Replacing 30yr Central AC with Mitsubishi Ducted-Ductless

D107

Original issue was there was no AC in 500 sq ft basement but a contractor has offered a proposal to replace the existing 30 yr old 3 ton Bryant AND add minisplit to basement all running off one compressor:

(His original choice was 24K btu Hyper H3at trizone. MXZ3C24NAHZ2-U1; then upgraded it.)

On main and second floor, system will use existing ducts.

House is 1700 Sq Ft. 1924 Insulated Walls Spray Cellulose;
Attic eaves (6") and Gable walls (4") sprayed closed cell foam.
Basement plus first floor bathroom in same cooling zone = 600sqft 80"H;
1st floor: 600sqft 9ft H.
2nd Fl: 535sq ft 8ft H.
(Heatloss about 35K)

They will do an AC calc––I was a little concerned that the unit will be too big. Contractor states that the controls on this system can scale the capacity down if I understand him right.

I also don't know if it's worth it to have the Hyper Heat on this. We have a new hot water boiler atmospheric that's dialed in quite nicely, and even in shoulder seasons, with our electric bills, it would seem cheaper to always use the boiler for heat and not the heat pump.

Our reasoning is that with a 30 year old AC that could go any time doing this all at once saves $$ and foregoes having to have a separate compressor added for just the basement AC and then in a year or two swapping the old handler and compressor out with a Bosch system when that breaks and end up with two disruptions and having to squeeze a 2nd compressor in.

Sound good?

pecmsg

Basements have a different heat load loss calculation. You can require heating while the 1st floor needs cooling.
a small mini split heat pump and dehumidifier are all that's needed.

pecmsg

2 stories

2 systems

Hot_water_fan

I don't like that combination at all.
1. That seems way too big. I have a similar sized house with no insulation (but two shared walls) and my 24kbtu hyper heat is more than enough when the outdoor temp is in the single digits.
2. The 1-outdoor unit to 2 indoor units perform worse than 2 outdoor to 2 indoor units. 9000kbtu/h is huge for 500 sqft below grade and the multi-splits can't turn down as well as a one to one can. However, a 1-to-1 9kbtu/h unit has great turndown. A 1-to-1 24kbtu/h unit's turndown is decent.
3. you have a 3 ton AC - on the hottest afternoons, does it turn off? That's a simple way to check sizing.
4. you also have a backup heat source, so even less need to oversize.
5. your mileage will vary, but I save substantially using electricity vs. gas.
6. a proactive replacement does seem best - you never know what equipment availability will be like during an emergency repair - then you'll be stuck with a multi-decade compromise.

How much gas do you typically use per year? Therms, not dollars.

D107

@Hot_water_fan While I've learned a lot about heating on this site, I haven't focused much on heat pump AC, so the advice is especially appreciated.

To answer your questions:

"I don't like that combination at all. 
1. That seems way too big. I have a similar sized house with no insulation (but two shared walls) and my 24kbtu hyper heat is more than enough when the outdoor temp is in the single digits. 
__Agreed and hopefully he wasn't sizing for heat at all since I will likely never use it for that. We are north of nyc and electric costs are even higher than the comparatively astronomical prices they've always been. Last bill came in @33¢ per kwh all charges; gas also jumped: $2.68/TH all charges. I suppose it's possible price of gas will stay high or rise and make electric more feasible….
2. The 1-outdoor unit to 2 indoor units perform worse than 2 outdoor to 2 indoor units. 9000kbtu/h is huge for 500 sqft below grade and the multi-splits can't turn down as well as a one to one can. However, a 1-to-1 9kbtu/h unit has great turndown. A 1-to-1 24kbtu/h unit's turndown is decent. 
__Yeah, another offer spec-ed 6000 btu for basement unit. With our small house and narrow pathways around it that second outdoor unit would really be cumbersome. If its sized right, then would turn-downs be that necesssary?
3. you have a 3 ton AC - on the hottest afternoons, does it turn off? That's a simple way to check sizing. 
__Been here for 25 years with the same AC. I don't think it ever ran continuously even before we insulated walls and attic—where the handler worked in 100 degree heat. Climate Zone 6b, 7a. In fact I thought it might be too big —or it needed some kind of humidistat to dehumidify more even when temp was below t-stat setpoint.
4. you also have a backup heat source, so even less need to oversize.
5. your mileage will vary, but I save substantially using electricity vs. gas. 
6. a proactive replacement does seem best - you never know what equipment availability will be like during an emergency repair - then you'll be stuck with a multi-decade compromise.
__Agreed. Proactive is best—for heating as well.

How much gas do you typically use per year? Therms, not dollars.
__In the year leading up to the latest March bill—a little warmer than usual probably—we used 710TH, 220 of which was for hot water (reverse indirect), dryer and stove, so less than 500TH for heating.

The alternative proposal was for a separate basement minisplit and replacement of central AC:
1-Mitsubishi 9mbh wall-mounted inverter system only for basement PLUS
2-Replace our attic Bryant handler and outdoor compressor with Bosch 18 SEER condensing unit and Bosch Bosch BOVA-36 1-3 ton 9.5 ASHP inverter heat pump and matched Bosch 3 Ton 20 Seer Air Handling heat pump. Vertical unit, BVA series, Multi-Speed.

Hot_water_fan

Yeah, another offer spec-ed 6000 btu for basement unit. With our small house and narrow pathways around it that second outdoor unit would really be cumbersome. If its sized right, then would turn-downs be that necesssary?

The outdoor space is a significant constraint for your situation unfortunately. I have two mounted side by side in a narrow space, maybe the installer can get creative? If not, this is a better option. It can turn down twice as far.

https://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006\M_SUBMITTAL_MXZ-3C30NAHZ3_en.pdf

In the year leading up to the latest March bill—a little warmer than usual probably—we used 710TH, 220 of which was for hot water (reverse indirect), dryer and stove, so less than 500TH for heating.

That's not much usage for a 35kbtu/h heat loss. The thermostat is set to ~70? Any supplemental heating sources?
















D107

@Hot_water_fan you wrote:

Yeah, another offer spec-ed 6000 btu for basement unit. With our small house and narrow pathways around it that second outdoor unit would really be cumbersome. If its sized right, then would turn-downs be that necesssary?
The outdoor space is a significant constraint for your situation unfortunately. I have two mounted side by side in a narrow space, maybe the installer can get creative? If not, this is a better option. It can turn down twice as far. 

https://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006%5CM_SUBMITTAL_MXZ-3C30NAHZ3_en.pdf

Thanks for the link. Are you spec-ing this for the two indoor/one outdoor option? So the turndown is useful not only as a protection against oversizing but also to modulate when temps are only moderate? Two fan speeds on a handler should help too.

Anyway the size seems right. We keep the summer t-stat at 78 recently. In the past maybe 75 or 76 so maybe we really didn't accurately 'test' that 3-ton unit. Wouldn't 72º setting be a better test? All the more reason to do a good cooling calc I guess….?

Is the unit you're recommending likely to be more costly?

In the year leading up to the latest March bill—a little warmer than usual probably—we used 710TH, 220 of which was for hot water (reverse indirect), dryer and stove, so less than 500TH for heating.

That's not much usage for a 35kbtu/h heat loss. The thermostat is set to ~70? Any supplemental heating sources?

T-stat at 68 24/7 no setback. Peerless MI-03, Hydrostat 3200+, Turbomax 45 with TACO VR1816, and for heat circ we use VT2218 with 3 Taco ZVC Zone Sentry Zone Valves…Adey MagnaClean….No other heating except for stove, dryer, incidental….

Hot_water_fan

Thanks for the link. Are you spec-ing this for the two indoor/one outdoor option? So the turndown is useful not only as a protection against oversizing but also to modulate when temps are only moderate? Two fan speeds on a handler should help too. 

Anyway the size seems right. We keep the summer t-stat at 78 recently. In the past maybe 75 or 76 so maybe we really didn't accurately 'test' that 3-ton unit. Wouldn't 72º setting be a better test? All the more reason to do a good cooling calc I guess….?

Is the unit you're recommending likely to be more costly?

yes, a different multi-split option. The modulation is most important for mild temperatures. Multi-splits have bad turn down as a rule, so the modulation is useless if you're too far oversized. Since this one can turn down to 11kbtu, it'll run more efficiently. It's smaller, so probably less costly but probably not a meaningfully amount cheaper. If you keep it at 78 and you're comfortable, that's good enough. 3 tons of cooling for 1100 above ground sqft is absurd! Luckily, this can turn down to 1 ton.

T-stat at 68 24/7 no setback. Peerless MI-03, Hydrostat 3200+, Turbomax 45 with TACO VR1816, and for heat circ we use VT2218 with 3 Taco ZVC Zone Sentry Zone Valves…Adey MagnaClean….No other heating except for stove, dryer, incidental…

Small heat loss then, which is a good thing! It won't change anything, just that the heat pump I linked to is more than enough to heat the whole house.




D107

@Hot_water_fan: Some more info for you:

Upon careful measurement I found the current main AC enclosure measures 55"W x 46"D. The available area is actually 80"W.

For the replacement, don't think we'd need the wood enclosure anymore. The actual circular pad the compressor sits on is only 36"diameter.

Best of all, around that corner to the right––a space for the smaller basement compressor––measures 55"w x 35"D which really wouldn't block foot passage.

Also:

• Walls 4" dense pack cellulose and 3/4" styrofoam plus wall assembly, vinyl siding over clapboards, = R25
• Basement windows are 2012 Andersen; house windows vinyl double-paned installed 1987. Manufacturer estimated those windows were rated 4R, U34, SHCC 32.
• Attic Rafters 6" closed cell foam R42; Gable Walls 4" clopsed cell foam R 28 + wall assembly = R30

Blower Door Test Results after insulation: Infiltration Rate 2700CFM50; ACH50 = 7.
(Blower Door Test Before Insulation: 3700cfm50, ACH50 = 9.)
Even with all the insulation, still far from airtight but a big improvement (30%). The 2700 and 7 ratings are in the middle of the moderately tight house range.

The real bottom line I guess is the heating gas usage info I posted.