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Two Ducted Systems vs. One Whole House System

urbanpanda | Posted in General Questions on

Hi,

In the market for a new heat pump to replace my gas furnace. I have narrowed down to two options and would like to run them by you experts here for your valuable opinions.

I’m in Southern California inland (zone 3). My house (1250 Sq Ft on paper and 1150 based on my own measurement) is facing North. There are plenty of trees on the east and south sides, so the house (esp. the bedrooms, both on the east side) in general is/are very shaded. It is an 1950s house with possbile R11 or no insulation in the walls. There is a crawl space. However, I don’t plan to add additional insulaton to the walls but I will do attic and maybe the crawl space if it is not expensive. I may also seal the house and upgrade the patio door to double-pane (windows are already double-pane). There is also an attic fan.

The layout is like this – the living room, dinning room and kitchen are in one open area. There is a hallway connecting the living room to the office and the bedrooms at the end of the hallway.

I typically work/hang out during the day in the living room, dinning room, kitchen and the office, collectively, the day zone. Bedrooms is the night zone. Bedroom doors are always closed.

I envision the usage to be as follows:

Summer Day
===========
Outdoor Temp: average 80s to low 90s (95 = designed temp)
Day Zone Temp: 74
Night Zone Temp: 74 or fans only if the room is not used

Summer Night
===========
Outdoor Temp: average 70s
Day Zone Temp: Fans only
Night Zone Temp: 68

Winter Day
===========
Outdoor Temp: average high 50s to 60s
Day Zone Temp: 70
Night Zone Temp: 70 or Fans only if not used

Winter Night
===========
Outdoor Temp: 40s to 50s (35 = designed temp)
Day Zone Temp: Fans only
Night Zone Temp: 68-69

My objective is to lower the usage cost and maximize comfort. I don’t think it makes sense to run a whole house system when I use only one area in the house. For example, I require 68 in my bedroom when I sleep, which doesn’t mean the living room needs to be 68 or be on at all. It’d seem like a waste. Also with a whole house system, I imagine it would be hard to get the bedroom temp right, without a lot of efforts.

The first option is to use two Daikin FDMQ ducted heat pumps, one for each zone. This is in my mind the ideal solution because I can have zoning (i’ve ruled out the multi-zone system), redundancy, efficiency and comfort. However, the downside is it is more expensive in terms of the upfront cost and maintenance.

The second option is a whole house system such as Daikin Fit. The salesman told me that the system is extremely efficient so I don’t have to worry about getting big electric bills as I fear I may. I’m not sure how true that is. Perhaps, I can ask him to zone off the bedrooms. But then I am not sure how zoning off a whole house system works and whether it will work effectively or efficiently. I don’t like systems that are complicated and hard to repair. Most importantly, the bedrooms need to be comfortable.

I welcome feedback to both options and your opinion on which one makes sense. Was I wrong in assuming that it will cost me less to run two systems in the manner I described above, compared to running a whole-house system continuously to cover all my needs?

Thank you!

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Replies

  1. Paul Wiedefeld | | #1

    A house like the one you’ve described in that climate will use very little energy to heat and cool, so two systems probably won’t save any significant amount of energy, I’d suspect nowhere close to cover the increased cost. I wouldn’t bother zoning a single system either. If you want to further decrease operating costs, I’d add solar if you can.

  2. PBP1 | | #2

    From another post on heat pumps, https://www.greenbuildingadvisor.com/article/multi-zone-heat-pump-issue

    Imagine max load for heating in your SoCal house would not exceed 15k or so?

  3. urbanpanda | | #3

    Paul, thank you. That is sort of what I was thinking at this point. I felt that 2 systems may be a bit of an overkill for a small house such as this. There is also a possibility I'll get solar if/when they get cheaper as California is pushing really hard on the green initiatives.

    PBP1, as stated in my post, I ruled out the multi zone. I was talking about two ducted units - one for the bedrooms and one for the living space. The bedrooms can share a 9K compressor which has a minumum of 3,900 BTU heating output. The load test indicated that the bedrooms together would call for 9K at the design temp (35 -> 70 degrees) and above 3,900 BTU at 40s through mid 50s. The other unit for the living space has a minumum output of about 5K and the load test indicated usage of 15K at the design temp and around 5K at the average temp (61 degree). There isn't any unit with a lower minumum.

    1. Paul Wiedefeld | | #4

      Who did the load calc? That seems quite high for a 35 degree design temp. It might be better to size for the cooling load if that’s lower and use resistance to supplement at 35.

    2. PBP1 | | #7

      Understood, I have a three zone ASHP (heating 28k) that is not oversized for 2100 sq ft in MT climate zone 5b - no complaints (my office is in SoCal) and I didn't want another box on the exterior wall, only a single ASHP. If I read correctly, your heat load is 9k + 15k, which is 24k, being only 4k less than my house. That seems a bit odd. In the thread below, whole house (single ASHP) sounds like it may be a good fit sized according to cooling load.

  4. urbanpanda | | #5

    9K was both rooms together. An HVAC contractor did the load calc. I made him, actually. But I think I'm going to pay someone else to do it for me just to confirm his calculation. I did forget to say that one of the bedrooms has a bathroom. We included that because the door is never closed. Without it, it would be around 8K. Still high. I know. I do have a crawl space that is not insulated. I think that changed the demand a bit.

    So you're right. The cooling demand is lower. At the design temp of 95, the bedrooms is around 7K and the house as a whole (including the bedrooms) is looking at 24K = 2 tons based on cooling (2.5 tons based on heating). So every contractor who walked in my house put 3 tons in their quotes. I asked one of them why, as I thought 2.5 was already oversized. He said that I have an old house and he wanted to ensure that I have enough cooling capacity when we get heat waves. We do have heat waves. The highest recorded temp in my area is 117. It is not uncommon to hit 90s during the summer.

    Anyhow, the lowest capacity I could find in a ducted unit is 9K (to be shared by 2 bedrooms) so not much I can do there even if I need less. In my mind, that beats having a 7K wall mount in each room.

    After thinking and talking to you guys, I'm now leaning more toward the whole house system. I'm reviewing Daikin Fit and am having a hard time finding the mininum cooling and heating capacity. Any idea where I can locate that information?

    Thanks again.

    1. Paul Wiedefeld | | #6

      Northeast Energy Efficiency Partnerships has an online database of heat pumps with min/max outputs. The 2 and 2.5 ton Daikin Fits look like they can turn down to 6k btu heating and 7.5k btu cooling, which is great for a whole house. What size is your existing AC?

  5. urbanpanda | | #8

    The existing AC is 3 ton but it is old (from 1996) and inefficient so it is not a good comparison. Just curious, what is the turn down for a 3 ton Daikin Fits? I was not able to find the database on their website. Do I have to make a request? Thanks.

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