Big time energy waste

Brad White_9

And I thank you for it.

The Mass. Energy Code (780 CMR Chapter 13) requires heat recovery (energy recovery) when there is over 70% OA in a given system AND the system is over 5,000 CFM. 100% OA in a 4,900 CFM system would waste more but that is the code. Besides, unless there is a savvy AHJ, the energy code is policed by the designers via affidavits.

As responsible engineers, we do like to give the Owner the option; it is the right thing to do.

From the "code minimum" then, you have to make a payback case to the owner, an ROI if you will. With the current pinch they are feeling it too.

Here are some other more engineering-based thoughts, ones I used to NOT install heat recovery:

You need two things: 1) A continuous or predictable exhaust stream and 2) a place to put the recovered heat.

If you have a small split or packaged system with minimum OA, you can supplant that OA with what you have recovered (pre-conditioned OA) to take the edge off.

But if you have an all-air economizer system, in winter you will defeat this by pre-warming the OA which you need to make discharge temperature, causing you to bring in more OA to compensate. Therefore only in summer when your OA will be at code-minimum will you "have a place to put it".

Occasionally you can leverage this to your advantage:

One of my current jobs under construction, Tufts University Music Building, I designed a heat recovery unit to take the heat from a constant source, the toilets. The system is only 2,500 cfm roughly, well below the code threshold. Even though the system has economizer free cooling, I am using the recovered air energy to ventilate high-population classrooms. This way I do not have to enrich the entire air system for the sake of a few high occupancy spaces that have sporadic use. The ROI is immediate in that I do not have to add and control the extra tonnage in a 30,000 CFM master AHU. No tail wagging the dog.

Just some thoughts... but I am glad you raised the topic.

Brad

EBEBRATT-Ed

One of my pet peaves is the commercial job where they are required by code (or otherwise) to have large amounts of exhaust air for whatever reason.

I understand the reason for this but why are we installing make-up air units and using exhaust fans?? Why are energy recovery units not mandatory????

If exhaust fans were outlawed and energy recovery units used it would go a long way to help the energy crisis.

If I'm not mistaken a heatwheel will recover about 80% of the exhausted heat and even a pipe type will recover 60-70%.

I realize exhaust fans have there place and that some air streams may be two dirty and contaminated to recover.

Some laws need to be passed about dumping heated (or cooled air)


If it's already in the energy codes it needs to be enforced.

ED