I need advice on HVAC

hardflex · 11-20-2010, 07:23 AM

I have an older house about 1000 sq ft that's been rented but now vacant and as part of the rehab and other considerations my wife wants to install a forced air heat/ac system. It currently has a wall unit gas heater and window unit a/c's.

I have a chance to buy a working used system at a good price. My sense says the gas heat has not gotten more efficient in the last few years, the A/C prob has. I'm thinking the gas mechanisms are very reliable with few working parts, so maintenance is not a factor there. The A/C newer and probably more efficient than the unit in the larger house i live in now, so the savings on the more efficient system would be negligible.

Is there anything I should know about the older systems vs newer ones?

Zeke · 11-20-2010, 08:12 AM

I think you nailed it, efficiency. But, the manufacturers have been cranking out efficient units for 10 years now. How old? If it uses PVC for the exhaust gases, it's new enough. They all still have relays and blower motors. There's a small SS circuit board in them, but largely serviceable in all aspects.]

Designing the ducting will have as much or more to do with what you get out of any FAU.

scottmandue · 08:53 AM

What Milt said...

New heaters are much more efficient than those of 10 plus years ago... same with A/C.

Making sure the house is well insulated will be your biggest cost savings with HVAC.

And have someone who knows what they are doing design the duct system... it is not DIY friendly. When I was in the biz I had to gut a few systems that homeowners tried to cobble together.

Edit to add: Is it a package unit or split system? If it is a split most if not all of the A/C efficiency in in the compressor design, that is the unit that sits outside.

Hugh R · 11-20-2010, 09:23 AM

Unless you have attic over the entire living space you will have issues running duct work. No provisions were made in the walls for running duct work.

scottmandue · 11-20-2010, 10:02 AM

Quote:

Originally Posted by Hugh R

Unless you have attic over the entire living space you will have issues running duct work. No provisions were made in the walls for running duct work.

Or if it is a raised foundation and there is room you can install heater/duct under the house.

Just my preference but I think an under house perimeter grill system works better than an in ceiling grill vents.

Boy, I sure don't miss installing HVAC systems!

MysticLlama · 11-20-2010, 10:14 AM

How about a ductless mini-split with multiple of the interior side unit and a single compressor?

It's a lot like Forced air with a heat pump without ripping the place apart to build ductwork.

Zeke · 11-20-2010, 10:16 AM

Don't know what the crawl space is in a typical TX home on a raised floor construction, but here in SoCal it's either slab or 18" in most cases. Not enough room.

Hugh, I remember some small diameter high velocity systems for retrofit. Saw them on "This Old House."

billybek · billybek's Garage

There is a residential system available that utilizes high velocity duct work so the size of the ducts is considerably smaller than conventional systems.
Really handy for old homes with small trusses and very little room to retro fit a heating/ac system into.
Can't remember the manufacturer but it could be Lennox.

Hugh R · 11-20-2010, 11:59 AM

Milt, yes you're correct, but you still have to deal with fire stops and such between joists. The mini splits aren't bad, and I've considered one for my 81 y/o MILs room so that I don't have to cool/heat the whole house when everyone else is gone during the day.

john70t · 11-20-2010, 01:52 PM

Make sure it's sized correctly.
The folks had a system too large for the house that would kick on and off constantly. Heat bills were through the roof. Smaller can be be better sometimes.

The two-stage systems are supposed to be very efficient. Small need, partial use.
A few hundred bucks now equals a lot more comfort and savings later on.

red-beard · red-beard's Garage

Quote:

Originally Posted by john70t

Make sure it's sized correctly.
The folks had a system too large for the house that would kick on and off constantly. Heat bills were through the roof. Smaller can be be better sometimes.

The two-stage systems are supposed to be very efficient. Small need, partial use.
A few hundred bucks now equals a lot more comfort and savings later on.

+1

An oversized system can be bad...

For 1000 sq ft, I would go with several of the splits. It will cost less to install.

MysticLlama · 11-20-2010, 02:38 PM

And you should only need multiples of the inside half, they make outdoor condensers that will run multiple units.

hardflex · 11-20-2010, 05:51 PM

I heard the figure of a ton for every 400 sq ft, Does that sound right? It's an older house, insulated but still has the older windows. That may change too.

MysticLlama · 11-20-2010, 05:58 PM

That can't be right, ours is a new 4 ton, which is a touch oversized, 3.5 was right on the line. It's 2850 sqft.

Does have somewhat recent windows and such, but still, that's one ton per 700.

Zeke · 11-21-2010, 09:00 AM

Quote:

Originally Posted by hardflex

I heard the figure of a ton for every 400 sq ft, Does that sound right? It's an older house, insulated but still has the older windows. That may change too.

Lots of factors beyond sq. footage. Volume is used as well as R value calcs.