Hard to give answers without knowing more about the building. Where is it located? How many square feet is the building? Amount of insulation...etc.

You note that an engineer has designed the AC/heat system for you. If the engineer did a thorough study of your needs I'd probably go with that recommendation.

If the space is wide open I wouldn't worry about zoning. If there's a number of rooms I'd be more concerned about zoning for the single compressor or possibly using two compressors. A three compressor system based on either of the btu's recommended doesn't make any sense to me unless there's something unusual about the configuration.

This is a residence or commercial?

What size and tell us about the design/layout of structure.

What fuel sources are available other than electric?

Insulation properties?

Your location?

Post back.

IMO

This is a residential, 3 family house, located in Brooklyn.
We are doing a gut renovation, so we will have spray foam insulation.
There is a basement, and 2 floors above that: top floor has 2 1-bedroom apartments, first floor has 3 bedrooms and is connected to the basement. Each floor has about 1,300 square feet.

No other heating system. We are removing an old hot water heating system, and replacing it with this mini split system.

The engineer designed 4 separate compressors, 36,000 btu each for the 1-bedroom apartments, 48,000 BTU for the first floor (with 3 bedrooms, kitchen, living room), and 36,000 BTU for the basement.

I've talked to 3 separate contractors and no one wants to do this. They eacg said that this is too expensive, and that these units are not for residential, and that I should go with smaller ones.

But I'm now terrified that it will be all wrong.

Who pays the HVAC bills?

I don't believe you are being sized correctly.

The mini splits will be heat pump models?

Why mini splits and not split systems?

IMO

Is this an attached brownstone house? If it is I find it hard to believe the cooling load requirements especially if you are adding foam insulation.

Is your "engineer" an HVAC contractor, because if he is you need to find one who knows what he is doing. You need a heating and cooling load calculation based on the renovations you are making. The proposed AC sizes don't make sense.

So 2 of you said the sizes don't make sense. Do you mean they are too big?

This is a stand alone brick building, not attached. So it will be losing both heat and cold from all sides.

And yes, the mini splits will be heat pump models, they will be used for both cooling and heating.

The engineer was just that - an HVAC engineer.

But in general, do bigger compressors work better than smaller compressors? As in - when it's 0F outside, will the 48,000 BTU still be working while the 24,000 BTU stops?

Let's step back a minute since you are making some major changes.

I grew up in Brooklyn so I am familiar with the construction and old heating plants. It sounds like you are doing a major and expensive renovation so you want to do this carefully and not have any regrets.

The original heating plant was hot water radiators. Was the boiler oil or gas fired? It breaks my heart that you took out the old radiators. I think should have kept them, but that is my opinion.

The AC sizes seem too big for the size of building you described. If you are installing mini splits then they have to be sized for cooling and not heating. Oversized AC condensers will not be able to lower the humidity. They will short cycle and run inefficiently. You will feel cold and clammy. You are correct that a smaller heat pump will put out less heat at zero degrees than a larger size. The fix for that is to have a back heating system to supplement the heat pump. Many houses use electric heat strips for this purpose. But at the rates Con Edison charges this becomes very expensive. In Brooklyn it is much cheaper to heat houses with natural gas than heat pumps and electricity. This is another reason to keep the old radiators.

Your HVAC engineer should have done a Manual J calculation. This is a heating and cooling load estimation. It is done with a software program. Ask him to show you a copy of the results and explain it to you. If he can't produce this then he is guessing and you need to hire someone else.

"when it's 0F outside, will the 48,000 BTU still be working while the 24,000 BTU stops?"

No, but heat pumps have lower and lower output as the outdoor temp drops. It is just an inherent characteristic.

Is the basement subterranean or above-ground? What kind of mini-split system is proposed, high wall units, recessed ceiling units, small ducted air handlers,...?

It all seems to much for cooling and so much that it will not function well. I don't know how much heating you will need with heat pumps in that climate, but my question is, why not heat with gas? With gas prices being what they are, I don't think it makes sense to heat with heat pumps where you are. Note that I am not an HVAC pro and I have not lived in the NE costal plain for some time. Honestly, I'd consider keeping the hydronic system with a new boiler if it fits your needs otherwise (decor and whatnot).

OK. Now that we know your looking at mini splits oversizing is much less of an issue to be concerned about. Inverter model mini splits can reduce their output to approximately 1/3rd of the rated output if full design capacity is not needed...so the issue of short cycling and excessive humidity goes away.

During the winter the full capacity would be available to handle heating needs (assuming the unit was sized correctly).

Even 1/3 of what is proposed seems like way too much cooling.

Thank you so much for all your answers. This was really really helpful!

This is really bothering me. Install an oversized heat pump system to get sufficient heating capacity when natural gas is readily available? That will cost in equipment upfront and in energy costs later.

I think what they have spec'd is the commercial City-Multi type product from Mitsubishi. Daikin, Fujitsu and tohers make these systems also and they are very popular in the cities. These systems can heat and cool at the same time. In commercial that is important as one side of a building may require heat while the other needs cooling. I would suggest that you have your engineer look at Fujitsu's Hybrid Flex Inverter system. It can either cool or heat, but not do both at the same time. They are a 48kbtu condenser and can accomodate up to 8 indoor evaporators. You can connect anywhere from 36k-64kbtu worth of indoor units. There is only 48 k worth of output, but usually not every evaporator is on at the same time. I prefer, and have done this in my own home, to have a multi minisplit hp for the bedrooms and go with the highest efficiency units in the primary living areas. For instalnce my upstairs multi will operate at 16.5 seer and 8.8 hspf. My downstairs unit will run at 25 seer and 12 hspf. I run the downstairs for heat and cool and run it frequently whereas the upstairs units are operated only infrequently as needed. Good luck with your new home. Simply eliminating duct work significantly increase system efficiencies

In my mind, the trouble with putting a lot of indoor units on one big outdoor one is that you lose the advantages of the variable speed compressor. If they can throttle to 30%, and you are running one 9000 BTU indoor unit, once you get over a 27,000 "compressor" size, you have no benefit. You are running at 9,000 or nothing. In contrast, f you have a 1:1 system, you can go to 3,000 BTU. Obviously, if all of your indoor units are running closer to flat-out, there will not be an issue. In a residential situation, you might be running one unit out of eight at night (low load time) so you want to be able to cut back the output.

Eliminating ducts improves efficiency by a great deal if the ducts are outside of the building envelope. if the ducts are inside, the benefits are lower.

Ionized, I think that while there is certainly a difference in the exterior or interior duct systems, the HVAC industry has absolutely swept duct performance under the rug. In my view, the unitary manuf (Carrier, Lennox, Trane, goodman, etc) can put all the technology they want into their products. Their problem is that they cannot deliver it. This is due to the absolutely terrible performance of probably 99% of all residential duct systems. Take a 90% efficient gas furnace and put motion a 60% efficient duct system and what do you get? Google "DOE duct leakage" and read over the first 4 or 5 posts. Amazing...and sad!

I know that great number of residential duct systems leak a great deal. What do you mean by "terrible performance", and "60% efficient"? Leakage alone? If that is the case, and a system is not zoned, what's the big deal if the leaks are to the living space?

I know that great number of residential duct systems leak a great deal. What do you mean by "terrible performance", and "60% efficient"? Leakage alone? If that is the case, and a system is not zoned, what's the big deal if the leaks are to the living space?

What happens in leaking duct systems is that you can find zone of differential pressures that can significantly increase infiltration. As well, the "it's leaking into the house" is the rationalization for it not being right. If a system is designed to deliver air to a specific place why should a paying customer get less than that?

We are one of the few nations that heats and cools by blowing air through tubes. We should at least do it well. After a lifetime in the industry I have, as you can tell from my posts;), gone over to the "net to the space" idea. I heat with mshp's and Rinnai Energysavers. No transmission losses and I only heat the areas I'm in. Very comfortable and economical.

Also, mshp's are very close to the efficiencies of water source heat pumps without the enormous costs.

What, you don't want to have a continuous blower test being run on every room? Seriously, I forgot about the pressure differentials. They are a serious problem when humid air is being sucked in and hits a cool, organic surface. Fungus happens.

This post makes me wonder if HPMS make much sense in cold climates, given the low cost of natural gas, and the relatively high cost of HPMS equipment, coupled to the sizing of the equipment necessary to heat compared to cool. Lots of homes where I come from have nice, hydronic heat and no AC. Keeping the hydronic and retrofitting HPMS for cooling makes a great deal of sense in that situation.

This post was edited by ionized on Fri, Jul 19, 13 at 10:28

I understand your question on the heat side in northern climates. I represented Fujitsu for 12 years and covered the 6 New England States. Serious heating territory. I am no longer affiliated with any mshp manuf. What is actually happening in that territory is that people are buying mshp's for the heat. I told my contractors and consumers to "buy it for the heat. I'll give you the cooling for free". The heat savings are where the return on investment comes in. I have a friend who has a big old farm house overlooking Lake Champlain just south of Burlington, VT. He has oil fired radiant and a quad Fujitsu. With the specs on his system, he runs the mshp down to 15-20* and olny fires up the oil below that. As the vast majority of heat hours, even in VT, is above 20 he is running the mshp probalby 80% of the winter. He has over the past 5 years reduced his oil consumption by 70%. I don't know his dollar ot dollar savings but he says they are significant. He paid for the system in 3 years according to him.

As to whether the single unit operation on a multi is efficient, I would say that while there may be a small penalty, it is unlikely that a unit would be run in that configuration for extended periods. If it is, then it is mis application. You have to understand the way people are living as a part of equipment selection. For instance, I put in a 24 dual for my upstairs bedrooms. I run the one bedroom this time of the year (100* today), but didin't turn the heat side on all winter. The 25 SEER/12HSPF unit downstairs runs every real hot afternoon in the summer and every day in the winter. That is the best combination for comfort and efficiency for my home and use.

When I talk about the duct tests, it is going to become a part of the codes. It is in CA all ready. to replace a furnace or heat pump you have to have a third party come in and test the duct. New construction must achieve no more than 6% leakage. Retrofits must achieve 16%. These duct repairs are costly to do. I spoke with the local fellow who does these tests and virtually every house requires significant repair. The unitary manuf can put all the technology the want into their boxes. The problem is they cannot deliver it as the duct systems stink. Consider the national energy impact of that. It is a big issue that we have been sweeping under the rug for a long time.

I don't doubt the numbers on duct leakage.

Beating fuel oil cost as a source of heat is a low hurdle. Beating nat gas is much higher. Natural gas heating equipment is less costly than heat pumps so initial installation costs are lower.

Jackfre, what are the heating vs. cooling loads for homes in that area? What I wonder is if it is possible to "right size" heat pumps for both seasons in that region (especially away from a large lake that will moderate winter temps).