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Community and Q&A

Whole home HVAC system and hot water

chrisdrake105 | Posted in General Questions on

We just purchased a home in Connecticut.  The house is a 2 story, 2400 sq ft cape.  We are doing some pretty major renovations and will be addressing the insulation issues that the home currently has (though I don’t have specific R-values yet).  We are also replacing the heating and hot water systems.  Currently, the home has cast iron radiators on the first and second floors that we have heard give off a very comfortable heat.  We are also looking for a cooling solution with mini-splits because retrofitting ductwork seems extremely challenging with the way the home is set up.  One quote we got for a solution was an oil boiler / indirect hot water tank system for heating and hot water along with mini splits for a/c and heating in shoulder seasons when they are at their highest efficiencies. I’m curious if it seems like overkill with the mini-splits though as the quote was for a 22K btu condenser attached to two heads in the first floor (18K for living room and office and 9K for our breezeway/kitchen).  On the second floor which is very sectioned off they recommended another 22K btu condenser attached to 3 heads (9k) in each bedroom.  The radiators on the second floor may have to be ripped out as part of the renovations so we are thinking of heating the second floor just with the mini splits.  The whole project was quoted for around 25K.  A few questions:
1. Is there a big difference in the “type” of heat from cast-iron radiators and mini-splits?
2. Is this an oversized proposal? (nobody has performed a manual J on the home and the contractors haven’t offered)

We liked the idea of having back up options in case the price of oil or electricity went up relative to the other but also aren’t thrilled with burning oil.  The house is for 5 people so our hot water usage is quite high and we figured the indirect water hater was a relatively efficient and cost effective solution

Any thoughts on any of the questions are welcome.

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  1. Expert Member
    Akos | | #1

    Cast iron rads tend to be more comfortable in older leaky houses as they are usually place where cold air comes into the house. Since they tend to run at steady temperature, there isn't that temperature drop between thermostat calls.

    With newer well insulated and air sealed houses with correctly sized HVAC, the above benefit disappears. I always struggle with taking out hydronic heat, but without comfort benefit, it is hard to justify the cost of dual heating systems.

    If you are running on oil, unless your hydro is very expensive, mini splits tend to be significantly cheaper for heat.

    The min split setup proposed is oversized by at least a factor of 2x. Multi split with a head in each room is a very bad idea. Each head is about 3x to 4x oversized for the load of a typical bedroom, which creates comfort and efficiency issues. There has been numerous threads on here with people that have had issues with this.

    If you want to keep your rads for backup heat, a reasonable setup is a single centrally located wall/ceiling mount unit on the main floor and slim ducted unit for the bedrooms upstairs. You want each of these on their own outdoor unit, this tends to be about the same cost but significantly more efficient and much better modulation range compared to a two zone multi split.

    If you want to retire the hydronic system, install a ducted unit for the main floor as well.

    For sizing, get an HVAC engineer to run a proper heat load calculation on your house with post reno upgrades.

    Indirect hotware heater off oil is VERY expensive to run. Lowest operating cost option is a heat pump water heater in the basement. Even a standard resistance tank would probably have similar operating cost as the oil indirect especially off heating season.

    An option would be to keep the existing boiler and install the mini splits. See how your house runs through a couple of winters. If you find that you need the rads for comfort, then replace the boiler, if not, take it out and enjoy the extra wall space.

  2. chrisdrake105 | | #2

    Akos - thanks very much for the thorough response. Our boiler has actually already been removed. We had to have an asbestos team remove it because of the age and asbestos contained in the unit. Our home owners wouldn't insure us without a new system anyways so we are essentially starting from square 1. The manual J could be an issue because we dont have any heat or hot water currently in the home so we were planning to install the HVAC system along with the renovations to the home. I figured we would be able to save by combining our heating and cooling needs into one system but the quotes I'm getting don't back this up. Finding an HVAC contractor that will do a manual J (Estimate) is also an issue.

  3. bfw577 | | #3

    CT resident here as well. I heat and cool my entire house with 2 12k mini splits and use a heat pump water heater for hot water. It's all powered from my solar panels and my electric bill is $9.62 every month.

    CT has extremely generous incentives for solar, heat pump water heaters, heat pumps, insulation, etc. Have you looked at the CT Greenbank website?

    I have probably received close to $10k in incentives through energize CT. It's a energy efficiency fund that is paid for through a charge on everyone's gas and electric bills. Seems like its flush with cash.

    There is currently a 100% off insulation incentive for a limited time.

  4. Expert Member
    Dana Dorsett | | #4

    Is the oil boiler/radiator system steam, or is it hot water?

    How big is the upstairs, and with how many doored-off rooms?

    Is there a full basement? If yes, are the foundation walls, sill, & band joist air sealed and insulated?

    A typical 2x4 framed 2400' cape with fluff in the wall cavities and R20-R25 in the attic and clear glass storm windows will have a design heat load in the ~40-45KBTU/hr @ +8F (Hartford's 99% outside design temp- got a ZIP?) range if there is no foundation insulation, 30-35KBTU/hr if the foundation is well insulated and the house is pretty tight. That makes even the smallest oil burners sub-optimally oversized for the whole house load, and LUDICROUSLY oversized for just the first floor's load.

    ASHRAE recommends holding the line at 1.4x the 99% design load. Say the design load for the whole house comes in at 38,000 BTU/hr after renovations, and fully half that is the first floor (not likely, but maybe if the first floor is bigger than the second). The tiniest oil burners out there put out ~60,000 BTU/hr, more than 3x the load of the zone it's serving.

    A typical bedroom would have a design heating load of less than 3000 BTU/hr. A 9K head attached to a cold climate multi-split compressor puts out about 11,000 BTU/hr at your design temp, so it's really about 4x oversized for the likely loads, which is a grotsque oversize factor likely to lead to comfort problems, both summer & winter. Even a 6K head would be oversized for the typical bedroom. An insulated 1200' upper floor would likely be adequately served by a single 1 ton ducted mini-split, provided it has a vapor injection type compressor. Fujitsu's AOU/ARU12RLFC puts out 16,500 BTU/hr @ +5F, the AOU/ARU18RLFCD puts out nearly 20,000 BTU/hr @ +5F. Mitsubishi's SUZ-KA12NAHZ+ SEZ-KD12NA puts out 15,000 BTU/hr @ +5F, the 1.5 ton SUZ-KA18NAHZ + SEZ-KD18NA puts out over 21,000 BTU/hr @ +5F:!/product/25312!/product/32101!/product/31988!/product/31990

    There are others.

    If your house has the typical cape kneewall attic spaces and only about 900' of conditioned space upstairs you're probably looking at a 3/4 tonner, and would likely have reasonable spaces to install the cassette & run the ducts behind the kneewalls, making it well worthwhile to fully insulate those attic spaces at the roof rather than insulating the kneewalls & floors. This is a lot easier to do BEFORE you start doing the finish work on the renovations.

    Suffice to say, the 2 ton multi-split compressor is just as ludicrous for 3 cape-sized bedrooms as the 9K heads. The heads may be able to modulate down to the design load of a single room, but the compressor might not- they have limited turn-down ratios compared to most ducted single zone mini-splits, and the cycling eats into efficiency & comfort.

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