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General: Do heat pumps modulate?

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1 to 24 of 24

  1.  
    • CommentAuthorcjard
    • CommentTimeOct 4th 2015
     
    Just wondered if a heat pump of X kW can modulate down when heat demand is lower, rather than cycling on and off?
    • CommentAuthorgravelld
    • CommentTimeOct 4th 2015
     
    I believe the inverter heat pumps do.
  3.  
    Yes, the more expensive ones do e.g. Mitsubishi EcoDan.

    I am in the process planning to install a solar PV and an ASHP, and am looking around for an ASHP where you can control the modulation, so I can match the heat pump's consumption to the solar PV, minimising consumption from the mains at 15p/kWh versus export at 5p/kWh.

    Does anyone know of a heat pump where it is possible to easily control modulation directly? I was thinking about a Raspberry Pi connected to the heat pump and an export meter, but for this to work well I would need to be able to control the modulation. I presume I might be able to fudge it by nobbling the output of a thermostat?

    I have looked at the Panasonic ASHP/PV controller which appears to do exactly this but my local supplier is reluctant to install it because of unspecified issues with spares.

    Does anyone have some ideas?
    • CommentAuthorJonG
    • CommentTimeOct 4th 2015
     
    Inverter driven heat pumps do modulate, but not to the extent that a gas boiler will but any cycling that does occur is far more detrimental to a heat pump than a boiler.

    They need a minimum flow rate and a minimum flow volume and typically this is achieved with the use of a buffer. With the advent of better quality insulation and air tightness we are also starting to install gas boilers with buffers because it can be very difficult to cover each end of the heat load spectrum.

    Some manufacturers do make boilers with a 10:1 turn down ratio but they are few and far between and choice is limited.

    With a heat pump that is cycling the risk is that the compressor meets an early demise, plus a fridge circuit typically takes several minutes to stabilise and become efficient. If you have a fully zoned system and only a small zone or limited zones in the shoulder months are calling and the load is below the heat pumps minimum ramp down it won't end well. Mitsubishi used to espouse this approach but we have changed several compressors as a result.

    If you want to link a heat pump to PV look at unit with the software on board already would be our advice, this is feasible with IDM and NIBE, there will be others (Ochsner from memory) but it might be an expensive experiment to mess with a package unit, we have seen Mitsis run rich when linked to TPI stats.
    • CommentAuthorTriassic
    • CommentTimeOct 5th 2015
     
    I thought the inverter was designed to do the modulation?
    • CommentAuthorJonG
    • CommentTimeOct 5th 2015
     
    Hi Triassic yeah the inverter controls the modulation usually via a separate PCB that responds to changes in the flow and return temperatures amongst other parameters via the heat pumps on board sensors. This is all set up and encoded by the manufacturer, it is not something we have any influence over at commissioning.
    • CommentAuthormike7
    • CommentTimeOct 5th 2015 edited
     
    <blockquote><cite>Posted By: ActivePassive</cite>
    Does anyone know of a heat pump where it is possible to easily control modulation directly? I was thinking about a Raspberry Pi connected to the heat pump and an export meter, but for this to work well I would need to be able to control the modulation. I presume I might be able to fudge it by nobbling the output of a thermostat?</blockquote>

    On my aircon-type heat pumps the modulation is linked to a thermistor. The resistance of this varies with the temperature of the room air, so the HP can compare that temperature with the 'set' temperature and decide whether to turn on or not, and if so by how much.

    If your HP works similarly then by substituting a different value thermistor or by adding ordinary passive resistors in series or parallel you could get the HP to do what you want.
  8.  
    A good thermostat will control the modulation for you. You need a "multi-stage" one. This will sense the rate of change of heating (or cooling) and adjust that modulation accordingly. I believe these are more common for air-to-air heatpumps, where the staging is important especially for cooling. If you cool too quickly, you don't dehumidify.

    Paul in Montreal.
    • CommentAuthorJonG
    • CommentTimeOct 5th 2015
     
    mike7, the resistor in the room sensor is only 1 piece of the control strategy, the heat pump will also be reading the external temperature, flow and return temperatures and its internal software regarding curve setting, degree minutes (or equivalent) current fan speed (if ASHP), compressor frequency, deforst logarithm settings etc. you do need to be careful modifying any part of the "package" in case you really upset the manufacturer's original design.
    • CommentAuthormike7
    • CommentTimeOct 6th 2015
     
    True enough JohnG - they are certainly sophisticated gadgets, as I learnt when I tried to adjust the range of fan speeds offered using some rather unsophisticated methods. It knew something was up, and just tried to compensate. I gave up on that. However, my thermistor tweaks were only deceiving the HP's brain by a few degrees C and have created no problems, perhaps because the lie concerned a variable, whereas with the fan speeds the brain was looking for constants.
    • CommentAuthorcjard
    • CommentTimeOct 7th 2015
     
    When I hear inverter, I think of a device that turns DC into AC .. How does this apply to a heat pump, and why is it necessary for modulation ?
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 7th 2015
     
    Odd that, I always think of turning something upside down.
    A quick search showed this:
    To turn inside out or upside down: invert an hourglass.
    To reverse the position, order, or condition of: invert the subject and predicate of a sentence.
    To subject to inversion
    • CommentAuthorEd Davies
    • CommentTimeOct 7th 2015 edited
     
    AIUI they basically rectify the mains to DC then invert to (3 phase?) AC to run the motor. Adjusting the frequency of the output AC is used to control the motor speed hence get the modulation.

    I think they're called inverters because they do the inverse of the normal rectification of AC to DC. But they do also turn the voltage upside down, half the time.
    • CommentAuthormike7
    • CommentTimeOct 7th 2015
     
    Here's a trace of the current used by my Hitachi HP running at part load, plus the sinewave of the supply. I've an idea it's regulating the power in by something called a 'zero crossing switch', but I could be wrong. Whether it does as Ed says after that I didn't get to see, but I presume so.
      2012 07dexter and bristol 042.jpg
    • CommentAuthorEd Davies
    • CommentTimeOct 7th 2015
     
    How horrid. But pretty much anything which rectifies mains but doesn't take power across the full cycle will do that, I imagine.

    https://en.wikipedia.org/wiki/Variable-frequency_drive
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 7th 2015
     
    What happens to the power factor of the motor when you modulate down low?
    • CommentAuthormike7
    • CommentTimeOct 8th 2015
     
    Ed - why horrid? Seems rather nifty to me. And I think the circuit does that in order for it not to take power across the full cycle, rather than the other way about.

    ST - From the position of the little bump (indicating the current) near enough right under the peak of the supply volts wave I'd say we can take it that the power factor remains close to one.
    • CommentAuthorevan
    • CommentTimeOct 8th 2015
     
    As JonG says, unless it's specifically designed to respond to a "power" command, options are limited. The parameter you can always control is the set point - heat pumps tend to have a ramp down response as the return temperature approaches the set point, so you can play in this area to modulate the power.
    The response is always going to be quite slow though, especially to turn up.
    • CommentAuthorEd Davies
    • CommentTimeOct 8th 2015
     
    All the odd modern power supplies are a bit horrid in that they tend to only take current for a small part of the cycle causing all sorts of harmonics. With a high power device like your pump you can see how it's clipping the voltage waveform due to the resistive voltage drop which will likely cause all sorts of effects on harmonic frequencies with who knows what consequences for other equipment (or even people - but let's not get derailed by that). Luckily, of course, most other things these days are also using switching power supplies so are likely to be fairly tolerant. Still, I'm a tiny bit surprised they're allowed to do this with such high-power equipment.
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 8th 2015
     
    Posted By: Ed DaviesStill, I'm a tiny bit surprised they're allowed to do this with such high-power equipment.
    I wonder if that is part of the reason that some DNOs are not keen on people having HPs if they are at the end of the line.
    • CommentAuthormike7
    • CommentTimeOct 8th 2015
     
    Ed - ah yes, I hadn't thought of that, although the power at the time was 490W, so not all that high. The flattening of the voltage waveform was evident and constant all the time btw, including when I plugged in a 3kW heater in place of the HP, so it's a coincidence that it appears to tie in with the current draw.

    ST - maybe that disapproval relates to high-power on/off HPs that have momentarily high starting currents rather than the likes of my 1.1kW max input tiddler with its modulation and slow ramp-up?
    •  
      CommentAuthordjh
    • CommentTimeOct 8th 2015
     
    Posted By: mike7The flattening of the voltage waveform was evident and constant all the time btw

    Something's wrong there then!
    • CommentAuthormike7
    • CommentTimeOct 8th 2015 edited
     
    Posted By: ActivePassive
    I am in the process planning to install a solar PV and an ASHP, and am looking around for an ASHP where you can control the modulation, so I can match the heat pump's consumption to the solar PV, minimising consumption from the mains at 15p/kWh versus export at 5p/kWh.

    Does anyone know of a heat pump where it is possible to easily control modulation directly?


    Posted By: evan The parameter you can always control is the set point - heat pumps tend to have a ramp down response as the return temperature approaches the set point, so you can play in this area to modulate the power.


    I can indeed control the set point, but only by personally pressing the button on the remote! Presumably ActivePassive will want to do something automated. That would be beyond my pre-digital-era brain to devise, whereas if it's a question of tweaking a resistance value I'm in with a chance, though still not clear to me how I might link that to PV output.
    • CommentAuthormike7
    • CommentTimeOct 8th 2015 edited
     
    Posted By: djh
    Posted By: mike7The flattening of the voltage waveform was evident and constant all the time btw

    Something's wrong there then!


    Have just looked at the supply waveform - the one in the photo is 3yrs old. Just the same now if not worse. I think our local transformer has atrial fibrillation. Needs a cardiologist to interpret?

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