Ducted Heat Pump Airflow/Air Volume Question

Forums › Home Workshop DIY › Ducted Heat Pump Airflow/Air Volume Question

Paul1977

4983 posts

Uber Geek

#287124 8-Jun-2021 09:36

I've been having continuing issues with my ducted heat pump ever since installation. The installers have been back to re-balance it a few times, but the underlying problem remains - it just seems to take a very long time to heat or cool (but costs a small fortune to run). I suspect this is actually being caused by a significant amount of conditioned air leaking into the roof space. But I'm hoping someone a little more knowledgeable could, based on what I'm describing below, confirm if my thinking is sound before I go back to the installers again.

Installer did airflow measurements with a hood after the last re-balance, and based on the size of the exit pipe on the hood I converted his m/s reading to L/s.

On high fan speed the combined air exiting all the vents is around 380 L/s, but the unit (fixed fan speeds) is rated at 700 L/s on that setting. Doesn't that mean that that 320 L/s are leaking into the roof space? The installer said something along the lines of the spec being based on standing right next to the unit, but that by the time it gets to the end of the ducts it's less - but that doesn't make sense to me. Surely if 700 L/s is exiting the heat pump then how could 320 L/s of it just disappear before getting to the vents?

Yesterday morning the outdoor temperature was a steady 10 degrees. As a second test I put a remote temperature probe in the roof cavity and then turned the heat pump to heating and high fan speed. The temperature in the roof space increased by 2 degrees within 30 minutes, and when I turned the heat pump off again the temperature up there dropped back down at a similar rate. To me this would also indicate a significant air leakage into the roof space.

Thanks

Home: Work:
Home Work

1 | 2 | 3 | 4 | 5

20424 posts

Uber Geek

Trusted

Lifetime subscriber

#2720603 8-Jun-2021 10:33

I have some limited experience. I had one ducted heat pump put in, it didn't perform well, I had them take it out. I have a new ducted system going in late June, Daikin with AirTouch4 from Blizzard in Wellington. I spent a lot of time researching and learning, what I say below is based on my limited experience.

Restricting the output of the heat pump with ducting / diffusers will reduce the throughput. Imagine blowing through a long pipe compared with blowing without. The system with no pipes might do 700L/s, but once ducting is installed the resistance of the pipes, the weight of the air, etc, will reduce it significantly. I don't think that is an issue.

Ceiling ducting is R0.6 so there will be some heat loss from the ducting, particularly since it has a large surface area. You should be able to tell if there's an actual air leak by physical inspection, particularly around the joins.

One thing I found with the ducted system I had removed is we had bad heat stratification - the ceiling was 30 degrees while the floor was 21 degrees. The default diffusers used are not really that good, they push the air out sideways, it flows across the ceiling, then into the ceiling return vent. You either need a floor level return (best) or diffusers that push the air down - downjet diffusers. I'm getting a look at a different square diffuser once Blizzard turns up which lets us change the direction of the air. Can you check for this? An easy way to check is to take the center plastic out of the diffuser if it's a screw fitting and see if things work better - it did for us. An IR thermometer is a useful tool, not too expensive.

AliExpress

Shop now on AliExpress (affiliate link).

Paul1977

4983 posts

Uber Geek

#2720616 8-Jun-2021 10:55

Thanks @timmmay.

I don't fully understand about the air volume reduction. Because of all the duct work does the heat pump not manage to create the full 700 L/s in the first place? So rather than being lost in transit, it was never created to begin with? Most of the duct work is pretty big - isn't a reduction of nearly half a lot?

What about the rapid increase and decrease in temperature in the roof space when the heat pump is used?

The rooms with high ceilings have rectangular grills blowing straight down, and the other rooms have these ones which we currently have locked in the "open" position.

timmmay

20424 posts

Uber Geek

Trusted

Lifetime subscriber

#2720625 8-Jun-2021 11:05

My understanding is the 700L/s is never created due to the air resistance. If you turn it on without pipes attached it will do 700L/s, but as soon as it has to push the air through the pipes, around bends, through diffusers, draw the air across the house, and back up the return vent the volume will be a lot lower. The weight of the air creates resistance against the motors.

Ceiling temp you'll have to check for leaks. If you've had them back to check it I doubt it's that. The pipes do have a large surface area and are only R0.6 so quite a bit of heat will escape.

The holyoake diffusers are the same ones I have, they're good in theory but my guess is 80% of the air goes across the ceiling and 20% max goes down into the room. Unscrew the center part and see if it improves things. It made a huge difference for us, and also reduced the noise significantly. Give this a shot to see if it helps much. With the rectangle "straight down" grills stand under it and see if you can work out where the air is going. It could be going straight down in a narrow column, across the floor, and up to the return vent without mixing properly. In that case you'd want a downjet style diffuser which spreads the air somewhat but still pushes it down, something like this.

Also, ducted systems do seem slower to heat or cool than high wall. A high wall pulls air in the top and blows it directly around the room which is very effective, and since it's inside the insulated envelope there's no heat loss. The ducted system pushes air in through poorly insulated ducts which has to mix with the air and be drawn across the room, it's reasonable volume but it's not like a high wall blowing it to all corners of the room. The best approach seems to be to leave it on most of the time. Personally in winter I turn heat pumps on 5am at the target temperature say 22 degrees, turn them down to say 18 degrees during the day, then assuming I was getting home about 5pm I'd have it come back up about 3pm, then off about 10pm. Leave it going like this for a week and the whole house will be heated through. When it gets really cold outside just never turn it off.

Paul1977

4983 posts

Uber Geek

#2720657 8-Jun-2021 11:31

We have R1.0 ducting, and a 2 degree temperature increase in 30 min sounds very fast to be a result of radiated heat from the ducts (to me anyway). We certainly don’t get a 2 degree increase in 30 min in the actual house.

timmmay

20424 posts

Uber Geek

Trusted

Lifetime subscriber

#2720660 8-Jun-2021 11:34

R1.0 is a little better, but still not a lot of insulation. Some heat will escape. I guess you need to examine the ducts for a leak, or get someone else qualified in to do some diagnostics.

I am interested to hear if unscrewing the diffuser centers helps. It helped a LOT for us. I'm getting rid of those diffusers in the lounge, might keep them in the bedroom because I put return grills in low down so the air is drawn across the room.

jonathan18

7413 posts

Uber Geek

ID Verified

Trusted

#2720672 8-Jun-2021 12:57

As another person who’s documented their many problems with a ducted system here on GZ, I feel your pain!

We’ve run our system for years without the central diffuser in place - as with Tim’s system, we got incredible heat stratification, which now I understand more makes complete sense: how the installers expected those diffusers to push air down from a 3m ceiling height...

We’ve finally bitten the bullet and had another company redesign the set-up, which will result in all ducting, dampers, diffusers and inlet grills being replaced - essentially, all that will remain are the indoor and outdoor units. This comes at a significant cost - well over $6k - but hopefully it’s worth it, as the system has never lived up to what it was supposed to deliver (and apparently never could, based on how it was designed and installed by the original company).

I guess we could have looked to find another company that can do this cheaper, but that’s what got us in this situation to begin with. I always warn those looking to have a ducted system installed that it’s a complex beast that can easily go wrong, to do due diligence on the company being used, and not to be swayed by price.

The two retrofitted systems I know that work well both use floor vents, and I agree this does appear to make it more likely it’ll work well.

timmmay

20424 posts

Uber Geek

Trusted

Lifetime subscriber

#2720675 8-Jun-2021 13:08

jonathan18: We’ve finally bitten the bullet and had another company redesign the set-up, which will result in all ducting, dampers, diffusers and inlet grills being replaced - essentially, all that will remain are the indoor and outdoor units. This comes at a significant cost - well over $6k - but hopefully it’s worth it, as the system has never lived up to what it was supposed to deliver (and apparently never could, based on how it was designed and installed by the original company).

@jonathan18 can you tell us more about the details of what you're having done, to help me and others avoid similar issues? Why does the ducting need to be replaced? What diffusers are you using? What have you learned in the process generally? Are you doing floor level returns with ceiling diffusers or vice versa, or just using different diffusers for better mixing / distribution of air?

decibel

311 posts

Ultimate Geek

#2720683 8-Jun-2021 13:38

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.

Reading the above comments though, we have our main return (520x520mm) mounted at floor level in the hallway and a smaller one (about 200x200) in the ceiling but in an out of the way area near the front door.

The main heated rooms have no returns and the air has to flow through doorways and around corners which we find OK as we seldom close internal doors.

timmmay

20424 posts

Uber Geek

Trusted

Lifetime subscriber

#2720684 8-Jun-2021 13:43

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.

Reading the above comments though, we have our main return (520x520mm) mounted at floor level in the hallway and a smaller one (about 200x200) in the ceiling but in an out of the way area near the front door.

The main heated rooms have no returns and the air has to flow through doorways and around corners which we find OK as we seldom close internal doors.

One vendor told me that a floor level return is really key to good performance, because otherwise the heat all just stays up by the ceiling. They said in some houses they can have air go into a grill in a hallway cupboard then out the top of that but that doesn't work for some homes. We're going to consider a floor level return later when we do some renovations in a suitable area.

Paul1977

4983 posts

Uber Geek

#2720685 8-Jun-2021 13:44

jonathan18: As another person who’s documented their many problems with a ducted system here on GZ, I feel your pain!

We’ve run our system for years without the central diffuser in place - as with Tim’s system, we got incredible heat stratification, which now I understand more makes complete sense: how the installers expected those diffusers to push air down from a 3m ceiling height...

We’ve finally bitten the bullet and had another company redesign the set-up, which will result in all ducting, dampers, diffusers and inlet grills being replaced - essentially, all that will remain are the indoor and outdoor units. This comes at a significant cost - well over $6k - but hopefully it’s worth it, as the system has never lived up to what it was supposed to deliver (and apparently never could, based on how it was designed and installed by the original company).

I guess we could have looked to find another company that can do this cheaper, but that’s what got us in this situation to begin with. I always warn those looking to have a ducted system installed that it’s a complex beast that can easily go wrong, to do due diligence on the company being used, and not to be swayed by price.

The two retrofitted systems I know that work well both use floor vents, and I agree this does appear to make it more likely it’ll work well.

@jonathan18 what city are you in? Who were the original installers (you can PM this if you prefer), and who the new installers are? And how did you decide on the new installers?

geoffwnz

1548 posts

Uber Geek

ID Verified

#2720713 8-Jun-2021 14:27

Interesting to keep an eye on these threads. Having had my ducted system for a few weeks now, I'm pretty happy with it. Seems to happily maintain 19ish degrees without issue.

I am wondering about maybe putting insulation blankets over the top of the ducting in the roof cavity to more or less include them in the thermal envelope.

Would the Downjet Diffusers fit into the same holes already cut for the circular "standard" diffusers I currently have? The current ones are circular with the screw in centre disc that I suspect will do the thing mentioned above and send air across the ceiling rather than downwards.

MotorDrive Rallying

timmmay

20424 posts

Uber Geek

Trusted

Lifetime subscriber

#2720718 8-Jun-2021 14:50

geoffwnz:

Interesting to keep an eye on these threads. Having had my ducted system for a few weeks now, I'm pretty happy with it. Seems to happily maintain 19ish degrees without issue.

I am wondering about maybe putting insulation blankets over the top of the ducting in the roof cavity to more or less include them in the thermal envelope.

Would the Downjet Diffusers fit into the same holes already cut for the circular "standard" diffusers I currently have? The current ones are circular with the screw in centre disc that I suspect will do the thing mentioned above and send air across the ceiling rather than downwards.

You can probably get downjet diffusers for the same cutout size. Just be careful not to install them over places people are sitting as it might be uncomfortable.

I thought about insulating over ducts as well, not sure if practical, interested to hear thoughts.

geoffwnz

1548 posts

Uber Geek

ID Verified

#2720727 8-Jun-2021 14:59

timmmay:

geoffwnz:

Interesting to keep an eye on these threads. Having had my ducted system for a few weeks now, I'm pretty happy with it. Seems to happily maintain 19ish degrees without issue.

I am wondering about maybe putting insulation blankets over the top of the ducting in the roof cavity to more or less include them in the thermal envelope.

Would the Downjet Diffusers fit into the same holes already cut for the circular "standard" diffusers I currently have? The current ones are circular with the screw in centre disc that I suspect will do the thing mentioned above and send air across the ceiling rather than downwards.

You can probably get downjet diffusers for the same cutout size. Just be careful not to install them over places people are sitting as it might be uncomfortable.

I thought about insulating over ducts as well, not sure if practical, interested to hear thoughts.

Looks like the ones you linked come in the same 150, 200 and 250mm sizes so *should* fit.

Yeah, dunno either but if I was doing it, I'd probably just blanket over the ducting and not the entire area, mainly so I can still easily see where to crawl without falling through the ceiling.
not sure if the weight of any insulation blanket would squash the ducts over time or not.

MotorDrive Rallying

jonathan18

7413 posts

Uber Geek

ID Verified

Trusted

#2720740 8-Jun-2021 15:45

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...

No, we’re sticking with ceiling-mounted returns, as the house design (a 1920s villa) doesn’t have any space that would work for floor-level returns; I get that this may be a compromise, but hopefully the re-work goes a long way to fixing the system’s flaws. 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. Here’s a link to the page on the thread I started in 2015, which is when it starts documenting the problems; scanning through it I see at least one other reporting similar hassles - https://www.geekzone.co.nz/forums.asp?forumid=141&topicid=175823&page_no=4

A word of caution about replacing the ‘mushroom’ type defusers with the downwards-firing type - I bought a bunch of these to do this a couple of years back, but the only one I could easily swap out was the 200mm one (which I think was simply a matter of replacing the centre piece). IIRC the external diameter of the 150mm mushroom type (ie the hole needed) was greater than that for the 150mm downwards type, or it could have been for another reason that I now can’t recall - but I do remember it was too tricky to solve! (As a result I have a few 150mm downwards firing diffusers if anyone is interested...)

Paul1977

4983 posts

Uber Geek

#2720762 8-Jun-2021 16:09

@jonathan18 I was quite interested in know who the new installers were as the may be able to recommend other good installers in different locations.

I'd like to get a second opinion on our current setup from someone other than the people who installed it, but I just don't know where to go. If it's behaving as expected, then it would be good to have someone independent confirm that. Likewise if it's not performing properly, I'd like someone independent to tell me why.

I personally don't think it's doing the job it should, but I don't have enough knowledge or experience to back up that up. The installers have been professional and courteous, but I just feel it's problematic having the company who installed the system being the ones to assess if there is a problem with the installation.

If anyone could recommend a really good company in Christchurch who could give a second opinion on our install would be appreciated.

1 | 2 | 3 | 4 | 5