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Steam and Heat Pump

Si_zim Member Posts: 40

Bought an old house in Northern NJ. It has gas steam boiler and one-pipe rads on 1st and 2nd floors and in 3rd floor attic room... and a hot water zone on 1st floor extension. There are multiple old wall/window ACs throughout the house. They are noisy in use and are draughty in winter even with covers.

I would like to replace the ACs with a ductless mini split system because of the age of the house. Probably one or two outdoor units and about 7 or 8 indoor units. Then I started researching mini split heatpumps - and thought that would be worth looking at... with the HP handling moderate temperatures and the steam kicking in when really cold. Let me know if this makes sense.


1. Any way to automate it so the steam kicks in automatically for emergency heat - and the HP shuts down when too cold outside to prevent defrost etc?

2. Any way with Zigbee or Redlink etc that the two existing zone thermostats could interact with the HP indoor units in those zones so it would be more integrated and easier to manage?



  • Si_zim
    Si_zim Member Posts: 40
    Steam and Heat Pump - PS

    PS: I know a lot of thermostats have 2 stage heat or dual fuel/hybrid capabilities which would intended for this - for forced or central heatpumps. But I dont know if you can integrate these ductless heatpumps indoor units (like Mitsubishi's, Fujitsu's etc) with it since they all seem to use IR/wirelss remote controls with proprietary interfaces.
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,573
    Mini-split+steam control

    There is a cost difference between the heat-pump version and the ac only of the mini-split.

    I think you would do better to spend the difference on making your steam as balanced and responsive as possible.

    What fuel are you using, gas, oil?

    I am in the same boat as you are, and my system has no problem with maintaining comfort at the cool temperatures of early and late winter. My thermostat is a Honeywell visionpro, with remote indoor sensor in the coldest bedroom, therefore it reacts more quickly to a drop in temperature. I do not use any setback, so can therefore keep a lower constant temperature setting.

    The mini-splits I have seen use a proprietary thermostatic control, which would be hard to integrate with the steam control.--NBC
  • SWEI
    SWEI Member Posts: 7,356
    Do any of the mini-split controls

    have terminals for emergency baseboard heat?
  • Si_zim
    Si_zim Member Posts: 40
    edited May 2013
    Steam and Heat Pump


    It's natural gas-fired... and yes I am still tweaking it. Getting 2 main vents installed before this winter. And I need to move the main thermostat upstairs - leave the hot water zone stat controlling 1st level. This will all help with balancing I am sure.

    I need to look at the cost difference of the AC only versus HP mini splits... and I presume there are performance implications too. I was thinking that in milder weather making steam seems bit inefficient... especially as not all rooms will be occupied for now. But I value all input on this as it will be a big, expensive project so I'd like to get it right!

  • fixitguy
    fixitguy Member Posts: 91
    No Problem

    Mitsubishi Mr Slim (the only brand I sell) multis are only offered as heat pumps., I have Mini's for cool & well balanced steam in my own house and it works great. My wife loves the arrangement and that says it all.
  • Si_zim
    Si_zim Member Posts: 40
    No problem

    Good to know. But you are only using the mini split for cooling right? So I guess it's pretty easy when the season changes to switch off the steam thermostat and start using the Mitsubishi controllers for cool.

    Might get interesting (and probably be very inefficient) if both steam and heat pump were trying to control heating which is why it'd be great to have a stat that knows about both and can call on them as needed.
  • JStar
    JStar Member Posts: 2,752

    Right now, there is no way to integrate both systems. I wouldn't do it anyway. Steam needs its own cycles per hour. You're better off doing it manually.
  • heatpro02920
    heatpro02920 Member Posts: 991
    edited May 2013
    I have made trickier things work

    Tis is actually easy I have done it with fujitsu units... Just use a t-stat that supports heat pump, aux heat, and out door temp sensor.. And wire the heat pump leads to a switching relay that breaks what ever terminals you need to break on your heat pump to shut down, you could use a 200v switching relay and shut the units power rite down, but a low voltage unit is fine to shut down com between the in and out units...

    My house has a "real" heatpump and coil setup, plus a propane boiler, and a coal furnace... It works this way, the tstat controls the propane and heatpump, so that when it is above 47 degrees the heatpumps run, if it is under 47 the propane runs, BUT if the coal furnace's heat exchanger rises above 120* it shuts down the heat pump and propane unit and lets the coal heat the house.... when the coal drops below it turns back to the heat pump or propane depending on the outside temp... Simple... And I am soon going to be adding geothermal heatpumps, that will be the new primary....

    But anyway, its simple to do, pick a tstat that has an od sensor and controls heatpump switching, that you like, then pickup a switching relay for each out door unit and your installer should be able to do the rest...

    Good luck, I wouldn;t throw away the heatpump on mild days idea, I love it and it saves a lot of fuel... You wont be sorry that is for sure, Im not a mini split fan, but they have there place, If I were your contractor, I would be doing my best to sell you a "real" system... the cost is normally the same when you are doing an entire house, you save when you just do one room, but you start buying 8 in units and a few outs and it adds up fast... 1 armstrong heatpump and 1 air handler with a plenom kit, a few boxes of insulated flex duct, some boxes, takeoffs, and registers and you have a "real " system that isnt a mini split... Just feels temporary to me, I know they are nice, I almost put one in my office, but couldnt bring myself to do it, I install a ton of them every year, people do love them, and they go in fast and easy, very forgiving, but to me its just not rite... Im obviously wrong because people love them and I get more and more calls for them sooo, can't be rite all the time...

    PS honeywell used to make an outdoor tstat, that would make a-b and break c-d upon temperature rise, you set the temps diff... But I havent seen them in a long time, they were mercury and I dont think they ever replaced them with a new model, I believe it was called a t-238 or t23a something like that... that would have worked too...
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