HVAC sizing

[Spfrancis]

So I have discussed my 2 systems here on various threads for our house in Delaware. I have a 70,000 BTU Ducane propane based furnace, and a 2.5 ton AC system. The old house was one level and the basement, and it was poorly insulated. When I blew in some insulation in the attic, the AC seemed to keep up okay.
The furnace is probably about 12 years old, and the AC is 8 years old. We have added a 2nd floor, for our master suite, which is about 400sqft, with a Mitsubishi ducted system, so that they can hit the main bedroom area, and then a duct to hit the bathroom. The HVAC guy was making a push to upgrade the existing system for the main level, and the basement. The main level expanded by about 150sqft, and the ceiling height is going to be 9ft. We are getting all new windows, 6inch depth on most of the walls, and all new insulation. The house will be alot tighter, than the old house. The new house will be about 1500sqft on the main level, and an additional 400 sqft in the basement. The HVAC guy was pushing a new system to replace the old system...but I really don't think I need it right away. He was pushing a larget Mitzu system, to drive both zones. I'm sure it would be more efficient, and I would save a few bucks on my montlhly bills. I said that I would like to just do the one mitzu system, and to improve the ducting in the main level of the house, since everything is down to the stud right now. The HVAC guy was stating 500sqft per ton, and I seem to recall on this forum that that is not the case, especially when everything is new. I did ask for the ManualJ results...but I may not get to see them. I did a rough estimate using a site that I found, and I think that I will be fine. I do like the idea of getting away from the propane based system..but we have so many expenses right now, and I just don't want to take that expense during the renovation

 

[Dana]

500' per ton is insane for cooling. For a 1500' house + 400' basement with new low-E windows, fluff in the attic & walls you'd more likely be looking at 1- 1.25 tons of cooling load with a competently executed Manual-J, and a heat load of less than 25,000 BTU/hr, maybe even less than 20,000 BTU/hr. A ton per 500' of conditioned space is ludicrous- that's the cooling load of an uninsulated tarpaper shack in Alabama, a good example of why rules of thumb must die. Check out the Manual-J results of square feet per ton plotted against house for a few dozen jobs performed by an outfit in Georgia:

Since you have a heating history on the house, run a fuel-use based heat load calc by comparing propane use against heating degree-days.

Or, run your own Manual-J using sorta-reasonable freebie online tools such as LoadCalc or CoolCalc, but be aggressive rather than conservative on inputs (per the instructions in the Manual) or it can be crazy-oversized using those tools too. Being conservative rather than aggressive on R-value and air tightness & shading assumptions is the most common error for newbies & pros alike.

 

[Sponsor]

 

[Spfrancis]

Yeah thanks Dana for the support. I just knew from spending enough time on this board that the sizing is more than adequate for the size of home we have. Even with the increase in height, and the additional sqft we picked up, so more than be offset by the new better framing, brand new windows, thicker walls, and all new insulation in the home. Really I kept pushing back that we did not want to tie the home renovation to needing a new system for the existing area. They were trying to explain a benefit to having a single outside mitzu unit, tied to both the new upstairs area, and the existing main level and downstairs area. In my mind, I don't see a huge benefit of having one outside unit versus having a second outside unit with what we are currently doing. In my mind being in the IT field, it seems more HA(highly available) if we have 2 different outside units, so that if one stops, the other zone will continue to work. Yeah aesthetically it doesn't look as nice, but I can live with that. I will give another crack at that LoadCalc site, to see if I can come up with a heating and cooling btu.

 

[Dana]

They were trying to explain a benefit to having a single outside mitzu unit, tied to both the new upstairs area, and the existing main level and downstairs area.

Did they explain that it helps the contractor stay current on their boat payments?

They're either BS artists, con artists, or idiots (possibly all of the above?). What they are not is competent.

Multi-splits (as a general rule) suck, due to low turn down ratios and high minimum modulated output. It's a recipe for short-cycling into lower than nameplate HSPF or SEER numbers, and poor comfort.

Even the smallest cold climate MXZ compressor probably has enough output to cover the design load, but has only about a 2:1 turn down ratio. With a minimum output of 11,000 BTU/hr @ 47F being several times the average heating or cooling load of the 400' basemen zone it would short-cycle like crazy serving either (or both) zone(s) most of the season.

The smallest non-cold-climate MXZ that still has a reasonable shot at covering your 99% design load has a higher turn down ratio than the cold climate version, but an even higher minimum modulated output, and would suffer even more severe short-cycling issues.

A separate -FH06NAH (the full cold climate version, with a pan heater for defrost ice management during cold snaps), would more than cover the basement zone.

The full-on cold climate 1.5 ton ducted KA18NAHZ / KD18NA is also fully modulating with about a 3:1 turn down ratio, and a minimum output of 8800 BTU/hr @ +47F which is probably still less than your load a +47F, yet is capable of delivering 21,600BTU/hr @ +5F, which is probably pretty close to the load at that way cooler than 99% outside design temp temp. The KD18 can't handle high static pressures on the duct system, but it doesn't take gia-normous ducts to get those static pressures under control- just go for an oversized 4" or 6" pleated filter, and design for <400 feet per minute duct velocities and use only hard-piped radiused turns and it'll be fine. Even oversized flex all coming from a short-fat plenum and a common return can work fine in a house that size.

These are just best-guesses as to what the most appropriate Mitsubishi equipment might be. A real room by room Manual-J performed by a competent engineer or RESNET rater would be useful for narrowing down on it.