Click here to Find a Contractor in your area.
Welcome! Here are some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.

Most Efficient way to run mini splits with steam boiler

Kjmass1Kjmass1 Posts: 149Member
I just had a 4 head Fujitsu mini split system (not the XLTH) installed. They work really well currently. Our home is pretty well insulated so they do a good job keeping the majority of the living areas at set point.

We have a 200K steam boiler that is probably 20 years old but runs perfect and gives us nice even heat in the winter.

What the most cost efficient way to run these in the winter? I believe they give out 100% heat down to 5 degrees. Heat load per manual J is about 55K BTU/hr and specs for these should put out 34K BTU/Hr (not including one in attic space not used a lot) at design temp. Do I just leave them on all winter or at a certain point should I just switch them off?

I also have an ecobee thermostat with 4 wireless sensors. I could run sort of a hybrid system where I shut off the radiators that overlap with the mini splits in the main living areas, and use the wireless temp sensors/steam heat in the other rooms that are a bit removed.

Any mathematical forumla to figure this out? I just don't want to run these for a month in the dead of winter and then get a huge electrical bill.

Some interesting cost comparisons on this site:
https://newenglandcleanenergy.com/energymiser/2017/02/20/how-ductless-mini-splits-beat-oil-heat-even-in-a-new-england-winter/

Thanks.

Comments

  • JUGHNEJUGHNE Posts: 4,586Member
    I assume you have not gone thru a winter heating season with these mini HP.
    Most mini HP will put out some heat down to 5 degrees, then they shut themselves down. The 100% output is debatable at temps that low.
    IIWM, in that situation I would go with HP down to about 25-30 Degrees. Once a HP starts to ice/frost up they need to go into some form of defrost. This is not only a little hard on equipment but you may get cool/cold air delivered into your house.
    There is no question of economical heat delivered, the first repair bill for some component out of warranty will get your attention and any efficiency savings will seem to evaporate.

    The comfort level of heat pump air will never match that of your steam heat system.

    I have a 40 bed nursing home with mini HP/AC in each room, they have baseboard electric heat for back up and were instructed to switch over at about 30 degrees. If any continue to run in cold temps they will ice up and maybe the fan blades will scratch on ice in the bottom of the unit and the lower fins get distorted from ice buildup.
  • Kjmass1Kjmass1 Posts: 149Member
    Lots of good points, thanks. I think that seems like the best action to start with and go from there. If it's 5 degrees out I want those radiators cranking. I originally bought them for AC and shoulder seasons so I'll plan on that.
  • ZmanZman Posts: 4,306Member
    Even though your mini splits will run at lower temps, their efficiency will fall off. I would start with your local energy prices and the attached spreadsheet. Then look at the performance curves of your mini's as the temp drops. You could absolutely do a control setup that automatically changes from mini split to steam when the outdoor temp drops below a setpoint.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Turbo DaveTurbo Dave Posts: 47Member
    edited September 2017
    With the spreadsheet above, you could have a 2-stage thermostat, such as a Tekmar #553 switch over automatically at the Balance Point. You can tweak this for the increased steam comfort and less cold-temp HP stress.
  • ratioratio Posts: 1,569Member
    Some mini splits can control auxiliary heat with an added relay kit. I'm not sure how it would do with steam, but it's something to look in to as well. Maybe the aux relay could start a one hour cycle of the boiler or something like that.
  • Kjmass1Kjmass1 Posts: 149Member
    Thanks for the advice- I’ll look in to that spreadsheet.

    I could easily add a IFTTT controller with ecobee so it turns on the boiler once the exterior temp hits a certain range.
  • HVACNUTHVACNUT Posts: 1,406Member
    Ok, I'm lost.
    How do you use an Ecobee thermostat to control 4 ductless HP heads that are wireless remote controlled?
    Use an isolation relay to break 220v to the condenser?
    Can the Ecobee be programmed to switch stages at a set outdoor temp?
    What am I not seeing?
    Help.
  • Kjmass1Kjmass1 Posts: 149Member
    Sorry- they are 2 separate systems. Each of the four mini split heads has a remote to control it.

    The steam system is controlled by an ecobee thermostat. Ecobee can use 4 wireless sensors to average out the temp in your house better than just the temp at the thermostat.

    IFTTT (if this than that) is a free web service that you can create triggers to control the ecobee. “If outside temp is below 45, set ecobee to 71 degrees.” All sorts of different ways to set triggers. “If indoor temp is below 71 turn on boiler”, etc.
  • HVACNUTHVACNUT Posts: 1,406Member
    > @Kjmass1 said:
    >
    > IFTTT (if this than that) is a free web service that you can create triggers to control the ecobee. “If outside temp is below 45, set ecobee to 71 degrees.” All sorts of different ways to set triggers. “If indoor temp is below 71 turn on boiler”, etc.

    That's fine, but you still have to manually turn on/off the ductless heads, correct?

    I wonder if you could wire an isolation relay to W1, breaking 220v to the condenser at an outdoor set temp and kicking in W2 for steam.

    Is there a common at the Ecobee, or is it power robbing?
  • JUGHNEJUGHNE Posts: 4,586Member
    It is usually advisable to keep the 240 VAC to any heat pump on all winter long. Crankcase heater may need to be on at all times.

    IIWM, I would just do a manual adjustment for the combo.
    Experience will tell you when to switch from HP to steam.
  • John MillsJohn Mills Posts: 838Member
    Most high end minis are variable capacity so an on/off stat won't work. The remote control is necessary and the indoor unit talks digitally to the outdoor unit to set the capacity level. Unless the mfr. has a conventional stat option, buying an Ecobee and breaking the high voltage isn't wise.
  • JackJack Posts: 1,029Member
    So, your heat loss calcs say 55kbtu and your steam boiler is 200kbtu? Fujitsu has output by degree graphs by model. Get them! That is complicated somewhat with multis, but worth digging into to assist your decision. With a boiler that is 4X the heat loss of the building, I would run the boiler as little as possible.
  • radmixradmix Posts: 194Member
    the extra low temp units will produce heat down to -15* if sized correctly with a heat loss and then going to the heat capacity charts. I would use these units as the primary heat source. I now have dozens of homes heating with these units as the only heat source with excellent results. I have a 900 sq ft cottage heating with a 15,000 btu unit and the electric bill for the mouth is around $35-$40 a month. I live in upstate NY. I'm getting incredible feed back from customers that are telling me about the savings there getting from the units.
  • Kjmass1Kjmass1 Posts: 149Member
    November will be the true test with some colder weather. October I saw huge reduction in nat gas but electric went up maybe $30. I have 2x 12k’s that will keep the house at 70 with the boiler as a backup at night at 68. Boiler will run a couple cycles over night when temps get in to the high 20s. The splits will go in to defrost cycles so I’m sure that will hit their efficiency. I’ll report back after my next bill to see how they did.

    Even with 40 year old boiler that is probably 2x oversized for the rads, innefficient, and probably 3x my house heating load, with gas prices so cheap it may still be cheaper than electric at $.20/kwh.
  • mikeg2015mikeg2015 Posts: 380Member
    Depending on electric rates and gas rates and the COP of those mini splits, plus some other factors, balance points economically is probably around 20-35F where the boiler is cheaper to run.

    For comfort, I like the radiant a LOT better. My heat pumps are great over 40F, but below that air gets cooler and they do into defrost sometimes. My economic balance point is around 30F for my downstairs system, 35F upstairs. But I can zone the temperatures better with the heat pumps and I'm not heating my basement.

    My homes heat loss i estimate from run times of the heat pumps, is around 105k BTU. Total radiation is 172k. Steam boiler is 375k input, 300k output. It's oversized by almost 100k. A 275k would be just about right.

    The 200k boiler is 164k output, with 33% pickup factor, it's designed for 123k of radiation. Which is pretty close to what you'll see, nearly 2x the radiator capacity as the homes heat loss. But keep in mind these were for coal systems that burned at a variable rate. You might want to heat the house up quick sometimes and let it burn down other times.
  • se1961se1961 Posts: 1Member
    edited June 10
    Kjmass1 said:

    I’ll report back after my next bill to see how they did.

    Hi! We just installed a Mitsubishi hyper-heat mini-split in my father-in-law's apartment downstairs, primarily for air conditioning, and I found this thread after asking the installer the same question. We have a relatively new Burnham Independence steam boiler, rated at 82% efficiency, installed about 18 months ago when the 50 year old boiler cracked. We are not too worried about costs, since electricity is high in our area and gas seems pretty reasonable. We live in Massachusetts, so winters can get pretty cold. I gather from this thread that it makes sense to run the mini-splits from the spring through the fall, until the temps drop below 35 (ish), and then switch over to steam. I'm curious to know what you ended up doing, @Kjmass1, and/or any other thoughts from folks who have experience running two systems. I would like to keep the heat/ac choices SUPER simple for my aging father in law. Thank you!
  • hvacfreak2hvacfreak2 Posts: 451Member
    Reading through the thread just now , I didn't know Ecobee could do all of that .

    Hope this may help someone ...in the case of Mitsubishi you can interlock indoor units by breaking S1 to the indoor unit ( do not mess with S3 ). There are also Bacnet interface modules available for indoor units but they are rather pricey.
    hvacfreak

    Mechanical Enthusiast

    Burnham MST 396 , 60 oz gauge , Tigerloop , Firomatic Check Valve , Mcdonnell Miller 67 lwco , Danfoss RA2k TRV's

    Easyio FG20 Controller

  • mikeg2015mikeg2015 Posts: 380Member
    se1961 said:

    Kjmass1 said:

    I’ll report back after my next bill to see how they did.

    Hi! We just installed a Mitsubishi hyper-heat mini-split in my father-in-law's apartment downstairs, primarily for air conditioning, and I found this thread after asking the installer the same question. We have a relatively new Burnham Independence steam boiler, rated at 82% efficiency, installed about 18 months ago when the 50 year old boiler cracked. We are not too worried about costs, since electricity is high in our area and gas seems pretty reasonable. We live in Massachusetts, so winters can get pretty cold. I gather from this thread that it makes sense to run the mini-splits from the spring through the fall, until the temps drop below 35 (ish), and then switch over to steam. I'm curious to know what you ended up doing, @Kjmass1, and/or any other thoughts from folks who have experience running two systems. I would like to keep the heat/ac choices SUPER simple for my aging father in law. Thank you!
    Yes, simplest solution is to just use mini splits until about it's consistently below 35F overnight, then fire up the boiler.

    However you might still use it for zone control if you want some rooms warmer without overheating the whole house. In that case you could use a second remote temp sensor with a Honeywell Visionpro and set the average to be say 69F, but set the heat the downstairs family room and kitchen that are occupied all day to 70 or 71F.
  • mikeg2015mikeg2015 Posts: 380Member
    ... I'll probably leave my boiler off altogether until it's consistently under 35-40F at night. Usually around late October or early Nov here. Then I turn it off some time in March.
  • Kjmass1Kjmass1 Posts: 149Member
    There is no doubt radiant heat is a better comfort, especially on really cold days when they always stay warm and the boiler is cycling every hour. Our floorplan allows for the splits to be centrally located so there isn't too much temp difference throughout the house. Since I have 2 condensers with a min mod range of 7K btu's, I run just the first floor unit above 45 degrees, and then turn on the second floor unit below that as that will split the load and allow them both to modulate @ 50% load sweet spot. I will say I had trouble keeping a steady setpoint in the shoulder season causing overshooting on warm days. A wired thermostat would help that but also a negative to mini split design.

    My boiler is a 199k/157k good for 654 EDR not including pickup losses. The radiator EDR is about 444 sq ft, but that includes basement and attic radiators which aren't used so it's closer to 300 EDR 99% of the time. Huge basement loses as it isn't insulated.



  • Kjmass1Kjmass1 Posts: 149Member
    Are you strictly looking for the break even for a cost perspective? The great people at greenbuildingadvisor told me it would be around 20-30 degrees with my particular set up with multsplits. They are also not hyperheat so your set up should even more efficient and 1 head units are also better.

    We only have one per floor and one on the master, so a couple rooms would get a little cooler than the others. I put ecobee sensors in those rooms and set the temp a degree or two lower as a backup to trigger the steam as needed. I did completely turn the splits off under 20 just as I prefer the steam. Your unit should do fine at those temps.

    They are awesome this time of year. Don’t use auto mode though as it generally has too wide of a temp swing for me.

    Easiest thing for your father would be keep the split on all the time, with an ecobee averaging a couple living areas and bedroom a degree or two below set point. I’d just let the split run all the time and not worry about it- cost is negligible at that point.

    They are certainly more green and use a fraction of BTU’s with no basement and pickup losses.

  • NY_RobNY_Rob Posts: 1,120Member
    Kjmass1 said:

    ....I will say I had trouble keeping a steady setpoint in the shoulder season causing overshooting on warm days.

    The mini-split system overshot setpoint?

  • Kjmass1Kjmass1 Posts: 149Member
    Yes- I had issues with my compressor having a minimum modulation that was higher than my heating load on certain days. The split was also high up on a wall in a little alcove so it would have trouble getting a true sense of the room temperature. I installed a wired thermostat and it works perfect now. Some brands have the thermostat built in to the remote so that can help as well.
  • NY_RobNY_Rob Posts: 1,120Member
    edited June 11
    ^ thanks for the explanation...

    I just installed a 3-head LG mini-split and have used the heat feature a few times for cold mornings. The one indoor unit that has been getting used for heat is mounted according to LG specs.. about 6.5ft off the ground and not closer than 8" to the ceiling.
    Results are mixed according to function and the flap position...
    On cooling if the flap is pointed downwards you can set the A/C at 75F and the room temp will go down to 71F at the height level of the heating system t-stat. If you level the flap to more or less horizontal output then the room temp is closer to the desired temp setpoint on the indoor unit.

    On heating, you need to point the flap down or the air temp at the height of the indoor unit quickly rises to setpoint and places the indoor unit in fan-only mode till it falls below setpoint again.
    I've also seen the indoor unit stop heat output completely and go into "defrost mode" which I believe means "pre-heating" mode for heat applications. The manual states "Defrost/Pre-heat" can last 15min or so... which means no heat during that cycle.

    With the flap positions being very critical, minimum turndown rates and no heat during "Defrost/Pre-heat" cycles it's no wonder it's difficult to find consistent temps with mini-split heating. My older Panasonic Mini-split does have the temp sensor in remote can control the wall unit temp feature, but sadly the new LG doesn't have that feature.

    How frequent were your defrost cycles during the colder days?
  • Kjmass1Kjmass1 Posts: 149Member
    Flap position is definitely critical- I keep it horizontal for AC and maybe 45 degrees for heat so it can push it down to the next room. My unit has maybe 2" clearance to the ceiling above a window, really the only place it could go. I actually believe it is within Fujitsu clearances surprisingly.

    My Ecobee/IFTTT trial worked so so (this was also when I was dealing with the min modulation rates issue) so I was trying to overcome that. Fujitsu sends codes in Celsius and the tie in from Ecobee was in odd numbers F so it was hard to get a tight temperature band.

    I then bought a Sensibo Sky which works really well- it essentially is an IR blaster with a iPhone app so you can control all the functions away from home. You can also make quick toggle iOS widgets so one button will turn it off, turn on AC, turn on heat. With Boston temps dropping in to the low 50s at night but 70s in the day, I could set a schedule to turn on the heat 6-7AM and that was enough to heat the house most of the day.

    Sensibo also has their own "climate react" feature they call it. So if the temperature at remote IR unit hits 68, turn on heat to 70. If house gets to 74, turn on AC to 72. Only downside is it needs to be plugged in and in clear view of split head...which is usually in the path of the air. It can also tie in to IFTTT, so you can set triggers based on outside air temperature for example.

    In the end, a Fujitsu wired thermostat made it work better, and the Sensibo is how I control it. Pretty much set it and forget it at this point. My 3 other units work fine solo.

    They will defrost below 30 degrees- usually only 5 minutes or so. They actually run in reverse and blow warm air outside to melt down the icing/freeze up. You'll hear a compression release valve sound that startled me the first time at night.
  • NY_RobNY_Rob Posts: 1,120Member
    Sounds like you put in a lot of effort to get it working to your satisfaction.... nice job!

    From my admittedly short time usage of mini-split heating... it seems small areas are problematic vs. large areas with less dramatic and slower temp excursions.
Sign In or Register to comment.

Welcome

It looks like you're new here. If you want to get involved, click one of these buttons!