Questions re: Hyper Heat ducted heat pump system for large leaky house

ac21

Our 18 year old natural gas furnace died and so we are debating whether to convert to an all electric ducted heat pump system, stick with natural gas, or go hybrid.

The weather here averages a low of 36°F to a high of 48°F in December/January, and a low of 54°F and a high of 86°F in July/August. There have been the occasional freak 100°F days in the summer and sub 20°F days in winter. The house is a 1960's, two story, walkout basement, 3,400sf, with tall ceilings, lots of windows, and insufficient insulation and air sealing in the walls (although the roof has rigid foam insulation). With the walkout basement and shade from tall trees we rarely use AC so this system would be used almost entirely for heating. Prices for electricity and natural gas here seem to be below the national average.

We were recommended to go with a Mitsubishi P-series Hyper Heat ducted heat pump system and am wondering if it is a good choice for our situation. We may move to a smaller home in two to three years so from a cost standpoint it doesn’t seem to make sense, but we do want to do our part to help with climate change.

The specs for the system are below (full spec sheet attached).

* Does the EER of 10 and COP at 47°F of 3.7 seem too low?

* Is there an easy way to find out the COP at 30°F and 40°F?

* Does the system have enough flexibility in adjusting the motor speed to deal with any static pressure issues that arise? A HVAC installer cited this as a drawback and said we should consider an alternative heat pump that would adjust automatically.

Specs

Indoor model: PVA-A42AA7
Outdoor model: PUZ-HA42NKA1

AHRI Certified Reference Number : 206223059
Cooling Capacity (A2) - Single or High Stage (95F), btuh : 42000
SEER : 15.40
EER (A2) - Single or High Stage (95F) : 10.60
Heating Capacity (H12) - Single or High Stage (47F) : 48000
HSPF (Region IV) : 10.00

COP at 47°F: 3.7
COP at 17°F at Maximum Capacity: 2.1
COP at 5°F at Maximum Capacity: 1.9
COP at -13°F at Maximum Capacity: 1.6

Jamie Hall

Those specs are fine -- certainly not too low. From what I've heard it's a good system. In your mild climate it should work just fine. Now whether you want to go for the extra capital cost is your call...

And yes, your energy costs are well below the national average.

Hot_water_fan

Full specs found here: https://mylinkdrive.com/USA/P_Series/R410A_Systems-4/Indoor_Equipment-10/Multi_Position_Air_Handler-2/PVA_A42AA7?product&categoryName=Multi_Position_Air_Handler-2

the engineering manual has all COPs. It’s a good fit. I have the ducted M-series version and recommend it. 

How much gas did you use last year? 

ac21

Thanks so much for commenting guys!

In response to the question of how much natural gas we used in 2021, we used a total of 1,135 therms, and spent a total of $1,355. We used natural gas for a 100k BTU/4 ton 93% AFUE furnace, a cooktop, and a water heater.

In terms of electricity, over the past 12 months we have used 12,240 kWh and spent a total of $1,286. (We have one plug-in hybrid car.)

Hot_water_fan

@ac21 42,000 btu looks like it’s on the larger size for that gas usage, you could probably use the 36kbtu unit. 

ac21

@Hot_water_fan Thank you very much for the advice!

We had heard that the biggest mistake that people make in selecting heat pumps is overestimating what a house needs. But the HVAC rep who recommended this system at first seemed iffy on whether the 42k btu version would be sufficient - he was talking about the possibility of adding on heat strips later if need be. He has been doing this for many years and did enter a lot of info into a computer program which spit out some numbers but he didn't tour the entire house or ask about our gas usage. Is there anything else we can point to to make a case for the 36k btu unit?

Should we try to hire someone to do the full Manual J? Although we are short on time due to the dead furnace, and it seems like if we hired 5 people to do a full evaluation we'd get 5 different answers.

TAG

Hot_water_fan said:

Full specs found here: https://mylinkdrive.com/USA/P_Series/R410A_Systems-4/Indoor_Equipment-10/Multi_Position_Air_Handler-2/PVA_A42AA7?product&categoryName=Multi_Position_Air_Handler-2

the engineering manual has all COPs. It’s a good fit. I have the ducted M-series version and recommend it. 

How much gas did you use last year? 

When you say "M" are you speaking of the minisplit ducted (low or high static) or the USA style air handler like the OP?

I'm not overly impressed w/ the air handler. We have a family member with one and I was just in a house that my builder finished with two 2T's linked to a single outdoor.

The multi stage or full variable US systems work better IMO .... I have used the Carrier 5 speed.

Mitsubishi's mini split head units are the best and they have made improvement to the ceiling units and I hear the newest floor unit has the same "brains" -- the ducted units just don't have all the brains of the others.

Hot_water_fan

pumps is overestimating what a house needs. But the HVAC rep who recommended this system at first seemed iffy on whether the 42k btu version would be sufficient - he was talking about the possibility of adding on heat strips later if need be. He has been doing this for many years and did enter a lot of info into a computer program which spit out some numbers but he didn't tour the entire house or ask about our gas usage.

Contractors struggle with heat losses. The programs are time consuming but aren’t very accurate and the inputs are fudged a lot, especially the air infiltration which has a huge impact on the final result! 

This method doesn’t rely on faulty measurements (for infiltration, sounds like they did zero measurements) or “experience” - it looks at how the house has historically performed. Nothing more, nothing less. https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

36,000 plus the option to include some heat strips to start or later on is a conservatively sized option. A manual J isn’t necessary. 

Hot_water_fan

When you say "M" are you speaking of the minisplit ducted (low or high static) or the USA style air handler like the OP?

The SUZ air handler - the US style one. Since the climate isn’t that cold, a carrier variable speed would do great too! 

ac21

@Hot_water_fan I love the article and method, thanks so much!

If our furnace is two stage (65k 1st stage/100k 2nd stage), do I use the highest stage for the calculations?

Using the highest stage, I get an implied heat load of 37,056 BTU/hr at 65°F, and 27,778 at 60°F.
With the ASHRAE sizing factor 1.4, I get 51,879 BTU/hr at 65°F, and 38,889 BTU/hr at 60°F.

Assuming I did the calculations correctly, would you still recommend the 36k P-series if we go the all electric heat pump route?

And if we went the hybrid/dual-fuel route, would this mean we should try to talk the contractor down to a 60k BTU/hr furnace paired with a 3 ton heat pump?

Here are the numbers I used:

Furnace:93% AFUE, 100k BTU/hr, variable speed, two-stage
Input BTU/h rating: 100,000 (2nd stage)
Output BTU/h rating: 93,000
99% outdoor design temp: 22
Therms used 12/14 - 1/18: 202.6
Sum daily base 65°F: 911
Sum daily base 60°F: 731

Hot_water_fan

99% outdoor design temp: 22
this was colder than I was expecting 

Assuming I did the calculations correctly, would you still recommend the 36k P-series if we go the all electric heat pump route?

I think so - the 36kbtu outputs 38kbtu at 17 degrees. Worst case scenario, the smallest Mitsubishi resistance heat strips is 3kw, which puts you at 48kbtu. 

Or if you’re getting the furnace anyway, 60kbtu is fine with the 36kbtu heat pump. It might never run! 

Does the 202 therms include DHW? 

ac21

@Hot_water_fan The 202 therms includes water heating and a cooktop.

For the calculations was it correct to use the higher of the two furnace stages (100k)? (It has 65k first stage input BTUH and 100k second stage input BTUH.)

Is there a 15-minute way to calculate cooling load when sizing replacement equipment?

GW

Even the best manual J users need to guess at this and that. Everyone guesses at infiltration (unless they own a blower door) and who really scans all the walls with an IR can to see (not see) the insulation? Just go with the guy that makes the most sense. 

Hot_water_fan

For the calculations was it correct to use the higher of the two furnace stages (100k)? (It has 65k first stage input BTUH and 100k second stage input BTUH.)

If you know the efficiency, then this step is unnecessary. The DHW probably knocks off 10-20 therms, so 5-10%. 

The 202 therms includes water heating and a cooktop. 

ac21

@Hot_water_fan D'oh! I see now re: the furnace stage. Thanks! Also, I found an error in my calculation at 60°F.

I get an implied heat load of 37,056 BTU/hr at 65°F, and 40,811 at 60°F.

With the ASHRAE sizing factor 1.4, I get 51,879 BTU/hr at 65°F, and 57,135 BTU/hr at 60°F.

Does this revision affect your conclusion that a 36k BTU heat pump would be sufficient?

Is there a good method as user-friendly as this to calculate cooling load?

Hot_water_fan

Does this revision affect your conclusion that a 36k BTU heat pump would be sufficient?

Nope, it's a great fit - you'll be subtracting 5%-10% for DHW and it's extremely easy to add resistance backup or even gas backup if you want.

Is there a good method as user-friendly as this to calculate cooling load?

Not that I'm aware of - electricity use is spread across more appliances and cooling load changes significantly with solar gains, which seemingly wouldn't be captured well with CDD. Cooling efficiency also changes as the outdoor temp changes so it's less linear than a furnace/boiler/resistance heater. What the fuel usage method is really doing is tracking runtime - which can be done manually, but the meter does it automatically. So an equivalent method would be to track the percentage of time that the AC ran on the hottest afternoon of the year.

ac21

@Hot_water_fan Thank you again! I have another quote for an American Standard Platinum 19 low profile heat pump system that would be about the same price as the Mitsubishi P-series Hyper Heat system.

The model # is 4A6L9036A1000A with Air Handler #TEM8A0C36V31DB

Is there a clear winner between these two options? One advantage of the American Standard is the warranty is transferrable.

Hot_water_fan

Is there a clear winner between these two options? One advantage of the American Standard is the warranty is transferrable.

The main difference is the American standard isn't a cold climate model so would require backup heat of some kind. If you're including that anyway, it won't be an additional cost but it could cost more to run on the coldest days. Not the end of the world, the coldest weather is a small percentage of the year.

ac21

@Hot_water_fan Okay, so with the 36k American Standard we would need to install heat strips from the start (and not wait and see)?

Hot_water_fan

Okay, so with the 36k American Standard we would need to install heat strips from the start (and not wait and see)?

For sure. You might find it easier to install them initially with both, but you’ll definitely need them with the AS. 

ac21

@Hot_water_fan Thank you again for all your help!

I am now trying to understand static pressure and air flow as one contractor said there may not be enough flexibility in the P-Series Hyper Heat system to adjust the motor speed to deal with any static pressure issues that arise.

The 36k Hyper Heat system has "selectable external static pressure: 0.30, 0.50 and 0.80 in.WG with 3 fan speeds at each static setting." The indoor unit airflow rate is 788-956-1125 CFM and the outdoor unit is 3530 CFM. I am assuming you feel these figures are more than sufficient?

pecmsg

Static pressure is determined but the duct size.
If the equipment requires X cfm's and the ducts can't handle it time to rework the ducts!

GW

static pressure- loosely compare it to blood pressure.

The systems are design to overcome "x" static pressure as you aptly stated and quoted.

People that build duct system have calculators/programs that indicate sizes. That may or may not be exactly what happens during installation.

So at the end of the day: the duct design was 'theoretical" and the actual duct static pressure is 'real life". So, design world meets real world.

Seems like the Mits your quoting is a true Variable Speed ECM motor. Most motors these days are basic ECM motors (or worse- basic PSC). ECM motors--we needs a chart- the more static pressure, the less CFM. The Variable speed motors are built to put their dukes up.

TAG

I have been most happy with single head to condenser mini splits. And of those I would say the Mitsubishi wall units with the eye have the edge. Having used floor, ducted and ceiling cassette in the Mitsubishi line I don't find they have the same control as the wall heads. It seems the newest Ceiling units with the eye are better. Same with the USA style air handler ... they don't seem to have the same "brains" as the wall units. I was playing with one last week .... two air handlers and one condenser. It must be the controls -- but they don't work as well as the variable speed Carrier system with the Infinity thermostats

I also have two 3 head system that all tie back to a single condenser -- these systems work best when all the heads are "on". In my new house I went with three system vs doing a 3 head multi.