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A big issue right now is the oversizing of heat pumps by HVAC contractors who are used to sizing gas furnaces using very simple btu/hr/sqft calculations. A con

by danans 3y ago

A big issue right now is the oversizing of heat pumps by HVAC contractors who are used to sizing gas furnaces using very simple btu/hr/sqft calculations. A contractor should do some kind of heating need analysis, at the least by studying old utility bills, and ideally by doing a Manual J heat loss calculation for the house. But almost none of them do that. There are some startups attempting to handle these design steps as a service, but the construction industry is slow to adopt new technology.

jacobolus 3y ago
Can you explain why this is a big issue?
- zbrozek 3y ago
  The simplest devices are on or off. If they are grossly oversized they cycle a lot and make homes less comfortable by pushing a huge volume of hot or cold air briefly. Variable speed compressors are better, but blowers may not also be variable speed. So you'll get better efficiency but may still suffer a feeling of draftiness. A properly sized variable speed unit will operate within some optimal band of efficiency and constantly output nearly the minimum necessary air volume to achieve the target temperature.
  - jlawrence6809 3y ago
    When I was shopping for a mini split recently all the models I could find seemed to be variable speed compressor and blowers, unless I was reading something wrong. So maybe this isn't an issue anymore? I oversized mine but wish I went even bigger after a cold snap we just had in the pnw this last winter.
    - zbrozek 3y ago
      Mini splits are better in this regard than traditional ducted installations.
- danans 3y ago
  Higher up front equipment costs , discomfort, potential humidity issues, higher energy costs, shortened equipment life, increased noise.
- supertrope 3y ago
  Combustion furnaces deliver very hot air so they can cycle on once in a while and then turn off. Heat pumps deliver warm not hot air so they have to run longer cycles. The slower rise in room temperature can even mean that it makes more sense to set a constant temperature on your thermostat instead of letting the temperature fall when you're away during the day (the conventional logic to save energy).
  - notyourwork 3y ago
    Why is this an issue? We have a ducted/ductless system. Air handler for ducts on first floor and basement. Upstairs has a mini split in each room. Having air handler constantly push air at a reasonable temp makes for a very comfortable living condition. Compared to a furnace that’s on/off with wider temperature range and more divergent temperatures throughout the area.
- moribvndvs 3y ago
  I’ve had one for three years. While I generally love it, it cools too fast, which means it sucks at removing humidity, actually increases it a little. The installer over-provisioned.
  - Tarrosion 3y ago
    I've heard this before -- that oversized cooling units (whether standalone AC or part of a heat pump) mean muggy interiors in the humid seasons. But...why? I'd think that a fixed amount of air compressed in the compressor means a fixed amount of condensation runoff from the unit, and it wouldn't matter much whether it's a big unit running occasionally or a small unit running frequently. Why is that wrong?
    - lbotos 3y ago
      I don't think it's about duty cycle -- it's seriously about speed of temp change and I _think_ dew point. But yes, I have a heat pump and in NYC Summer I cannot run it on anything but low otherwise it increases the humidity. It took me a few weeks of looking at the temp humidity graphs to understand that point.
      - BenjiWiebe 3y ago
        As it was explained to me, it is about duty cycle. The condensation doesn't instantly accumulate enough to make droplets and run all the way down the drain, so if the compressor only runs briefly the condensation is still on the fins and evaporates again. You need to keep the compressor going long enough that you actually have water running down the drain, instead of condensing/evaporating cyclically.
        lbotos 3y ago
        Yes, I believe you are right!
    - throwup238 3y ago
      It's not a fixed amount of condensation because the air around the AC only has so much humidity. It quickly condenses just a little bit of water and then shuts off before the humidity in the rest of the house can redistribute to replace the now dry air. By running for longer it allows the water in the rest of the house to actually make it to the compressor.
Mountain_Skies 3y ago
Other than higher cost for the heat pump unit, are there any other issues with having one that's oversized for heating? My understanding is that for cooling, having one that is oversized can be a problem because of humidity issues, which also makes me wonder what can be done if a space has an imbalance between the size needed for cooling versus what it needs for heating.
- danans 3y ago
  > Other than higher cost for the heat pump unit, are there any other issues with having one that's oversized for heating? - Short cycling leading to lower equipment life (also true for gas furnaces, although heat pumps have more moving parts) - Greater discomfort as the house heats up rapidly then cools down rapidly (especially if it has a leaky building envelope). - Higher peak electric loads, possibly during hours of high electricity prices, leading to higher electricity costs. > My understanding is that for cooling, having one that is oversized can be a problem because of humidity issues, Yes, because moisture will build up when it isn't running. > makes me wonder what can be done if a space has an imbalance between the size needed for cooling versus what it needs for heating. Many heat pumps have different output ratings for heating and cooling modes to deal with this. Often however, this has as much to do with the distribution of the heated/cooled air and placement of supply registers, which is often an afterthought when the system is purposely oversized (which is presumed to make up for lack of air distribution design).
- bluGill 3y ago
  No there is not IF the heat pump is variable speed/flow. Many modern ones are, but most old ones are not, and it is really hard to tell if a new one is. In fact you want one oversized as if it is the correct size for cooling it cannot make enough heat when it is cold (it can make heat, just not enough) If you get a normal single speed heat pump see the other reply - there are significant downsides.
deleted 3y ago
[deleted]
seltzered_ 3y ago
A possibly helpful thread related to the heat pump retrofit process and the need to assess context for each home: https://twitter.com/karakara98/status/1598718540365938688 https://twitter.com/karakara98/status/1598718540365938688 The other thing I've kept thinking about is the replacement interval , material input (compressor, refrigerant, etc.) and ease of maintenance. They generally have a lifetime of 10 years IIRC.
- danans 3y ago
  > The other thing I've kept thinking about is the replacement interval , material input (compressor, refrigerant, etc.) and ease of maintenance. They generally have a lifetime of 10 years IIRC. The compressor and refrigerant should never have to be replaced as a maintenance item. The compressor should be cleaned off occasionally since it's outdoors, especially if you live in an area with high dust/pollen, but that's no different than an AC. The interior air handler maintenance is the same as for any furnace or AC system. The lifetime is also similar to a similarly built AC.
bagels 3y ago
I'd much rather have oversized than undersized and still be sweating inside when it's hot outside.
jillesvangurp 3y ago
It's a big topic everywhere. A lot of early adopters of heat pumps end up with relatively expensive and inefficient solutions because the people that install them barely know what they are doing. There's a lot of demand for heat pumps so most people doing the installation work are very new to this. The good news is that they are probably learning from their mistakes and gaining a lot of knowledge. The bad news is of course that a lot of people are getting a bit of a bad deal. Things will improve in a few years but until then you really need to be careful with making sure you get the right stuff installed properly by the right people for the right price.
notyourwork 3y ago
Why is oversizing a problem? Is it cost or something else?

by danans 3y ago

jacobolus 3y ago

Can you explain why this is a big issue?

zbrozek 3y ago
The simplest devices are on or off. If they are grossly oversized they cycle a lot and make homes less comfortable by pushing a huge volume of hot or cold air briefly. Variable speed compressors are better, but blowers may not also be variable speed. So you'll get better efficiency but may still suffer a feeling of draftiness. A properly sized variable speed unit will operate within some optimal band of efficiency and constantly output nearly the minimum necessary air volume to achieve the target temperature.
- jlawrence6809 3y ago
  When I was shopping for a mini split recently all the models I could find seemed to be variable speed compressor and blowers, unless I was reading something wrong. So maybe this isn't an issue anymore? I oversized mine but wish I went even bigger after a cold snap we just had in the pnw this last winter.
  - zbrozek 3y ago
    Mini splits are better in this regard than traditional ducted installations.
danans 3y ago
Higher up front equipment costs , discomfort, potential humidity issues, higher energy costs, shortened equipment life, increased noise.
supertrope 3y ago
Combustion furnaces deliver very hot air so they can cycle on once in a while and then turn off. Heat pumps deliver warm not hot air so they have to run longer cycles. The slower rise in room temperature can even mean that it makes more sense to set a constant temperature on your thermostat instead of letting the temperature fall when you're away during the day (the conventional logic to save energy).
- notyourwork 3y ago
  Why is this an issue? We have a ducted/ductless system. Air handler for ducts on first floor and basement. Upstairs has a mini split in each room. Having air handler constantly push air at a reasonable temp makes for a very comfortable living condition. Compared to a furnace that’s on/off with wider temperature range and more divergent temperatures throughout the area.
moribvndvs 3y ago
I’ve had one for three years. While I generally love it, it cools too fast, which means it sucks at removing humidity, actually increases it a little. The installer over-provisioned.
- Tarrosion 3y ago
  I've heard this before -- that oversized cooling units (whether standalone AC or part of a heat pump) mean muggy interiors in the humid seasons. But...why? I'd think that a fixed amount of air compressed in the compressor means a fixed amount of condensation runoff from the unit, and it wouldn't matter much whether it's a big unit running occasionally or a small unit running frequently. Why is that wrong?
  - lbotos 3y ago
    I don't think it's about duty cycle -- it's seriously about speed of temp change and I _think_ dew point. But yes, I have a heat pump and in NYC Summer I cannot run it on anything but low otherwise it increases the humidity. It took me a few weeks of looking at the temp humidity graphs to understand that point.
    - BenjiWiebe 3y ago
      As it was explained to me, it is about duty cycle. The condensation doesn't instantly accumulate enough to make droplets and run all the way down the drain, so if the compressor only runs briefly the condensation is still on the fins and evaporates again. You need to keep the compressor going long enough that you actually have water running down the drain, instead of condensing/evaporating cyclically.
      - lbotos 3y ago
        Yes, I believe you are right!
  - throwup238 3y ago
    It's not a fixed amount of condensation because the air around the AC only has so much humidity. It quickly condenses just a little bit of water and then shuts off before the humidity in the rest of the house can redistribute to replace the now dry air. By running for longer it allows the water in the rest of the house to actually make it to the compressor.

Mountain_Skies 3y ago

Other than higher cost for the heat pump unit, are there any other issues with having one that's oversized for heating? My understanding is that for cooling, having one that is oversized can be a problem because of humidity issues, which also makes me wonder what can be done if a space has an imbalance between the size needed for cooling versus what it needs for heating.

danans 3y ago
> Other than higher cost for the heat pump unit, are there any other issues with having one that's oversized for heating? - Short cycling leading to lower equipment life (also true for gas furnaces, although heat pumps have more moving parts) - Greater discomfort as the house heats up rapidly then cools down rapidly (especially if it has a leaky building envelope). - Higher peak electric loads, possibly during hours of high electricity prices, leading to higher electricity costs. > My understanding is that for cooling, having one that is oversized can be a problem because of humidity issues, Yes, because moisture will build up when it isn't running. > makes me wonder what can be done if a space has an imbalance between the size needed for cooling versus what it needs for heating. Many heat pumps have different output ratings for heating and cooling modes to deal with this. Often however, this has as much to do with the distribution of the heated/cooled air and placement of supply registers, which is often an afterthought when the system is purposely oversized (which is presumed to make up for lack of air distribution design).
bluGill 3y ago
No there is not IF the heat pump is variable speed/flow. Many modern ones are, but most old ones are not, and it is really hard to tell if a new one is. In fact you want one oversized as if it is the correct size for cooling it cannot make enough heat when it is cold (it can make heat, just not enough) If you get a normal single speed heat pump see the other reply - there are significant downsides.

deleted 3y ago

[deleted]

seltzered_ 3y ago

A possibly helpful thread related to the heat pump retrofit process and the need to assess context for each home: https://twitter.com/karakara98/status/1598718540365938688 https://twitter.com/karakara98/status/1598718540365938688 The other thing I've kept thinking about is the replacement interval , material input (compressor, refrigerant, etc.) and ease of maintenance. They generally have a lifetime of 10 years IIRC.

danans 3y ago
> The other thing I've kept thinking about is the replacement interval , material input (compressor, refrigerant, etc.) and ease of maintenance. They generally have a lifetime of 10 years IIRC. The compressor and refrigerant should never have to be replaced as a maintenance item. The compressor should be cleaned off occasionally since it's outdoors, especially if you live in an area with high dust/pollen, but that's no different than an AC. The interior air handler maintenance is the same as for any furnace or AC system. The lifetime is also similar to a similarly built AC.

bagels 3y ago

I'd much rather have oversized than undersized and still be sweating inside when it's hot outside.

jillesvangurp 3y ago

It's a big topic everywhere. A lot of early adopters of heat pumps end up with relatively expensive and inefficient solutions because the people that install them barely know what they are doing. There's a lot of demand for heat pumps so most people doing the installation work are very new to this. The good news is that they are probably learning from their mistakes and gaining a lot of knowledge. The bad news is of course that a lot of people are getting a bit of a bad deal. Things will improve in a few years but until then you really need to be careful with making sure you get the right stuff installed properly by the right people for the right price.

notyourwork 3y ago

Why is oversizing a problem? Is it cost or something else?