Mitsubishi HPs (SUZ vs PUZ H2i)

rosco

We have two 25ish year old AC units which are still working, but our excellent service company which does a yearly spring tune-up has cautioned us that we are on borrowed time. The existing AC and heat (via condensing boiler) feed a hydroair systems. Given current rebates available in MA, it’s making sense to replace them with heat pumps. We also have an oversized solar array and over-generate on a yearly basis by around 2ish kWh but also don’t have other things that may use that energy like an EV which is likely in the next 5 years.

The service company is also a quality installer of all forms of heating & AC and I trust them to do an excellent job. They have quoted two different Mitsubishi HP systems. Each has two new outdoor units and matching air handlers. The existing hydronic heating would be used as the very cold weather crossover/backup. The first one is based off the Mitsubishi SUZ-KA**NA2 series (not H2i) and the second uses the Mitsubishi PUZ-HA**N*** H2i Hyper-heating units.

I know the PUZ series can in theory operate at lower outdoor temperatures, but also uses a bunch of more “advanced” technologies to achieve this. I also know the electrical cost to achieve these lower outdoor temperatures will cost more than using the boiler at current gas pricing. It’s hard to know gas & electric pricing in the future and feel the PUZ would at least give me more flexibility.

What I don’t understand is whether the tradeoff going with the PUZ vs SUZ makes sense from the standpoint of system reliability and comfort.

1) Seems like the PUZ adds a bunch more complexity in its implementation (PAM, PWM, Wave Eco inverter) and in general there is virtue in more simple systems. In reality is there anything to be worried about with the PUZ for reliability?

2) The Mitsubishi consumer brochure also talks about how their inverter based systems as running longer with a closer indoor temperature regulation. They always highlight this when talking about their H2i based units. In reality, do the SUZ and PUZ really operate the same except for the lower outdoor temperature capability? Improving indoor comfort would be a tangible benefit.

3) Am I missing anything else?

Hot_water_fan

My question for the installer would be: why not the hyper heat SUZ? Supply chain issues? If you’re keeping the backup, the non-HH options are a touch more efficient at the warmer temps. I wouldn’t expect any reliability differences between the two given similar operating conditions. 

rosco

@Hot_water_fan:
The reason for not using the hyper heat SUZ is because the rebates in MA aren't qualified for those units. He said it absolutely doesn't make any sense, but given the generous rebates his hands are tied on this one. Our DHW production will be tied to the Vitodens in any case, so the boiler backup is pretty straight forward. There are 2 levels of rebate where the PUZ could qualify for $10K, but that may require going to an electric element backup which doesn't excite me. We have not infrequent power failures, and a generator to handle that would be massive vs still using the Vitodens in an emergency. The lower level rebate would still be $6250 and would have higher crossover temperature requirement (30°F vs extreme weather event ~0°F). He did say the PUZ is more in the small commercial space, but hopefully parts and service wouldn't be an issue given they are probably less popular.

https://masssave.com/saving/residential-rebates/heat-pump/heat-pump-qualified-list

Hot_water_fan

Oh these rebates are a disaster. So the HSPF is a fraction too low so it’s worth $6250 less? Ha! Just encourages manufacturers to meet the standard and not innovate. 

rosco

@Hot_water_fan:

The standard rebate is $1,250/ton. We are 5 ton = $6,250. The whole house rebate is a fixed $10,000 but has the requirement that you complete all the work in the Mass Save energy audit and you have a system sized as the sole source & old system removed unless homeowner commits to only use it in an emergency. So the difference is the PUZ system for whole house level of rebate would have an additional $3,750 over the SUZ. I assume if you wanted, you could have the PUZ installed while only claiming the $6,250. There is also a 7 year 0% loan available up to $25,000. Even with the warts, it's a pretty nice program.

Our home energy audit came back with some weird stuff like to place the same level of insulation in the attic floor as already exists and to put open blow cellulose where 9" of fiberglass is already in place. They also called for the installation of propavent & damming. I assume that goes in the between the roof rafters and where the rafters meet the floor area. It's still a cold space attic so it's not clear to me what benefit I'd get from those. Someday I'd like to insulate the attic, but the incentives won't cover that. I've tried to contact the person who did the audit to explain some of the items which don't make a lot of sense, but so far I've not heard back.

Hot_water_fan

It’s generous and I support the end, but that’s a poor structure. Paying by the ton leads to oversizing since costs aren’t linear, so there’s an incentive to oversize and the 100% coverage seems to encourage overpaying for reductions - for example a hybrid with 99% reduction should get 99% of the subsidy, but that’s but how it’s designed. Too bad about the SUZ HH, I have it and it’s great! It must be a utility’s dream: my winter and summer peak are both around 2.5kwh. Gentle on the grid.

GW

Yes, the kicker is there’s a math formula, cold weather heat output divided by mild weather heat output. I don’t have the structure memorized, because there’s a list (my brain is already full). It’s not just Seer and HSPF 

PUZ is their commercial series- it’s built more ruggedly. There has been some murmurings about this exact topic with hvac installers. Apparently there may be an adjustment that may allow some of these hyper duct systems in the not very distant future. 

There are some advantages to PUZ. From what I recall, from my training, they are a little easier to diagnose. I’ve only installed M series and a little bit of SUZ

Do you have a vitodens with natural gas? Yes, you’re gonna wanna hang onto that.

rosco

@Hot_water_fan
Your point around the incentive to oversize might happen with less honorable contractors. Luckily mine seems to play it very straight and 100% by the program rules. Having done a detailed Manual J in the past, what is being proposed seems reasonable.
@GW
The Vitodens is natural gas and is about 17 years old. It also gets yearly service and has been reliable with only one more major part needing replacement plus one igniter and a replacement of that neutralizing canister. Our attached Vitocell just does its job and has been problem free.

GW

I don’t think the unscrupulous contractors will last long in the masssave program. 

pecmsg

rosco said:

@Hot_water_fan
Your point around the incentive to oversize might happen with less honorable contractors. Luckily mine seems to play it very straight and 100% by the program rules. Having done a detailed Manual J in the past, what is being proposed seems reasonable.
@GW
The Vitodens is natural gas and is about 17 years old. It also gets yearly service and has been reliable with only one more major part needing replacement plus one igniter and a replacement of that neutralizing canister. Our attached Vitocell just does its job and has been problem free.

Can you post that Manual "J".

rosco

@pecmsg
Here is the Manual J.
Note: The basement isn't heated beyond heat given off by the Vitodens, air handler and any duct leakage.

Hot_water_fan

Manual J seems pretty fat on the cooling side: 100 degrees for design temp in MA? That’s higher than the design temp for New Orleans. What is the output of the existing AC units? 

On the heating side: How much gas did you use last year? This is implying about 2400 therms if you’re in western MA. 

rosco

@Hot_water_fan
I'm pretty sure the 1st Floor AC is 3 tons and 2nd Floor AC is 2 tons.
We do have a much higher than average number of mostly double hung windows with the lower sash using IGU and the upper sash have an energy panel. The windows haven't gotten tighter in the 29 years we've lived here.
Here is a spreadsheet of our energy use this past heating season (Framingham is the closest city):

I can say that on those crazy cold -10F days we get on occasion, the heating is running almost non-stop. Typically we haven't run the AC in the summer all the time unless it's a heat wave. When it does run, it does a very nice job of removing the humidity and certainly doesn't bring down the temperature all that fast, but is far from running non-stop.

Hot_water_fan

@rosco This is usually how it goes! The data matches your own experience well, but the Manual J may as well be done for another house. The boiler is cycling on -10 degree days! Based on their manual J, your heat loss should be about 75,000*1.15 = 86.5kbtu at -10, which exceeds the capacity of the some of the Vitodens (not sure which size you have).

Reality says otherwise: in 8 months you used 1296 therms over 5466 Heating degree days. Subtracting 15 therms per month for domestic hot water gets you 1176 therms. Say 85% of those make it into the house, that puts you at (1176 therms*.85*100,000 btus/therm)/5466 HDD =18,287 BTUs/HDD, or a heat loss of about 50,000 btu/hr at 0 degrees (18287 Btus/HDD * 65 HDD/24hrs).

The point of the heat loss exercise is to make sure you have sufficient, but not grossly oversized equipment as it'll be more comfortable and quieter to have equipment fit the load. Since you're not lacking cooling capacity now, I can't possibly see how 5-tons of capacity makes sense, especially since their own cooling manual J (using 100 degrees as a design temp!) doesn't reach 4 tons. I wouldn't be surprising if 4 tons A. exceeds all cooling needs and B. meets 90+% of heating needs (if Hyper heat).

rosco

@Hot_water_fan
Maybe it's the same difference, but on those rare -10°F nights, the AH might cycle off, but not for long. My perception is from lying in bed and noticing that after a very long period, it would cycle off, but then within a couple minutes go back on.

The Vitodens 200 is model WB2 8-32. No doubt this was also oversized, but I'm not sure how much smaller they went at the time.

Could the 3 ton unit for the 1st floor just come from looking at the Manual J number of ~42,000 BTU/H. I looked at the Mitsubishi M & P catalog on their website which seems to be a mix of both marketing and technical information. The 3 ton air handler + hyper heating PUZ says it has a heating capacity of 38,000 BTU/H at 5°F. The PUZ HH option was proposed for a system that could do the whole job without going to any auxiliary. Seems like the HH capability is liked by folks here. Would it be wise to have them spec a smaller capacity HH, but in the lower rebate class where you're expected to use aux heat when it gets real cold.

For cooling, it does seem like smaller equipment would be better.

pecmsg

How is the total heat gain 1st floor 25,000- and the second floor 17,000? The 1st floor has No ROOF!
Basement heat # also look high considering its surrounded by 50° earth.
As stated, 100° summer is extremely high. 85 - 90 is more realistic.
Grains of moisture 89. No Way.
You don't size for the extreme, you size for traditional high and lows.

pecmsg

rosco said:

@Hot_water_fan:

The standard rebate is $1,250/ton. We are 5 ton = $6,250. The whole house rebate is a fixed $10,000 but has the requirement that you complete all the work in the Mass Save energy audit and you have a system sized as the sole source & old system removed unless homeowner commits to only use it in an emergency. So the difference is the PUZ system for whole house level of rebate would have an additional $3,750 over the SUZ. I assume if you wanted, you could have the PUZ installed while only claiming the $6,250. There is also a 7 year 0% loan available up to $25,000. Even with the warts, it's a pretty nice program.

Our home energy audit came back with some weird stuff like to place the same level of insulation in the attic floor as already exists and to put open blow cellulose where 9" of fiberglass is already in place. They also called for the installation of propavent & damming. I assume that goes in the between the roof rafters and where the rafters meet the floor area. It's still a cold space attic so it's not clear to me what benefit I'd get from those. Someday I'd like to insulate the attic, but the incentives won't cover that. I've tried to contact the person who did the audit to explain some of the items which don't make a lot of sense, but so far I've not heard back.

How does a questionable Manual "J" show 45,000 BTU/h cooling and your at 60,000 BTU/h? Be very very careful sizing for the heat load with cooling load equipment. At the very least 3 or more units so the cooling load is more in line!

Also be advised Heat pumps put out cool heat at low Ambiant's. Still not a replacement for Steam or HW.

Hot_water_fan

Also be advised Heat pumps put out cool heat at low Ambiant's. Still not a replacement for Steam or HW.

To be fair, hyper heats put out pretty high temperature heat all the time and can be supplemented during defrost if desired. And a hydro air handler can put low temperature air. Depends on installation quality!

Steam has no bearing here, as the OP has a hot water boiler.

pecmsg

Hot_water_fan said:

Also be advised Heat pumps put out cool heat at low Ambiant's. Still not a replacement for Steam or HW.

To be fair, hyper heats put out pretty high temperature heat all the time and can be supplemented during defrost if desired. And a hydro air handler can put low temperature air. Depends on installation quality!

Steam has no bearing here, as the OP has a hot water boiler.

Yes, Hyper Heat can supply heat to low temperatures but as that outside temp drops so does the heat output. Add to that the frequency of defrosts where your actually cooling I'm still not convinced the technology is there to replace steam or in the OP case Hot Water in northern climates!

Hot_water_fan

For ducted systems, replacement with a heat pump is very low drama. No need to fear defrost cycles! In fact, they're less common as temperatures get colder.