Dumping Excess Load into Attic

Dan_15

Hi--
I have had this question in my head for a long time, and figured it was time to ask the opinion of some experts.

I recently commissioned a completely new hydro air HVAC system for my 1930s brick Tudor. We abandoned all the original ducting. Intalled all new ducting properly sized, sealed, wrapped. The system consists of an air handler with heat exchanger in the attic that serves the second floor, and another air handler in the mechanical room that serves the first floor/basement. 2 high efficiency condensers for AC. Heat is provided by a Lochinvar Knight boiler, and indirect tank for DHW. There are 7 space heating zones with mechanical dampers, one radiant floor zone and one DHW zone.

As part of the project, we brought the attic into the thermal envelope. We installed open cell foam sprayed to R-50. Best thing we ever did from an efficiency standpoint.

Here is my question. The system is pretty well balanced, but there is one zone that ended up with a bit more forceful air flow than the other zones, just by nature of where it is located in relation to the blower. My installer tried tweaking the jumpers on the multi stage blower control, as well as the settings on the main bypass duct. He even installed another bypass duct just on that troubled zone. That helped. At the same time, he added a register in the attic that dumps excess air into the attic, which is tied solely to that troubled zone with a mechanical damper. This was just for insurance in case we need it. Currently I am not finding any need to use it, so I have disabled it permanently.

Question: I am wondering if I should actually use this damper for the attic? With the spray foam insulation, the attic is very tightly sealed now--it is fully within the thermal envelope of the house. Would I achieve any efficiency gain out of heating/cooling the attic at the same time as the rest of the house? In other words, if I put energy into the attic, would that have the effect of needing less energy in the living space...or does it all net out equally...or am I wasting energy in the attic. I'm not sure what is the right way to think about it.

Thanks! Dan

Spence

From what you describe, I believe any energy use would be outweighed by the comfort resulting from abating the excess air in your "trouble" zone.

Dan_15

Thanks for your reply. For certain, it would make the living space more comfortable to have less forceful air flow. Having said that, I am also really interested in the efficiency caculus.

It sounds like you feel that dumping excess air into the attic would be wasting energy.

I am still trying to understand the theory behind this question. I have to assume there must be a precise answer that engineers with PhDs are all probably aware of. I am not an engineer, and what I know just comes from basic observation of my world. It seems to me that it is reasonable to assume that putting energy into an unoccupied space is wasteful. But is that really true? I am guessing that less air flow in the living space will probably cause the thermostat to take longer to satisfy, thus using more energy. But since the spray foamed attic is basically a sealed barrier, wouldn't adding warm air on top of the living space have the effect of reducing the "stack effect", thereby trapping more BTUs inside the lower floors of the living space for a longer period of time---which saves energy---or at the very least nets out equally? Likewise, wouldn't adding cold air to the attic have the effect of keeping the lower floors cooler for a longer period of time?

Am I thinking about this the right way, or am I totally on the wrong track?

Bob Bona_4

My opinion is if the dump zone is within the envelope, and there's no negative effect to the things in the envelope, there is no waste or harm. My experience with full foamed attics is that the space is pretty temperate from thermal migration from the areas below, and loss from the hvac in that space. Can't tell in your case how long the dump cycle extends, or how extreme the temps get via the dump, but give it a try!

Is the blower PSC or variable ECM? ECM could give you more tailoring CFM-wise.

SWEI

Dan said:

I am guessing that less air flow in the living space will probably cause the thermostat to take longer to satisfy, thus using more energy.

Not as far as the heating source goes. In a conventional furnace, when the fan is moving less air, the rise across the HX increases, delivering hotter air to the space. With a hydro-air system running ODR, the water temp can be lowered to better match the lower airflow.

Has your installer considered choking down the problem duct to reduce the airflow?

Dan_15

Thanks for all the replies. My installer did not want to choke down the ducts for fear of freezing the AC coils.

I still cannot get my brain around dumping energy into a space that is not occupied. It is true that the attic is very temperate because of the that air migrates from the lower level.

I think I will try some experimentation with a combination of choking the dampers and putting a bit of air into the attic, and I will watch closely for freezing coils.

Spence

You should have a freeze stat on your system anyway, to protect the compressor when the smallest zone is calling. I don't believe I'd worry to much about sending conditioning unit the attic. The energy "portion" there probably has more value than over-pressurizing a small space and recirculating conditioned air back into the airstream.