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HVAC Discussion

Posted: Wed Apr 14, 2021 1:34 pm
Created this new thread for HVAC discussion. Will leave it here under solar.

Here again is a link to the Load Calculator I've been using:

I'm still tinkering with my numbers. Windows seem to be the biggest problem in my house. They were single pane but I've replaced two already with double and have two more on order.

The next biggest issue depends on heating or cooling season. For heating, my floors are the next biggest problem and I can't really do much. A slab is a slab. For Cooling season, the second biggest issue is either duct load or ceiling. I have to go up in my attic and decide what R value my ducts actually are. Currently I'm running with the setting at R-19 for attic, R-13 for walls and R-4 for ducts. These could all be wrong. Knowing your inputs are vital to get good numbers out of the calculator. I've measured everything else, windows, doors, etc. I've tried measuring the attic insulation but its a mix of blown and bat, and the blown in fiberglass is uneven. It is a late 80's house in the desert in CA so R-19, while it sounds low, might be correct.

For Design temp inputs for cooling I have put in indoor of 75f without Outdoor 110f, and for Heating outdoor 20f and indoor 68f.

Currently with these inputs I'm getting a Load of:
Total Btu's Cooling: 43347
Total Btu's Heating: 50479

More to come on my past usage for my NG furnace and systems I'm looking at, their design BTUs and DIY issues, and fitting it in with my Solar.

Re: HVAC Discussion

Posted: Wed Apr 14, 2021 1:55 pm
How many square feet are you heating and cooling? your numbers look about right for a 2000 square foot house......

Re: HVAC Discussion

Posted: Wed Apr 14, 2021 2:07 pm
dmacarthur wrote:
Wed Apr 14, 2021 1:55 pm
How many square feet are you heating and cooling? your numbers look about right for a 2000 square foot house......
1810 sqr feet, but I have high ceilings in several rooms. I have my cubic feet set at 21720, which is another number I need to spend more time on.

Re: HVAC Discussion

Posted: Mon Apr 19, 2021 7:20 am
Over the weekend I ripped out my old NG furnace. A bit of a pain because it was heavy, but my son and I finally lifted it out and carried it outside. An old Heil furnace, maybe 80% efficient, from the late 80's or very early 90s. The heat exchangers though looked to be in good shape.

So now I have until November to figure out what I am doing to replace it.

I'm still leaning toward a heat pump, I think its a good fit in our area.
Pouring through the spec sheets for some models' I've noticed just how different one unit to the next can be.

For example, if you look at a Mr. cool Universal 5 ton you see:

Outdoor Temp / Capacity (BTUs) / Input Watts / HSPF / COP

17f 49000 7100 6.90 2.02
32f 54000 6600 8.18 2.40

Then compare to a Goodman 5 ton:

17f 34600 4100 8.4 2.3
32f 41580 4600 9.0 2.65

As far as I can tell, HSPF is just the BTU output divided by the kW input. You can also look at COP, Coefficient of Performance, which looks at btu output / btu input (or kW output / kW input)

Anyway, either way, the Goodman appears to be more efficient at the lower temp BUT the Goodman has a significant reduction in output.
The Goodman unit falls well below my needed BTUs at the lower end of the temperature I'm looking at, which would mean installing the emergency heat strips, which have a COP of 1. So that higher looking efficiency of the Goodman would quickly dissapear when heat strips turn on.

As a side note, a "5 ton" unit theoretically should move ~60,000 btus or 12k btus per ton.

I think the takeaway for me is that yes, some heat pumps are much better than they used to be at cold temperatures. But they appear (per their spec sheets) to not be created equally. In fact, the 4 ton Mr cool Universal is almost as good at the 5 ton at those cold temperatures. Well, cold for where I am... YMMV.

Here is the 4 ton system:

17f 48000 6880 6.98 2.05
32f 48000 5800 8.28 2.42

So, if I'm reading the spec sheets correctly, it tells me one really needs to dive into literature beyond just the 1 page glossy product flier.

I have to go look to see how the commercially installed (non DIY) systems like Carrier and Lennox and Trane compare.

*IF* I went forward with the Mr. cool 4/5 ton unit, I'd likely install a 5 kW (small) heat strip, mainly for defrost cycles. But not the standard 15 to 20 kW strips, as I just don't need that much heat. I'm going to post more on my prior NG consumption with some estimates later based on my old furnace.

Re: HVAC Discussion

Posted: Mon Apr 19, 2021 7:43 am
I guess for those two systems I also should have posted cooling, though I have a whole house swamp cooler, so AC would matter less to me.

Mr. cool Universal:

Outdoor Temp / Capacity (BTUs) / Input Watts / EER

105f 54000 6650 8.12
115f 46000 5700 8.07

Then compare to a Goodman 5 ton:

105f 50800 5100 9.96
115f 47900 5750 8.30

But these may not be perfect comparisons, as the Goodman Spec sheet requires you to choose indoor wet bulb temp, while the Mr. cool does not list indoor wet bulb as part of its spreadsheet.