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Baseboard heat and minisplit heat

cbut8995 | Posted in General Questions on

This maybe a stupid question but is the amount the baseboard heat output in BTU/ft the same as the Heat in the Minisplit when sizing it. 

I have been told our small apt needs 6k BTU in the living room from the baseboard heat which requires about 10 feet at 180 degrees F. When I search for the minisplit sizing guides I am getting a lower number around 4K BTU heat needed for a minisplit and the smallest minisplit I was planning to get is a single zone 6K unit from mitsubishi as it has a low min BTU output range which would help, I hope, in preventing short cycling.

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  1. ohioandy | | #1

    cbut, (name?)

    You've come to the right place. No stupid questions here, as long as you're genuinely into figuring something out. There are all sorts of guides and charts and "rules of thumb" for figuring out how much heat you need. Some are helpful, none are accurate. To your question, yes, heating appliances sold in the US are often marked with a maximum power output in Btu/hr or watts (and there are 3.4 watts in a Btu/hr), so that's a standard thing. But as to how much you actually need: rather than relying on vague sizing guides that WILL give you widely different recommendations, the ONLY way to accurately figure out what's needed is to run a calculation based on the construction characteristics of the space you're heating and the climate in which you're located. Do a search on this site for 'heat loss calculation" and pick a method that looks doable for you. Or describe your apartment and your location more fully, and maybe someone will recommend something specific.

    1. Expert Member
      NICK KEENAN | | #2

      Let's stipulate that when he says "I have been told our small apt needs 6k BTU in the living room" that is the result of a Manual J or other reliable heat loss calculation.

      The answer to the question he's asking is "yes, but it's complicated." BTU's are BTU's. However, boilers run most efficiently when they are somewhat oversized, while minisplits are most efficient when they are slightly undersized.

  2. gusfhb | | #3

    I think oversizing of boilers is a significant problem

  3. Expert Member
    Akos | | #4

    Rules of thumb don't work for high density housing. You need somebody to run the numbers.

    For example, a small apartment in a typical all brick townhouse here would have around 18'x10' of exterior wall with 20% glazing. The rest of the walls are internal or against a neighboring properly so there is no heat loss there.

    A double brick is around R4 and cheap dual panes are around R2. So the heat loss with and outdoor design temp of 15F is around 3kBTU for the whole place. Air leaks add some more but it is hard to guess at this at is depends on the actual building.

    I would run each place through an online load calculator (coolcalc or loadcalc) and not guess at the load. Getting the numbers right means less equipment, smaller pipes and less baseboards, which adds up quickly if you are installing nice baseboards.

    A single Mitsubishi 6k wall/celing mount feels like the right size for these. I've installed 9K units in 450-650 apartments and they only ever run near full on places with a lot of western exposure.

  4. cbut8995 | | #5

    I was looking to put 1 6K single zone unit in each bedroom and living room from floors 2 and up.

    The first floor and basement I am still looking for the single zone concealed duct units that have a wide min-max range as well.

    Document Attached shows placement of everything.

    1. Expert Member
      Akos | | #6

      I think you are going a bit overboard.

      For the main floor unit, you should be able to supply the whole place with a single ducted unit. If the basement staircase is open, you should not need any cooling down there.

      You might need a dehumidifer in the basement, which you would have to add in even if you have a unit there as most will not be able to dehumidify a basement without over cooling it.

      If you want a bit of control over bedroom temperature you can add a dumb zone valve to duct feeding the bedroom. What you do is set up the bedroom flow to be about 120% of what the bedroom needs, then set the zone valves so that in closed position it supplies about 70% of the flow. A separate thermostat in the bedroom now drives this zone damper.

      For the rest of the units, look at the smaller dual zone multi splits like:!/product/25244!/product/26250!/product/30560

      (I think the Carrier and Midea units might be the same just different badge).

      They will be oversized for the load and will cycle, but who cares. You only need AC at best for a couple of months. Plus the efficiency is not your problem, the operating cost difference between a right size one and the oversized one is minimal if only used for cooling.

      If you really want efficient for the tenants, a single slim ducted unit in the ceiling of the hallways with a couple of feet of ducts is pretty easy to install if find the right installer.

      P.S. Make sure to only mount outdoor units on masonry. These are very quiet but still have a bit of vibration which will carry through any light construction.

      P.S.S. Just check the data sheet for the Daikin unit, looks like it supports dual 7K heads. That is probably as close as you'll get with a multi. The min load is 4K so that is not too bad either.

      1. cbut8995 | | #7

        Hi Akos,

        I will look into that, I dont understand that basement stuff but Ill speak with my GC to see if it can work. I have never had a dehumidifer in the basement.

        The mini splits I wanted to use for the smaller apts was a 6K mitsubishi with a min cooling at 1700 and heating at 1600 so I was hoping with these min BTU it wouldnt short cycle.

        Id like to stick to the brands like Fujitsu, Mitsubishi, or LG as we usually do the pro installers and get the highest warranty from their diamond installers etc. I know daikin is a big company but dont have any experience.

        I was looking at doing the slim ducts for the smaller apts and just have 1 unit as it would save me on an outdoor unit too but if theres maintenance I feel the wall mount units are better for the rentals.

        1. Expert Member
          Akos | | #8

          The one to one setup of wall mounts should be fine but I think you'll end up with one of these:

          Wall mounts are MORE maintanance. About 5 to 6 years down the road you have to clean the coil and blower on them which is not easy.

          With a ducted unit, you have inlet filter which just needs changing every couple of years.

          If you can get a ducted unit installed for reasonable price, go for it, it is a much better option.

          Make sure they install a filter on the return, it could be even one of those house return air filter registers, but a nice filter grill is better.

          I'm in colder climate than you, here basements tend to need a bit of dehumidification as they are colder than the rest of the house, without dehumidification the RH tends to be too high. High humidity in the basement is a recipe for mold.

          You are in warmer climate, so the humidity might not be an issue, but I still doubt you'll need much if any cooling there.

          1. cbut8995 | | #9


            For the concealed unit, I am still not sure how it would work. it seems there is zero to no maintenance if used correctly and a standard filter change every 1-2 years if the tenants were to only use it for cooling even if it can be used for heating as well as I am sure the tenants will stick with the baseboard gas heat since its cheaper.

            I can probably get the duct units installed somewhat cheaper given I can put it in the stair well and have a vent on the ceiling for access without going into the apts.

            So just to clarify the outdoor unit would connect to the indoor unit normally like any minisplit with the thin piping that can virtually fit and go anywhere. From the indoor unit, I would need to have our hvac guy build a duct from the indoor unit to where the linear slot diffusers end up mostly on the wall close to the ceiling. But the return air where would that be?

            I attached an image with what I think you might be saying. Yellow: single line linear lot diffusers, blue: ducting, Red: Indoor unit. This in turn would minimize our need for 2 outdoor units per apt if I went with the 1 to 1 ratio zoning for the 6k mitsubishi, minimize maintenance which would really be cleaning 1-2 years, and more modern seamless look with the linear slot diffuser, given this is 1 indoor unit feeding 1 rooms and 1 outdoor unit this wont short cycle if I find a single zone 9k, or 12 k outdoor unit that can go as low as 3900 BTU min on cooling and 4200 BTU min on heating which I doubt they will ever use.

  5. Expert Member
    Akos | | #10

    You are correct, the slim ducted units are the same install as wall mount, you just have to run ducts to it. Most also come with a built condensate pump so it is easier to route the condensate drain.

    I guess you can put it in the stairwell, just make sure that the fire separation is still met since these are technically part of the apartment.

    I usually install them in either a hallway inside the apartment or in the ceiling of the bathroom. With taller ceilings nobody notices the loss of a bit of height in there.

    For the return, simplest is to minimize the ducting and mount the filter grill straight to the unit with minimal/no ducting (see attached, the inlet is the black mesh on the bottom). This can be done if the intake side of the unit is near the main living space.

    Linear slot diffusers typically have a plenum that the supply pipe connects to, check that you have the space for that as well.

    Most slim ducted units have very low pressure capabilities, the supply ducting needs to be sized accordingly. You generally want to avoid a lot of bends and use larger duct than would be typical for forced central air. You'll also probably need longer slot diffuser length.

    The air filter should also be bigger ( 1.5x to 2x the rule of thumb for area/ton). This is a good thing as the filter will last much longer, so even less maintenance.

    Something like this would cover the cooling needs of even the large apartments.!/product/25310

    Although it is rated at 9K it can do 12k of cooling. The min is 3k which is not as low as some of the wall mounts, but should be MUCH better than the oversized multi.

    1. cbut8995 | | #11

      This is very helpful. So if I placed the unit in the hallway of the apt, have a grill on the ceiling where it can be removed to access the black mesh in the photo that is where the maintenance will be. I was going to have this unit hang below the cjoist of the apt and then run ducts hanging on the bottom of the cjoist to the linear slot diffusers which I would imagine would only be around 5 feet in length to the bedroom and even less to the living room so not too long. We could probably have easily 2 circular 8 inch in ducts for each room that lead to the plenum which should be enough I believe. ceiling heights are at 10 feet in the living room and bedroom so I would only lose about a feet or so at the entrance of the apt where the ducts and unit would be held.

      1. Expert Member
        Akos | | #12

        That is the easiest. The black mesh is the built in filter, it doesn't do much real filtering. Bellow the unit is the access panel which has the actual intake filter.

        You want an filter grill with a real filter, something like the 30"x12" or 36"x12" one from here:

        Provided the ceiling drywall is close to the unit there, you can just cut a hole in the drywall for the grill and seal around the perimeter of the grill against the bottom of the unit with a foam strip.

        Generally living space end up needing an 8" duct, bedrooms a 6" and bathroom 4". For some of the ducted units you can get a pre-made plenum kit as well to simplify the install:

  6. cbut8995 | | #13

    So there will be a grill on the ceiling where I can access the unit. The unit itself. Some type of steel attachment to the opening of the unit where I can run circular ducts from it probably an attachment with Five 8 inch holes.Then the 8 inch ducts to the attachment leading to the plenum (3 holes for the living room, 1 for the bedroom) opening to in this case a 11 feet of a singular slot diffuser at around 2-3 inches for the living room and 10 feet to the bedroom singular slot diffuser. If ever maintenance, we would just remove the larger grill from the ceiling where we can access the underside of the minisplit unit in the ceiling without removing drywall and making a mess.

    1. Expert Member
      Akos | | #14

      Sounds like a good plan.

      I'm not familiar with the slot diffusers, make sure those get sized correctly.

      Not sure why you would need five 8" ducts, more is always better though. Just make sure that you get right amount of flow to all the rooms.

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