Ducted Heat Pump Airflow/Air Volume Question

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17 posts

Geek

#2721238 9-Jun-2021 14:40

I would be interested in some more details on the size of your home and the equipment models used, etc. As a general rule, you should have the larger of roughly 33 L/s airflow for each 1 KW of heating or cooling or 1.5 L/s per m2 of floor area.

The return air grate should be located to constantly mix air throughout the house just like a heat transfer system. Follow those guidelines and everything should work great.

Duct sizing is important and maybe there are a few issues there. Any bends increase the duct friction substantially and air filters will greatly reduce airflow if your fan is not designed for higher static pressures.

Paul1977

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#2721251 9-Jun-2021 14:59

TechE2020:

I would be interested in some more details on the size of your home and the equipment models used, etc. As a general rule, you should have the larger of roughly 33 L/s airflow for each 1 KW of heating or cooling or 1.5 L/s per m2 of floor area.

The return air grate should be located to constantly mix air throughout the house just like a heat transfer system. Follow those guidelines and everything should work great.

Duct sizing is important and maybe there are a few issues there. Any bends increase the duct friction substantially and air filters will greatly reduce airflow if your fan is not designed for higher static pressures.

@TechE2020 It's a Mitsubishi PEAD 125. It's rated at 14kW for heating but (if I'm reading the specs right) can actually go up to 16kW. For cooling rated at 12.5kW but (again, if I understand the specs) can go up to 14kW. I'd have to do some more calculations for the house size as we have varying ceiling heights, plus part of it is a home occupation which we excluded from the ducted system and has it's own hi-wall unit.

TechE2020

17 posts

Geek

#2721259 9-Jun-2021 15:09

As long as the conditioned space is less than 300 m2 and is well insulated and double glazed, then that seems about right for the sizing. So likely a ducting issue with too much pressure loss which is reducing your overall flow.

Paul1977

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#2721272 9-Jun-2021 15:45

TechE2020:

As long as the conditioned space is less than 300 m2 and is well insulated and double glazed, then that seems about right for the sizing. So likely a ducting issue with too much pressure loss which is reducing your overall flow.

After excluding the garage and the home occupation space I think it would be under 180m2. Wall insulation is R2.8 on most walls (one wall is R4.0 as it had 140mm framing). Ceiling insulation is R6.0. Windows are low e excel with thermal break joinery. Ducting is all R1.0 instead of the usual R0.6.

We didn't really skimp on anything, which is why this is so frustrating!

The ducting all looks pretty big (but could be deceptive since it's R1.0). But if that's an issue hopefully the pressure setting on the unit can be adjusted to compensate. And I still suspect an air leak because of the rapid temperature changes I've record in the attic space. I'm no expert, but they seem to happen too quickly to be radiant heat from the ducting or heat pump.

If nothing else I'm learning a hell of a lot!

Thanks

TechE2020

17 posts

Geek

#2721276 9-Jun-2021 15:59

Paul1977:

TechE2020:

As long as the conditioned space is less than 300 m2 and is well insulated and double glazed, then that seems about right for the sizing. So likely a ducting issue with too much pressure loss which is reducing your overall flow.

After excluding the garage and the home occupation space I think it would be under 180m2. Wall insulation is R2.8 on most walls (one wall is R4.0 as it had 140mm framing). Ceiling insulation is R6.0. Windows are low e excel with thermal break joinery. Ducting is all R1.0 instead of the usual R0.6.

We didn't really skimp on anything, which is why this is so frustrating!

The ducting all looks pretty big (but could be deceptive since it's R1.0). But if that's an issue hopefully the pressure setting on the unit can be adjusted to compensate. And I still suspect an air leak because of the rapid temperature changes I've record in the attic space. I'm no expert, but they seem to happen too quickly to be radiant heat from the ducting or heat pump.

If nothing else I'm learning a hell of a lot!

Thanks

OK, that all looks great. This unit should be plenty big for your setup, so now it comes down to the pressure capability of your blower unit, the air filter pressure drop, and ducting friction losses. Did you have a maximum pressure for the blower unit or a chart like the one shared earlier? Looks like the installation manual requires a log-in.

openmedia

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#2721281 9-Jun-2021 16:07

Paul1977:

TechE2020:

I would be interested in some more details on the size of your home and the equipment models used, etc. As a general rule, you should have the larger of roughly 33 L/s airflow for each 1 KW of heating or cooling or 1.5 L/s per m2 of floor area.

The return air grate should be located to constantly mix air throughout the house just like a heat transfer system. Follow those guidelines and everything should work great.

Duct sizing is important and maybe there are a few issues there. Any bends increase the duct friction substantially and air filters will greatly reduce airflow if your fan is not designed for higher static pressures.

@TechE2020 It's a Mitsubishi PEAD 125. It's rated at 14kW for heating but (if I'm reading the specs right) can actually go up to 16kW. For cooling rated at 12.5kW but (again, if I understand the specs) can go up to 14kW. I'd have to do some more calculations for the house size as we have varying ceiling heights, plus part of it is a home occupation which we excluded from the ducted system and has it's own hi-wall unit.

We're looking at the same Mistubishi unit to drive 3 bedrooms as sadly with our 2 story house it isn't easy to reach downstairs.

Ideally we want individual per-room temperature management. Any other tips/guidance you can provide would be great.

Generally known online as OpenMedia, now working for Red Hat APAC as a Technology Evangelist and Portfolio Architect. Still playing with MythTV and digital media on the side.

Paul1977

5047 posts

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#2721283 9-Jun-2021 16:15

TechE2020:

OK, that all looks great. This unit should be plenty big for your setup, so now it comes down to the pressure capability of your blower unit, the air filter pressure drop, and ducting friction losses. Did you have a maximum pressure for the blower unit or a chart like the one shared earlier? Looks like the installation manual requires a log-in.

I think they left the installation manual in the attic, I'll have a look for it tonight to see if it has anything like that. The specs say high speed is 700 L/s and says "External Static Pressure: 35-50-70-100-125 Pa".

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Paul1977

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#2721287 9-Jun-2021 16:25

openmedia:

We're looking at the same Mistubishi unit to drive 3 bedrooms as sadly with our 2 story house it isn't easy to reach downstairs.

Ideally we want individual per-room temperature management. Any other tips/guidance you can provide would be great.

I'm not sure if that's directed at me, but my tip would be to learn all this stuff beforehand so you know all the right questions to ask. I'm trying to play catchup now - I thought I'd read up on it, but am learning there is A LOT more to it than I thought.

We also have AirTouch 4 zoning to give some individual temperature control. It's OK for what it is, but I think you'll get better individual control with separate high wall (or floor console) units - especially for non-communal areas like bedrooms. Probably cheaper too. We just didn't want a visible heat pump in every room, but if that doesn't bother you I'd look at that as an option.

I'm probably a little down on ducted systems at the moment though since I'm having issues, so take my opinions with a grain of salt!

SomeoneSomewhere

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#2721337 9-Jun-2021 19:16

That should be not too bad as long as they've calibrated it against a real one. That's basically all a real one is, anyway.

My understanding is that the airspeed is pretty dependent on exactly where in the pattern you measure the flow - the centre of the duct obviously has a much higher airspeed than the outsides. You can't really just go n square meters, and multiply it by the airspeed unless you take a *lot* of airspeed readings - this is the way they typically do large rectangular ducts, with holes in the duct and a pitot probe. But typically they do a 3x3 grid of values if not more, and apply correction factors.

dasimpsonsrule

147 posts

Master Geek

#2721342 9-Jun-2021 19:49

TechE2020:

Looks like the installation manual requires a log-in.

https://www.mitsubishielectric.com.au/assets/LEG/PEAD-RP71_140JAA_KD79D905H02_w(IM).pdf

Is probably similar enough to work. Page 21 has what you want.

timmmay

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#2721461 9-Jun-2021 20:59

Paul1977:

I'm not sure if that's directed at me, but my tip would be to learn all this stuff beforehand so you know all the right questions to ask. I'm trying to play catchup now - I thought I'd read up on it, but am learning there is A LOT more to it than I thought.

There's probably even more complexity than you have found yet, and more than I know too. I watch a 30 minute video on diffuser selection, my main takeaway was that to do this properly probably requires an engineer doing calculations rather than installers guessing. I guess that after doing a few dozen they'd find rules of thumb that mostly work, but there will be many variables. One technical thing I learned which I may or may not have mentioned is that larger diffuser = lower velocity air = quieter system but low velocity air doesn't mix as well as high velocity air and may result in heat stratification. So larger diffusers probably need to be downjet style to encourage mixing as the suction in the return duct pulls all that air across the room.

In the US in good houses (ie US$700K houses not massive mansions) they have engineers do the heating system design, specification, and such.

jonathan18

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#2721471 9-Jun-2021 22:08

timmmay:

…So larger diffusers probably need to be downjet style to encourage mixing as the suction in the return duct pulls all that air across the room.

Have any of you got any of this type installed? This is what will be going in to replace the current mushroom type. Provides for downward firing for hot air and sideways for cool - one variation switches automatically

https://www.holyoake.com/product-details/ECO-A_item.html?reg=NZ&ref_cat_id=ECO-Diffusers

timmmay

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#2721478 9-Jun-2021 22:23

Yes we have all of our diffusers that type from Holyoake. They push very little air downwards maybe 10% at most, most air goes sideways across the ceiling to the diffuser. Because the holes are small it's a fairly loud diffuser. We found them MUCH more effective and quieter with the center part of the diffuser removed. Before removal we had terrible heat stratification, with it out it was much better but it created a narrow column of air that reached the floor from a 2.8M stud. I think some diffusion would be better to spread the air.

I'm considering downjet and a square type of diffusers. I'm just trying to get hold of them to look at.

TechE2020

17 posts

Geek

#2721479 9-Jun-2021 22:32

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?

There shouldn't be a leak into the roofspace, but you may want to check that based upon your report that the roof temperature is going up. It would be pretty obvious with the system running if that much air is leaking out.

That said, the theoretical flow of 700 L/s is typically for a low amount of static pressure (say 5 Pa if you have it set to 50 Pa mode), so if you pressure is higher, then the flow will be less since the air will not get pushed through by the fan blades.

From the manual you linked to, you have a few static pressure settings. The default of 50 Pa would really be for a 0.5m duct when the unit is installed in a drop ceiling at a convention centre and a basic G2 filter that will catch big floating bits that you can see.

For a home installation with an F7 filter, you will want the 100 Pa or 125 Pa setting. This will consume more power and like be a bit louder, but it will likely be a necessity. Here is the table from the manual you linked to:

For a domestic, low-velocity system you will typically want to run it with a duct velocity of around 3 m/s with the blower at high speed. Using the 1.5 L/s per m2 of floor space rule of thumb, a typical bedroom will need to supply around 25 L/s. This can be supplied by a single 100mm diameter duct. Using a duct run of 5m and a flow rate of 25 L/s, the velocity will be 3.18 m/s and friction loss will be 8.64 Pa. See https://www.engineeringtoolbox.com/duct-friction-pressure-loss-d_444.html for the air-duct friction loss calculator.

If you can do this for all rooms and keep all of the ducts the same length and with gradual bends, then you do not need to balance the system or bring in an HVAC engineer to do more advanced methods such as equal friction or velocity reduction.

An air filter will have a very large pressure drop if you are using an F4 or F7 filter and this could easily be 100 Pa depending upon the size of the filter element. Take a look at one of these higher-end units that has an initial pressure drop of 85 Pa (and final of 300 Pa) for a flow rate of 3400 m3/hr (944 L/s): https://filtercorp.co.nz/ventilation-products/filtalon-compact-filter-f7-1

Assuming the air filter loss is 85 Pa and that you just have a plenum will small ducts off of it, then you total pressure drop is 85 + 8.64 = 93.64 Pa.

Have a look at the duct sizing, length, and number along with the pressure setting for your blower and report back.

The other part to consider is how you are using it. A ducted system like this is designed to be run continuously and will likely take 3 to 6 hours to warm up the house from 10C to 19C if it has been off. By comparison, mini splits are extremely oversized from an airflow standpoint (typically around 100 L/s per kW of heating) since they need to handle heating rooms quickly depending up on the room you are in. An acceptable time is typically around 1 hour to heat up a room to a stead-state temperature.

timmmay

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#2721562 10-Jun-2021 06:22

(@jonathan18 I updated this reply significantly so I've deleted the old one and added a new one)

All of our diffusers are that type Holyoake from Holyoake. They push very little air downwards maybe 10% at most, most of the air goes sideways across the ceiling to the return vent. Because the holes in the center are small it's a fairly loud diffuser as well. We found them MUCH more effective and quieter with the center part of the diffuser removed. Before removal we had terrible heat stratification, with it out it was much better but it created a narrow column of air that reached the floor from a 2.8M stud. I think some diffusion would be better to spread the air, but in my new system I'm planning on removing the Holyoake diffusers from at least the lounge, and probably from every room. They might be ok if you have a floor level return drawing the air down, but otherwise I would strongly recommend a diffusers that is made to push much more of the air downwards for most circumstances.

The two diffusers I'm considering are "MDO Multi directional outlet 263sq" and a downjet style. The Multi Directional lets you change the direction of each of the sections and it's what Blizzard recommends. The Downjet are omnidirectional but are made to push air down and out, so maybe not so good if you're sitting under them but better for mixing the cold air with the warm. The downside of the square one is if it doesn't work well it would be a heck of a job to replace them with a round style.

Images below if the link copy and paste works, otherwise look on the links above.

@TechE2020 you seem to be extremely knowledgeable about this and extremely technical, are you an HVAC engineer or similar? Do you have any thoughts on difffuser selection, both style but diffuser size? I have a higher 2.8m stud, ceiling return, diffusers are not quite over our sofa but not far off, I want to ensure the heat reaches the floor without sitting in a huge downdraft. The downjet would blow air on us, but the square diffuser look like they could blow air down but not directly at us. Blizzard in Wellington seem good but they seem to use more "rule of thumb" than any calculations about diffuser size as far as I can tell. I'm getting a Daikin 10kw with an Airtouch4, it's to heat 3 bedrooms (one usually turned off) and a lounge.

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