Comments on: The 3 Types of Heating and Cooling Loads

By: Emre Yurttas

Emre Yurttas — Sun, 23 Aug 2015 12:13:32 +0000

Hi Allison,
Hi Allison, Great blogs. very helpful in understanding the basics of HVAC design. I am living in Texas. as you can imaging humidity is the biggest evil associated with our weather. Our old HVAC system has been working overtime to keep our house cool and thus has been needing service every year. So we decided to have with the recommendation of our HVAC contractor, the system upgraded to a new unit. The new unit has been great. However, lately I have noticed it is running more than it has to and cannot keep the temperature more than 1 degree below the setpoint. even when it satisfies the setting, it cools very rapidly. We installed a 3.5 ton unit based on a Manual J load calc which is an upsize from a previous 3 ton unit . Now my impression is that the load calc was gamed and no duct testing was done at all. I think the contractor gave the impression that a bigger unit would cool better. Now I have a 90k 4 ton drive furnace which i think is more oversized than the a/c. homne is about 1614 sq ft. so thoughts ? Am I using an oversized system on undersized ductwork or is just the blower that is too big?

By: Emre Yurttas

Emre Yurttas — Sun, 23 Aug 2015 08:13:32 +0000

By: Morgan Audetat

Morgan Audetat — Sun, 25 Jan 2015 00:50:47 +0000

We have been installing
We have been installing radiant floors and panels with combi service condensing storage water heaters for over 20 years now and over the last 10 a sub-system that modulates flow on outdoor reset.

An ERV, high velocity AC and mini-splits round out the package, but all are sized to ACCA Manual 'J'; every one.

By: Morgan Audetat

Morgan Audetat — Sat, 24 Jan 2015 19:50:47 +0000

We have been installing
We have been installing radiant floors and panels with combi service condensing storage water heaters for over 20 years now and over the last 10 a sub-system that modulates flow on outdoor reset.   <br />  <br />An ERV, high velocity AC and mini-splits round out the package, but all are sized to ACCA Manual 'J'; every one.

By: Dale Sherman

Dale Sherman — Wed, 14 Jan 2015 22:08:38 +0000

At NESEA's BuildingEnergy
At NESEA's BuildingEnergy conference in Boston last year, Jonathan Wright told the story of moving into one of his own high performance homes in the midst of a severe winter storm. He promptly lost power for 3 days with outside temps in the 20's. After 3 days, his indoor house temperature drifted from 68 down to 65.
That's the kind of design load we should be striving for, with variable capacity equipment to match.

By: Brett

Brett — Wed, 14 Jan 2015 19:58:11 +0000

Thanks for this. I'm looking
Thanks for this. I'm looking to install A/C this year and got some quotes in the fall. Out of 3 companies, only one said a 1.5 ton condenser would be fine. Everyone else wanted to install a 2 ton unit for a 1000 square foot house that has R-49 (foam/cellulose) in the attic. I went so far as to purchase software and do my own manual J and sure enough, 1.5 tons is plenty.

By: bob

bob — Wed, 14 Jan 2015 19:42:10 +0000

IMHO Man J doesn't give
IMHO Man J doesn't give enough credit for "thermal lag". It affects peak load considerably on high mass buildings.

By: Mac Sheldon

Mac Sheldon — Wed, 14 Jan 2015 19:19:02 +0000

Perfectly stated once again
Perfectly stated once again Allison,
There's been a lot of chatter of late about over-sizing furnaces and air conditioners, and hopefully our mechanical contractors are getting more confident in home performance so they reduce capacity of the equipment to meet the actual load. As you point out, even a well designed system is over-sized most of the time unless it's capable of varying it's capacity, which is the wave of the future.
As we improve our heating and cooling systems and fit them carefully to the home or building loads it's imperative to consider th "V" in HVAC separately. As our H & AC loads shrink it's more and more common to require a second air distribution system for ventilation in order to get the air to and from the critical areas in the home.
Whatever it takes we must Build it Tight and Ventilate Right, Build it Tight and Ventilate Right, Build it Tight and Ventilate Right!

By: Dennis Celsor

Dennis Celsor — Wed, 14 Jan 2015 18:30:31 +0000

Allison,

Allison,
Thanks for a superb article. I find AC companies constantly trying to oversize systems in new homes. It seems that most Manual J's have a built in default to a ton per 500 sf. I will be passing this article on to clients to help them understand why too much AC is harmful in today's tight homes.
I encourage every one of my client's to upgrade to a 2 stage condensing unit with a variable speed fancoil. I'm not yet persuaded that the true variable speed mini-split system, even with some ducting, will deliver the air distribution to which we Americans are accustomed.
Thanks again for a superb article.

By: Dale Sherman

Dale Sherman — Wed, 14 Jan 2015 17:08:38 +0000
At NESEA's BuildingEnergy
At NESEA's BuildingEnergy conference in Boston last year, Jonathan Wright told the story of moving into one of his own high performance homes in the midst of a severe winter storm. He promptly lost power for 3 days with outside temps in the 20's. After 3 days, his indoor house temperature drifted from 68 down to 65.   <br />That's the kind of design load we should be striving for, with variable capacity equipment to match.