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RE: [EnergyPlus_Support] ZoneHVAC:LowTemperatureRadiant:VariableFlow


Are you sizing your slabs correctly. The radiant ceiling component in energyplus is a little delicate and I have found that the autosize is pretty unreliable. If you size your flow rates to create the correct capacity you might have some more luck with the system. The second place I would check with my radiant slabs is the construction, have you made the slab too thick, or got the materials layered correctly? I model radiant slabs fairly frequently and have found that in general compared to a single forced air system a DOAS and radiant system will save around 30% energy. I would also check that the pump and fan energy are as you would expect. Radiant systems obviously have large savings on fan power in comparison to forced air systems.

I hope that helps a little


To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: agerman@xxxxxxxxxxxxxxx
Date: Mon, 25 Mar 2013 17:00:05 +0000
Subject: [EnergyPlus_Support] ZoneHVAC:LowTemperatureRadiant:VariableFlow

I have a 2-story single zone residential home which I'm modeling with two distinct HVAC systems. One is a forced air system using a single speed heat pump with ductwork in conditioned space (no losses accounted for). The other is a hydronic system which uses radiant floor on the first floor and radiant ceiling on the second to distribute both heating and cooling. The distribution system is tied to a water-to-water heat pump and a ground loop. All other envelope characteristics are identical.

Operating efficiency of they hydronic system in both heating and cooling is 1.5-1.75 higher than the forced air HP. Provided this increase in system efficiency along with similar distribution efficiency I would expect to see significant savings for the hydronic system. However, I end up with similar heating energy and 35% higher cooling energy for the hydronic case. Cooling energy delivered to the space is 2.5x greater for the hydronic case (both systems have the same capacity). I calculated this difference in delivered energy from the "HVAC Input Sensible Air Cooling" for the FAU and "HVAC Input Cooled Surface Cooling" for the hydronic case, is this correct?

It seems to me that the radiant distribution delivery to the zone isn't performing correctly. During a heating or cooling event the two systems are turning on at similar times and have similar runtimes but the hydronic system apparently delivers a lot more energy to satisfy the load at a higher energy cost (based on the COPs input). I tried 1) making the model 2 zones and 2) eliminating the radiant ceiling and doubling the heat transfer area of the floor. Neither of which helped. Does anyone have any advice?


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