The Problem With Heat Pumps (Or Why We Won’t Buy One Yet)

2014-05-21 09.50.13

A year ago this month, Kate and I bought our first house together. Ultimately we’re planning for a very different configuration, but at the moment it’s a pretty standard single floor ranch. Though the original part of the house dates back to the seventies, we were pleasantly surprised when our inspection revealed that the property was both well insulated and included a furnace of recent vintage – albeit one that ran on expensive fuel oil.

The big problem with these two findings is that neither was true.

This past winter was hard on everyone, and compared to many, we don’t have much to complain about. So let me complain about the size of our heating bills. First, because the furnace was (theoretically) current and therefore efficient. Second, because we had locked in at a price that was $0.20 less than what heating oil eventually peaked at. Third, because we had invested ahead of the heating season in Nest thermostats to help maximize our savings. And last, and most importantly, because we keep our house cold. Not as cold as these guys, but cold. During the week, our program for the Nest is simple: we keep the house at 50 degrees except for an hour in the morning and three or four at night – when we splurge and bump the heat up to 60. Weekends are a little more liberal for obvious reasons, but we were in the top 25% of Nest users every month of the winter in terms of relative efficiency, and the top 15% for two.

While oil is expensive then, we didn’t expect to be paying hundreds and hundreds a month to keep the house just this side of a meat locker. In spite of the horrific nature of the winter.

Which helps explain why we engaged the services of an energy auditor (DeWitt Kimball out of Brunswick, whom I highly recommend). He delivered the first piece of bad news: our adequate insulation situation was considerably less adequate that we had been told. Attics, for example, have a recommended R factor of 40+. Ours was, charitably, in the high teens. Which doesn’t factor in the other various insulation failures we have in a few older windows, the basement door and so on. According the blower test, our house places in the top third of “leaky” homes. We were paying all season, therefore, to heat the outdoors. Which we’ll be addressing by insulating. Heavily.

The far more expensive problem, however, was one we discovered prior to our audit. The oil furnace we thought had been installed in 2005 actually dated – by serial number – to 1992. Our primary heat source, in other words, was 22 years older than we had believed. 22 years less efficient.

The auditor’s recommended solution for heating issue was to keep the oil furnace in place, but to complement it with a “ductless mini-split heat pump,” which are generically referred to as heat pumps or heat exchangers. Popular in both Asia and Europe, these systems are hyper-efficient because they don’t actually use energy to create heat, they simply move it from one place to another – much as your refrigerator does. If a propane furnace is 80 something percent efficient and an oil furnace 90ish (due to the nature of the two fuels), a heat pump is closer to 250% efficient. Couple that with the fact that electricity in Maine is much cheaper on a relative basis than fuel oil – and potentially can be generated on premise were we to invest in solar – and the heat pumps appeared to be a perfect solution. Oh, and you can reverse the direction of the heat exchange and use them as air conditioners in the summer.

The most obvious downside of heat pumps, the fact that they perform less efficiently as the temperatures decline – eventually ceasing to function well below zero – would be a non-issue for us as we already have an auxiliary heat system in place for the few days we see real, deep cold. For most of the winter when the temperatures are 20 degrees fahrenheit and above, we could heat the house using the cheaper and more efficient heat pump. For the day here or there that it got seriously below zero, we could fall back on the oil furnace.

The more I read about heat pumps, the better they sounded. One guy on Martha’s Vineyard heated his house for a year for the grand total of $250 – a fraction of what we paid per month. Another from Presque Isle, almost as far north as you can go in Maine without being in Canada, estimated that he’d save $1,000 a year.

And yet there’s effectively no chance we’ll invest in the technology this year. Why? Because their interfaces are entirely proprietary.

When doing the initial research, I assumed that if there was a problem integrating heat pumps into our existing infrastructure, it would be with our two Nest units. But as it turns out, Nest is more than capable of working with heat pumps as well as an auxiliary heat source like our oil furnace. Called “Heat Pump Balance,” it essentially allows you to use the heat pump until the outside temperature renders it inefficient, then kick in a backup. Perfect.

Except for the fact that, as nearly as I can determine, the best and most efficient heat pumps – manufactured by Fujitsu and Mitsubishi – cannot be integrated into existing thermostat systems, Nest or otherwise. This was the response I got from Fujitsu when I asked about whether their system could be integrated with the Nest or even a standard thermostat:

Our systems can only work with our controllers, they cannot be controlled with the Nest thermostat. At this time there is no way to connect it to the Nest unfortunately.

Essentially, these heat pumps have to be installed as a completely separate system, one entirely independent of your existing HVAC infrastructure – Nest or otherwise. The only way to control the devices is with specialized equipment supplied by the vendor. Which means that we would have two independent, unintegrated heating control systems. Honeywell customer support described the situation to one Mitsubishi customer as follows:

The MIFH1, although built by Honeywell, uses a proprietary Mitsubishi communication protocol to translate and transfer commands between the Mitsubishi equipment and the RedLink enabled products that can also be used with it.

In other words, Mitsubishi’s integration with Honeywell’s Redlink system is a one-off, non-standard connection. Unlike virtually every other piece of HVAC equipment you could buy – air conditioners, gas/oil/propane furnaces, etc – heat pumps cannot leverage standardized thermostat connections. Even if said thermostat, like the Nest, has built-in, native support for heat pumps.

It is somewhat ironic that the most state-of-the-art, technologically sophisticated heating system currently available is unable to integrate with something as basic as a thermostat, but that is the current reality. Which means in turn that as a potential customer, I’m being asked to invest thousands of dollars in a product that cannot be controlled remotely, cannot be leveraged in conjunction with other heating systems, and can’t report telemetry back to somewhere I could use it.

But maybe the real irony is that I won’t invest in these technological marvels for lack of such basic functionality. Even if I could make two independent heating systems work together, I’d be kicking myself next year, or the year after, or whenever Fujitsu, Mitsubishi et al figure out that people want their heating and cooling systems to a) be aware of one another and b) work with each other. It may cost us in the short term with higher heating bills, but it certainly beats paying for heat pumps now, and then again later when the vendors have seen the light and let them work with our Nest units.

In the (hopefully) likely event that Fujitsu and Mitsubishi eventually see the light, my message to them would be simple.




  1. Kind of chuckling. On the one hand, I completely agree with you, as I am greatly loathe to invest in any “system” (from electronics to ski bindings) that is not universally interconnectable. On the other hand, the ultimate “completely separate system, one entirely independent of your existing HVAC infrastructure” is of course a wood stove, and no one seems to mind that they don’t integrate into the primary heating system. In fact, having an independent system is part of the appeal. Isn’t there something to be said for being able to run the mini-split even if the Nest happens to melt down?

  2. There’s something to be said for redundancy, I suppose, particularly in an era in which network connected devices are increasingly likely to come under attack.

    That being said, we have a fireplace – which we use for some heating currently, and could be converted to something like a wood stove by way of an insert – if independence was the goal. For us, however, that’s not the priority. If I’m going to invest thousands of dollars in a digitally managed heating system, I’d like it to play nicely with our other digitally heating systems.

    Because if the Nest melted down and took the minisplits with it – assuming they could be independently run manually, of course – we’d still have a fireplace to fall back on.

  3. I stumbled on this post looking to see if anything was new with the Mitsubishi RedLink (programmable thermostat).

    I had the same thought before I installed the mini-splits in my house. I really wanted the Nest so I could simply program my system for my daily habits and so I could turn it on/off when away. It really frustrated me that I could use a normal thermostat. At one point I even switched gears and was going to install a standard Heat Pump with electric resistance backup.

    I had issues with the contractor for the standard heat pump and ended up going with the Mitsubisbi because the install was easier and I could do it myself.

    Here is what I have found out over the two years I have had the system – the programmable RedLink thermostat is nice but you can’t really have the temp swings you can with a hydronic, forced hot air systems do. Last year I had my rooms cool to 62 during the day and heat to 68 at night. Their were days she the system could reheat the house (like when it was less then 10 out). One issue I had was the unit would go into defrost mode about a 1/3 of the time, so it kept fighting the loosing heat. I had to use supplemental heat to keep the house warm.

    This year I set the thermostats at 68 and the home stays consistent temp. I dont go into defrost that often, even when it has been less then 5 degrees. My electric usage is way down over last year, compared to the electric baseboard I have saved enough that I paid for about 70% of my system cost (Four wall units).

    Back to the point of this post – If you keep the mini split units on at a set temp they turn completely off if the room gets warmer (about 2-4 degrees) then the thermostat is programmed for. If the Nest was programmed to turn on the aux heat at x outside temperature it would work as designed seamlessly and automatic. The Nest while smarter is no where near as smart as the controls on the mini split. It would be a step backwards. Remember part of the efficiency is the indoor units has a eye that scans he room and adjusts the vanes and fan force depending on what is hot and cool in the room.

    For reference I have poor insulated attic (R-19), about 1,400 sq ft in Northern NJ. I have easily heated my home down to -8. My average power bill is about $120 in the winter, all electric house.

  4. Not sure who at Fujitsu gave you that info, but it’s wrong. They sell interface kits that provide “dry contact” control to allow an external thermostat or building automation system to enable/disable the unit. The interface kit is part number UTY-TWBXF1, and the dry contact kit that plugs into it is part number UTY-XWZXZ5 (for most of their wall-mount units – there’s a compatibility chart that any contractor should have, to determine the right kit for the specific heat pump model in question).

    I’ll also note that there are downloadable “universal remote” apps for smart phones that can control these units, because folks have reverse-engineered the communications protocols. So, if Nest actually wanted to offer compatibility, they could – it’s not really Fujitsu’s job to proactively establish compatibility with third-party products, and there’s nothing at all preventing Nest from establishing fully compatibility on their own. But Fujitsu’s interface kits allow for basic compatibility with other systems, anyway.

    • Joe – can you point to a project or website where there is documentation that shows a reversed engineered Fujitsu or mitsubishi protocol?

      • If you do a web search for “fujitsu heat pump smarthpone app” you should find quite a few.

    • Joe – will this interface kit really work directly with a Nest thermostat? Can’t find any documentation online.

  5. I have been down this road as well and have 2 homes with Fujitsu Mini Splits, and another rental house with a 2 zone Panasonic. I use the Lowes Iris system for home automation, and certainly would like to see integration and have done a good bit of research. Practically all of the minisplits use RF remote control, and rarely lave controls on the indoor unit. This is similar to window air conditioners, space heaters, or for that matter TV sets; One could say that they all have proprietary RF controls, which currently do not connect to Nest or Iris.

    With that said, there are some 3rd party WiFi enabled RF “universal” controllers available out there. Think of it as a wifi equivalent of a TV universal remote, which can be synced to the proper RF frequency of the device. I have tried 2 different versions of these, both from Chinese providers and some as cheap as $30 including postage from China. However, I can’t say I am happy with them; their UI is rough and seem to work inconsistently, which is not what you are looking for with a remote thermostat app. Beyond that, I can’t understand why Nest and Iris haven’t come out with RF solutions; Granted Minisplits is a narrower market than general thermostats, but when you look at all the other RF devices that could be controlled is is actually a huge market.

    So for the moment my wifi based minisplit control aspirations are on hold. However, I feel confident that they will be addressed soon (Fujitsu prod mgmt has told me so much). I agree with post that generally Minisplits work best in steady-state mode with their variable compressors; They lack the high “surge” heat output like a furnace to bring a room up to temp, and running at full output to do so is not the most efficient way they work…look at it this way, they have a fixed coil size, so if the unit is running at less than full compressor capacity it has more heat exchange area per compressor input. Additionally, the latest minisplits like my Fujitsu 15RLS3 have built in motion sensors, which lower heating temp if there is nobody in the room, so they are effectively providing some level of self adjustment even WITHOUT Nest or Iris.

    These units are excellent heat sources even in cold weather, and with recent advances Would say they are the lowest cost per BTU you will find for any comparable heating system, and that includes . wood or coal; However No flames, no ash, etc More people should ditch their oil furnaces for these things asap. While I can understand wanting Nest to integrate, by not considering these units due to that believe you are missing out

  6. David Martin says:

    Mini-splits work most efficiently when they are left on a single temperature all day, not turned down at night, then warmer for a couple hours in the morning, then cooler, then warmer in the evening. They are relatively inefficient when they are in high power mode to increase the temperature of the house by several degrees. Instead they compute how much heat is necessary to maintain the selected temperature and run constantly at that level, not cycling on and off. So even if they were made compatible with the Nest it would not be economical to set back the temperature at certain hours.

    • David,

      Do you have any data to back this up? I have my mini splits programed to move between 68 and 60 depending on the time.



  7. Enjoy, Mitsubishi it producing a thermostat interface device! Model number is PAC-US444CN-1

  8. Steve says:

    There are lots of manufacturer apologists in these comments. Manufacturers deserve blame for making a home climate control product that doesn’t work with a product like Nest, which already works with 99% of existing climate control solutions. They also deserve blame for relying on RF communications between their products when RF is so susceptible to interference and are limited in distance. If you are making holes to install mini-splits, then including a wire for a thermostat is not any harder. There is no good reason to rely on proprietary devices other than greed. A mini-split needs two pieces of information: the current temperature of a space, and the temperature you want it to be. Anything else is a gimmick. This is the same as any other system in current use. Let’s stop pretending this is some kind of futuristic technology. The real improvement is that they use smaller inverter-based compressors to be more efficient. It is no different than using multiple window air conditioners. They just split the unit in half allowing you to locate the air vent and compressor in different locations. It is great because it allows multi-zone comfort, the ability to use it on-demand in rarely used spaces, no ducting, and no single point of failure. These units should be cheap, easy to install/maintain by a DIYer, and adaptable to existing infrastructure. There is lots of bullshit out there from manufacturers, trade organization, and hvac techs that love to protect their profits by deliberately misinforming customers, engaging in anti-competitive practices, and focusing more on profit that delivering the best possible product.

    • Steve, a Nest thermostat does not provide information about the room temperature to the appliance. The reason it is compatible with “99%” of equipment, is that that overwhelming majority of equipment only wants an “on/off” signal from the thermostat. If the room temperature is on the wrong side of the setpoint, the thermostat closes a relay until the setpoint is reached, and then opens the relay. The appliance just gets a “run” or “stop” signal, and that’s it. A few systems will have stages, so the equipment gets “off/low/high” or “off/low/medium/high” information. These are the most utterly basic sort of control available, and that’s why there’s such wide compatibility.

      The efficiency of an inverter-based mini-split derives from the ability of the control to vary the speed of the compressor to precisely match the demand. The system knows the desired set temperature, the current room temperature, the outdoor temperature, and a number of other parameters which it uses to control the speed of the compressor, the speed of the fan in the indoor unit, the precise orifice in the expansion valve, and sometimes the speed of the fan in the outdoor unit. The efficiency simply cannot be achieved without that information, which the Nest cannot provide.

      There are simple, cheap units which have the sort of easy compatibility that you desire, but they can’t have anywhere near the efficiency of the units being discussed, here. You’re asking for the efficiency of a hybrid car, but using a carburetor, and you also want it cheap and DIY. Those are not compatible parameters. Particularly the DIY part, since few homeowners have the vacuum pump and refrigerant gauges necessary to charge the system (I suppose those could be included with the purchase, but you want it to be cheap…)(also, the EPA would be inclined to chase you down for handling refrigerant without a license – they tend to get uppity about that).

      No, it’s not greed or protectionism, any more than cars using fuel injection (much more difficult for DIY repairs compared to a carburetor) is about greed or protectionism – it’s simply necessary to get the demanded efficiency levels. Nest may be fancy, itself, but the way it connects to a heating system is just about as basic as it gets, and it simply cannot act as the brains of a high-efficiency unit while communicating in that manner.

  9. Ryan Terwedo says:


    We are building out a new system and essentially want exactly what you need in a heat pump system. After researching extensively, I found an LG setup with something called a dry contact to work with a Nest. Any experience with something like this? I know its not one of the manufacturers you said you liked.

  10. Chris L says:

    I realize this is an old thread but the situation hasn’t changed much. I bought a Mitsubishi heat pump in 2012 and went through the same issues as you. It had the simplest of timers, which were next to useless. I wanted a way to control my heat pump over the internet, so I could come home to a warm house. The Nest wasn’t able to work with heat pumps and isn’t “supported” in New Zealand anyway. Why it isn’t supported is beyond me!

    Nothing existed at the time for remote control of this heat pump so I cobbled together a raspberry pi and IR transmitter to do what I needed. It worked but the IR transmitter was very unstable so I gave up on it. In the mean time a company called Sensibo ( has released a product that works in a similar way, it uses IR signals to control almost any heat pump. I’m not affiliated with the company in any way, I just like the idea. This might be a good option for people in our situation and it’s relatively inexpensive. Thought I’d share it here in case other people are in the same boat.


  1. […] was interesting, but given that they’re less viable in extreme cold and more problematically will not integrate into an existing thermostat system, they’re not quite ready in our eyes. We’ll probably […]

  2. […] hyperefficient relative to our old and inefficient oil furnace. Not believing the technology was quite ready, we instead swapped out our fireplace for a fireplace insert (a Jøtul Rockland 550, specifically). […]

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