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[EnergyPlus_Support] Re: Problem with removing reheat coils in VAV
Hi Dr. Li,
I have to simulate a real system, installed in 1996, with five RTUs (rooftop air handling units), each one with a air mixing box, two variable speed fans (one in the entrance of the RTU, right after the "Return Air Outlet" Node, and one at the exit, right before the "Supply Path Inlet" Node, but I only need to simulate the second fan at the exit) and one Heating Coil of Hot Water. There is then a Hot Water Loop, which connects the RTUs to the Boiler, and, of course, a Hot Water Boiler, with propane gas as fuel.
Since the cooling coil and reheat coils do not exist at all in the system, I don't simulate them: I deactivated the cooling coil in the schedule and I removed the reheat coils.
In the present moment I had to simulate with an electric heating coil to reduce and concentrate in only one problem. Now that I can finally heat correctly the zones, I have to remove the electric coil and insert the hot water coil, hot water loop and boiler. And when I do that, the zones are not heated at all.
The supply pipes are not very long, the boiler is not very far from the RTUs. I'm using the 80º-70ºC temperatures for the hot water, because they are the temperatures used in reality in my system.
I'm limiting the number of variables I change to limit the sources of error. I try to use only the templates and only change the variables I really need. I already made a drawing of the system nodes some time ago, to help me out at pinpointing the problems, but in this case it didn't helped me much. Strangely the hot water enters the heating coil at 80ºC and leaves it at 80ºC, not changing its temperature at all.
I can send you my file to your private e-mail, if you don't mind, but first I will try to analyse your file and comments to see what I can do first, so I don't waste too much of your time, because the building is quite big (4 stories and more than 2000 square meters and 5 VAV systems).
Once again I really thank you very much for your help and patience!
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> Dr. Li
> To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> From: n_junk@...
> Date: Fri, 22 Jul 2011 22:43:05 +0000
> Subject: [EnergyPlus_Support] Re: Problem with removing reheat coils in VAV
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> Hi,
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> In these days I have studied with careful the HVAC system I have to model in my thesis and for the problems I had with my file.
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> At this point I finally can mantain the zones' temperatures between 20-25ºC has I wanted. What I did was to remove some autosize fields and insert there the values I have in reality, and force the VAV damper to be always fully opened (which is the situation I have in reality). I also forced the system to insert in the zones the outdoor air I need.
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> The point that Dr. Li made about the internal loads made me understand better the problems I have. Both suggested programs were also useful, many thanks to you all!
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> I still have one big problem though. I'm currently using an Electric Heating Coil in each RTU, but I really need to use a Hot Water Coil and a Boiler. The problem is that when I switch from the electric to hot water coil the zones' temperatures drop below the setpoint and thermostat value (below 20ºC). More strange than that is that the setpoints are all the same, I only changed the coil type and add a hotwater loop and a boiler, but the outlet temperatures from the system drop a lot. If I rise the heating design temperature, the autosize feature increases the estimated heating coil capacity but the temperatures doesn't change at all... Do you know what's going on?
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> Note that this time I'm only using the HVACTemplates. To emulate the problem just open a example file with packagedVAV and just change the gas or electric heating coil for one with water.
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> If everything else fails (I'm running out of time), is there any known ratio between a electric coil's and a equivalent hot water coil's capacity? Like, for a same design heating temperature, a electric coil with x capacity is equivalent to a water coil with x/y capacity?
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> Once again many thanks to you all for all the precious help!
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