Ducted Heat Pump Airflow/Air Volume Question

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#2720880 8-Jun-2021 19:45

jonathan18: Essentially, the original design and install was apparently so poor they’ve determined it’s better to start from scratch. Basic flaws like the wrong type of defusers (sideways not downwards firing); a spaghetti of ducting that’s not sized or designed correctly (by this I mean the way the ‘tree’ of ducting is arranged, if that makes sense) or even laid out properly - this also provides an opportunity to improve R value of the ducting; wrong diameter ducting/diffusers meaning many can’t output the air volume necessary for the space (and will be too noisy in attempting to do so) - replacing the 150mm ones with 200mm, plus adding in another one in the lounge, and retiring one in the hallway; return vents that are too small in capacity to return the needed amount of air...

That thread you linked to is interesting, the comments by @gedc. I've found many similar things with the system I had taken out because the installers weren't experienced enough to fix problems. Heat stratification is a big problem unless you use the correct type of diffusers, careful duct layout is important for efficiency and reducing noise, etc.

Assume you're upgrading from R0.6 to R1.0 ducting? It looks to be about 25% more expensive. The ducting might cost $1000 so the 25% increase isn't really that material. I suspect good / poor installation could make as much difference. As for insulating over it, ducting can be 400mm, 300mm, or 200mm so it would be quite tricky to insulate over. In the US they design and build this stuff into their houses so the ducts travel in insulated spaces.

I have 200mm ducts but I'm trying to work out if I should go to 250mm or 300mm in the lounge. After watching some diffuser design videos on youtube I think larger is quieter but lower velocity so it won't mix as well with the cooler air in the room, creating the stratification issues that are quite common.

If I knew at the start of this journey what I know now I may not have bothered with this. I could have kept the high-wall in the lounge, had a mullti-split put into the bedrooms, and while we wouldn't get our wall space back it would be cheaper and probably more effective, just uglier.

gedc

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#2720886 8-Jun-2021 20:05

Hi Timmmay. Thx for the mention.

Good luck with it all. Ours still functioning well albeit the unit upstairs decided to leak water (a lot) from the condensation drain connection. Now fixed...and a hole in the plasterboard to patch :)

Jase2985

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#2720888 8-Jun-2021 20:09

To be far Timmmay you were rather picky in what you wanted and didnt want which may not have helped with things

doors shut, vents etc etc

dasimpsonsrule

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#2720898 8-Jun-2021 20:33

The missing part of your equation is static pressure. The 700l/s spec will only be able to be supplied with very low static pressure usually, ie no ducts or very little ducting attached. As static pressure increases, airflow decreases. Static pressure is affected by your ducts, how many bends you have in them and how big they are. Bigger, straighter and shorter duct runs have lower static pressure, and will allow you to get better airflow.

Here is what a typical airflow curve would look like:

So you are not really loosing that much airflow, the reduction is caused by increased static pressure.

timmmay

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#2720903 8-Jun-2021 20:46

Jase2985:

To be far Timmmay you were rather picky in what you wanted and didnt want which may not have helped with things

doors shut, vents etc etc

We told the installer what we wanted to achieve, they agreed they could do it and the system would work. The main thing was we wanted a solution that helped regulate lounge and bedroom temperatures summer and winter, they agreed they could do that, and it couldn't due mostly to the north facing room. The system also pulled ceiling cavity air down into the house which is not good at all (lots of rat poison and such up there), and it was really loud - indoor unit in the ceiling, outdoor unit, and diffusers. So while yes I can be somewhat pickey, I don't think many people would've been happy with that system, and even the vendor agreed that it wasn't up to scratch and removed it.

Doors weren't a consideration initially, we didn't know anything about this type of system. We ended up with wall vents because my wife didn't want an inch cut off the bottom of the doors, which is what's apparently required for them to work well. The wall vents work quite well, and with them installed low down they work like a floor level return for bedrooms when the doors are closed. I'm happy with that side of things. Having to have the doors shut for maximum effectiveness isn't ideal, but I might change the diffusers to ones that push more air down so it works either way. If we get a floor level return that be handy, and one vendor said they only do residential systems with a floor level return as they have too many complaints about heat stratification otherwise.

Like others have found, ducted systems aren't a panacea like some think, but with good system design, a method for individual room temperature control in some situations, and good install I think they can be an effective whole house heating solution.

Jase2985

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#2720904 8-Jun-2021 20:57

im not talking about how the system ended up you had a lot of extra specific requirements over and above what you would normally have, through door vents, wall vents, doors closed, placement of things etc etc it may not have attributed to all your issues but some of them may not have helped things out.

It does sound like you got a shoddy job and i would never use HRV after their business practices prior to getting into air conditioning but thats as different story there.

You are correct about good system design too many people dont know much about that which is why companies that deal in commercial units are often better to go with as they do this day in day out.

timmmay

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#2720914 8-Jun-2021 21:30

Jase2985:

im not talking about how the system ended up you had a lot of extra specific requirements over and above what you would normally have, through door vents, wall vents, doors closed, placement of things etc etc it may not have attributed to all your issues but some of them may not have helped things out.

Your posts seem unnecessarily rude, as well as being incorrect. When I was talking to the company up front my requirements were to heat or cool three bedrooms and a lounge, keeping the bedrooms a bit cooler than the lounge. That's it. They put everything where they wanted them. Once the system was in I realised that instead of cutting the bottoms off doors I could put door or wall vents in, so I decided on wall vents and did it myself - nothing to do with the company installing the system.

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#2720958 8-Jun-2021 22:21

jonathan18: This is in a provincial city, so not one you guys reside in; we went with a company that has a long history of installing ducted systems, both commercial and domestic - I think the problem is many companies approach domestic ducted systems as a more complex version of a back-to-back split units - well, that certainly seems to be the case with the first company, as some of their errors were just so basic it was kinda embarrassing.

Most heat pump slingers aren't competent to install ducted systems with any sort of complexity. Many commercial installers aren't a great deal better IME.

They don't do Heat Load Calcs or design the ducting, they just biff it in any old way they can so of course there will be a ton of airflow problems.

It's very difficult to get good control across multiple rooms at the best of times but crap design and installation practice make it impossible.

Paul1977

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#2720971 8-Jun-2021 22:39

dasimpsonsrule:
The missing part of your equation is static pressure. The 700l/s spec will only be able to be supplied with very low static pressure usually, ie no ducts or very little ducting attached. As static pressure increases, airflow decreases. Static pressure is affected by your ducts, how many bends you have in them and how big they are. Bigger, straighter and shorter duct runs have lower static pressure, and will allow you to get better airflow.

Here is what a typical airflow curve would look like:

So you are not really loosing that much airflow, the reduction is caused by increased static pressure.

@dasimpsonsrule the specs say “external static pressure 35-50-70-100-125 Pa”. Are multiple values listed because these need to be manually set on the heat pump to match the actual static pressure caused by the ducting?

If so, what happens if it’s set to low or too high?

dasimpsonsrule

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#2720979 8-Jun-2021 22:53

Paul1977:

@dasimpsonsrule the specs say “external static pressure 35-50-70-100-125 Pa”. Are multiple values listed because these need to be manually set on the heat pump to match the actual static pressure caused by the ducting?

If so, what happens if it’s set to low or too high?

Not 100% sure but sounds like it may have a setting.

Too high would make the unit really noisy, too low would not deliver enough airflow and could cause the unit to generate high/low refrigerant pressure or temperature errors

SomeoneSomewhere

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#2721006 9-Jun-2021 01:14

Yes, most units have some configuration of the static pressure. I'm not sure of how smart they are. Sometimes it's DIP switches, sometimes it's in the installer settings in the wall controller. It's likely it needs to be on high as flexible duct is pretty high resistance compared to solid ducting, especially if installed poorly. It should be tight and dead straight on the straight sections, and not squished around corners.

Were they using a proper flow hood (measuring volume), or just holding an airspeed meter up and multiplying it by the area? The latter is very inaccurate.

Get the return airflow measured too.

rogercruse

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#2721015 9-Jun-2021 07:31

decibel:...I guess we were lucky, this is the one and only time we had a ducted system installed and we are very pleased with it...

We had wall mounted heat-pumps installed at our previous house and this gave us instant control and feedback for that room. However, these wall-units simply recycle the same air and we wanted something better.

Our ducted system has a heat recovery ventilator to improve the air quality and was installed as part of our new build a year ago. Still very pleased with it.

Jase2985

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#2721023 9-Jun-2021 08:48

timmmay:

Jase2985:

im not talking about how the system ended up you had a lot of extra specific requirements over and above what you would normally have, through door vents, wall vents, doors closed, placement of things etc etc it may not have attributed to all your issues but some of them may not have helped things out.

Your posts seem unnecessarily rude, as well as being incorrect. When I was talking to the company up front my requirements were to heat or cool three bedrooms and a lounge, keeping the bedrooms a bit cooler than the lounge. That's it. They put everything where they wanted them. Once the system was in I realised that instead of cutting the bottoms off doors I could put door or wall vents in, so I decided on wall vents and did it myself - nothing to do with the company installing the system.

all im saying is some of that information (doors close wall vents etc) might have better helped an installer pick and setup your system.

geoffwnz

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#2721080 9-Jun-2021 09:48

Jase2985:

timmmay:

Your posts seem unnecessarily rude, as well as being incorrect. When I was talking to the company up front my requirements were to heat or cool three bedrooms and a lounge, keeping the bedrooms a bit cooler than the lounge. That's it. They put everything where they wanted them. Once the system was in I realised that instead of cutting the bottoms off doors I could put door or wall vents in, so I decided on wall vents and did it myself - nothing to do with the company installing the system.

all im saying is some of that information (doors close wall vents etc) might have better helped an installer pick and setup your system.

The trouble was, having followed the threads, that this was not information that was known prior to putting in the request. As he'd said, "knowing then what I know now...". If it hadn't been for Timmmay's questions and research, I would not have known to request a second return duct so there was one in each half of the house that is frequently separated.

I think a number of us learned some interesting and useful information on ducted systems during the various installs. Particularly that one size/solution does not fit all purposes.

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#2721099 9-Jun-2021 10:48

SomeoneSomewhere:

Were they using a proper flow hood (measuring volume), or just holding an airspeed meter up and multiplying it by the area? The latter is very inaccurate.

@SomeoneSomewhere not an expensive flow hood. Looked like one they constructed themselves which funnels the air into a 150mm pipe that has a hand held meter mount at the bottom. I would have thought that using the correct formula to then convert the m/s reading to L/s should give a pretty accurate indication of volume? i.e. (m2 area of a 150mm circle) x (m/s reading from meter) = m3/s flow. Then m3/2 flow x 1000 = L/s flow?

Or is that still inaccurate?

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