +49 30 6706662-23

On 26.06.2014, at 22:03, "HJ wanghaojie630@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

Yeah I am conerned this might be the problem especially you have a carpet on the floor. We use ceiling radiant system and hang them directly outside. That could be better but energyplus wont let you set the radiant system on the outside surface and they have to be inside one layer of construction. If i just use a layer with really high conductivity, energyplus will give me a warning saying it cannot handle it properly. It would be really helpful if I could know how the sizing of radiant system work. The sizing:zone or sizing:system or sizing:plant does not provide any information regarding to the radiant system sizing parameters. I wonder this might results in the undersizing?

HJ

On 6/26/2014 10:49 AM, 'Edward G. Lyon' eglyon@xxxxxxx [EnergyPlus_Support] wrote:

I’ll take a quick shot in the dark based on my own experience this winter with my home radiant system. The temperature you are setting for radiant may be the fluid temperature (I have used radiant surfaces in Energy+, but not radiant floor heat systems). I expected my system to work with a fluid temperature in the range of 50C. Good thing I didn’t rip out my electric baseboard, because I couldn’t keep the room warm. My wood floor is too insulation even with Al heat plates around my Pex pipe. I found I needed to run 80C fluid to stay warm. With that temperature, my floor surface was not much above 32C. I think you should check the floor surface temperature that you have. If you are not reaching mid 30’s, you haven’t given the system enough power.

Ned Lyon, P.E. (MA, WV)
Staff Consultant

SIMPSON GUMPERTZ & HEGER
781.907.9000 main
781.907.9350 direct
617.285.2162 mobile
781.907.9009 fax
www.sgh.com

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx]
Sent: Thursday, June 26, 2014 10:26 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] Radiant with DOAS sizing error

Thanks Jean. 17 C is VAV air temperature reset in winter. In summer, we use 13C for cooling. In winter, we dont want to heat the air too much because there will be core zones that still need cooling, so we use 17 C. Also, we dont want to supply air temperature too high in winter because radiant system will have direct radiation to people and they will feel too hot if we supply air temperature at 30 C. So basically, DOAS can only provide cooling, and radiant will pick up the rest of the cooling load in summer and all the heating load in winter except the ventilation preheat to 17C from the VAV main heating coil.
I agree there will be a limit posed by the total surface area of radiant system. But i tried with E+ example file and they also have the same problem. I wonder if they under predict the convective heat transfer too much so the air temperature wont go up in winter and down in summer.

HJ

On 6/25/2014 11:25 PM, 'jeannieboef@xxxxxxxxx' jeannieboef@xxxxxxxxx [EnergyPlus_Support] wrote:

Biggest tip for radiant systems is to control the surface temperatures. If they are correct, and you still get unmet loads, then you need an additional surface area or auxillary equipment in the zone like a fancoil. The convective and radiative heat transfer is ABSOLUTELY limited by the amount of area at temperature in the active surface. Check with hand calculations what W/m2 you can expect and if it is enough to meet your loads.

Side note:

Why is your limit of 17 deg C imposed? If outdoors is warm and you need heating inside, you could use warmer air, no?

Mit freundlichen Grüßen- Sent from my iPhone (excuse the brevity)

i. A.

Jean Marais

b.i.g. bechtold

Tel. +49 30 6706662-23

On 25.06.2014, at 21:17, "HJ wanghaojie630@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

Thanks Ery,

I used sized by ventilation because I want to use DOAS to provide ventilation only (at 13-17C all time) and use radiant system to provide the rest of heating and cooling. I also set the priority in zone:equipment to let DOAS meet the load first and then use radiant to meet the rest of the load. However, there are still tons of unmet load hours and the energy also seems to be higher than using VAV system alone. I also tried with radiant system without DOAS, so I only need to meet the building load, but still I have tons of unmet load hours. Do you have any sample files that you could share with the group? I would really appreciate if you could let us know how you set the parameter. Seems like there are other people having the same problems too. Maybe I did something wrong with the zone:sizing or system:sizing tab?

Thanks,

HJ

On 6/25/2014 12:22 PM, Ery Djunaedy ery.mailinglist@xxxxxxxxx [EnergyPlus_Support] wrote:

HJ

You need to play with a number of parameters in Sizing:System and
Controller:OutdoorAir objects for your DOAS system.

If you size your DOAS on "ventilation" then there will be excess
heating and cooling that cannot be met. You can see this in the HTML
report under unmet hours. But if you changet the sizing to "sensible",
all of the excess heating and cooling will be met by the DOAS.

You need to set the equipment connection to set the priority on
heating and cooling of the zone between the radiant and the DOAS.

If you have big DOAS coils as the result of your autosize calculation,
you need to say "No" to 100% OA when cooling and cooling. With this
setting, you will have DOAS most of the time but when there is extra
heating and cooling beyond the ventilation load, then the OA fraction
will be reduced to meet the load.

Ery

On Tue, 24 Jun 2014, HJ wanghaojie630@xxxxxxxxx [EnergyPlus_Support] wrote:

> Hi all,
>
> I have some projects that has radiant system with DOAS. I find
> EnergyPlus could not size this type of system properly it never try to
> meet the thermostat temeprature. When I look at the summer or winter
> design condition, the temperature could go as high as 30 or as low as 7.
> I used openstudio to export to EnergyPlus. For radiant system, it does
> not use thermostat but use its only temperature control, I wonder if
> that could be the problem for sizing? Does anyone have any successful
> experience with this type of system sizing and could share your idf file?
>
> Many thanks,
>
> HJ