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Old 03-11-2019, 10:07 PM   #1
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Can ducts/fans be added to use a Mini split for the whole house


Hi all,


I am trying to add a heat pump to my house which has no duct. It is a single level with about 1000 square feet and the basement is also a living area.


I live in Canada where temperature is around -5F (-20 Celsius) for several months in winter so heating is my main interest.



From what I read, Mini split heat pumps are the cheapest and most efficient these days so this is most probably what I would buy. Since these are mostly targeted at heating/cooling an open space area, I am thinking of adding some ducts to increase its range and ideally bring the heat everywhere in the house, including the basement.


One question I have is where would I put it for getting the most efficiency for my goal, first floor or basement?


From what I have read, installing just one unit for heating two floors is quite unconventional. Usually they would suggest a dual head for heating another floor. But the cost for those units are almost double so I don't think I want to buy a dual head. I would prefer a more powerful single head and move more air to reach everywhere.


One issue I see is how to design the ducts, since the heat source is not within a furnace because there would be no furnace at all. I could, in theory put a dedicated duct near the output of the mini split to get around this perhaps. But even with this, I would need to be mixing the return air (colder air) with the mini split air (hotter air) and recirculating the sum of the two which might not be warm enough. I am guessing this will put a big lag on the ability of the system to react to outdoor temperature changes.



Another idea I had was to have separate variable speed fans for each room fresh air vent. Then I could in theory increase CFM on demand for a room based on a temperature sensor (using house automation like z-wave or ZigBee). That could be too costly though.



One other issue I am trying to solve is how to connect a Heat Recovery Ventilator, a HEPA air filter and a dehumidifier within this setup. My latest idea was to let my locker room become kind of a 'clean room' and let all the ducks go in and out of that room which would become kind of the inside of a 'virtual furnace' where I could hook-up those units.


I still have quite a lot to learn so feel free to educate me!

Last edited by virtualpaul; 03-11-2019 at 10:34 PM.
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Old 03-12-2019, 05:17 AM   #2
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Re: Can ducts/fans be added to use a Mini split for the whole house


They make ducted mini splits. Use one of them.
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Old 03-12-2019, 05:33 AM   #3
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Re: Can ducts/fans be added to use a Mini split for the whole house


Yup, just get a ducted mini.
And at -5 for prolonged operation, Iíd want some sort of backup heat as well. Like electric baseboard radiators.
Youíll have to spec the unit carefully. As some of the cheap brands off the internet arenít going to heat well at that low of an outdoor temperature.
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Old 03-12-2019, 06:17 AM   #4
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Re: Can ducts/fans be added to use a Mini split for the whole house


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Originally Posted by beenthere View Post
They make ducted mini splits. Use one of them.

I had looked at one from Fujitsu but the brochure says that the wall unit can work down to -14.8F (-26C) but the ducted unit can only reach -5.8F (-21C). I confirmed with a Fujitsu support person. So I am guessing they are less efficient somehow. I am not sure why.



I will have a look at other duct units to see if one work down to -13F (-25C) since where I live it can go to much lower temperature (like -35F) on rare occasions, perhaps other brands do not have the same limitation.

Last edited by virtualpaul; 03-12-2019 at 06:32 AM.
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Old 03-12-2019, 06:29 AM   #5
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Re: Can ducts/fans be added to use a Mini split for the whole house


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Originally Posted by roughneck View Post
Yup, just get a ducted mini.
And at -5 for prolonged operation, I’d want some sort of backup heat as well. Like electric baseboard radiators.
You’ll have to spec the unit carefully. As some of the cheap brands off the internet aren’t going to heat well at that low of an outdoor temperature.

Yes thanks, I already have baseboard radiators so I would set them a bit lower than the wanted temperature so they only start when the system cannot supply enough heat.

Do you have any suggestion for selecting the unit BTU based on my house? It has R20 wall insulation and R40 in the ceiling. I live in climate zone 6.


I am using a list from neep.org that are supposed to work in cold climate region.
https://neep.org/initiatives/high-ef...urce-heat-pump

Last edited by virtualpaul; 03-12-2019 at 06:38 AM.
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Old 03-12-2019, 08:40 PM   #6
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Re: Can ducts/fans be added to use a Mini split for the whole house


I've used Fujitsu and Mitsubishi concealed ductless units. They are virtually silent with your ear against them. In the living space, you only hear air noise if the ductwork is small. (being variable speed, they are completely silent when not running full out.

I've also seen the ducted Mitsubishi zuma in action. It's one of the only models that can use backup aux heat, integrated into the system. It looks like a conventional air handler, but it's really expensive.

I've seen all the above run at -25*C out of the box. Using a mechanical thermostat and resister, it's common to trick it to keep running below that when needed. You'd most likely want to switch to your baseboards by then though.

For the size, you really need to do a load calc. There's some free and some paid versions available online, if you want to DIY. Www.loadcalc.net is a basic free whole house one, but far from perfect. I use it from time to time. Then once you have your numbers, add a buffer if the program hasn't already, and pick a unit that will output what you need at the design temp. (since you have backup, the design temp doesn't have to be the lowest temp you get, but just something reasonable.) You look at the performance charts to see how they perform at lower temps.

The ducted units lose a hair of efficiency due to the ductwork losses, but generally can be quite minimal in practice when done well. The spec numbers is their promise that it'll work at those temps. Due to potential ductwork issues, they give themselves a bit more buffer, even though they still can work. It's the same outdoor unit doing the hard work, after all.

I think you'd be fairly disappointed if you attempt to use a ductless model with a few fans to circulate the air, like in your plan. I don't think you'll get the results you're looking for and still be quiet and unobtrusive.

I suggest either the dual head, 2 separate systems, or a ducted option. Any of those will work quite nicely.

Cheers!
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Old 03-13-2019, 09:44 AM   #7
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Re: Can ducts/fans be added to use a Mini split for the whole house


Quote:
Originally Posted by supers05 View Post
...
For the size, you really need to do a load calc.
...
The ducted units lose a hair of efficiency due to the ductwork losses, but generally can be quite minimal in practice when done well. The spec numbers is their promise that it'll work at those temps. Due to potential ductwork issues, they give themselves a bit more buffer, even though they still can work.
...

Thanks a lot for all your suggestions.


I did a load calc using the site you gave me. So this is what I would need:
Cooling 10,398 BTU
Heating 37,244 BTU
This is higher that I thought.


I am puzzled why a ducted unit is less efficient than a wall unit. Unless the ducts are passing though the attic or a crawlspace where they would loose energy. Or if the air is not evacuated optimally because of a lack of CFM perhaps.



Seeing that 3 people recommends it, in my case, a ducted unit would optimize heat propagation in the whole house.


So in this case, the ducted unit would need to be installed in the basement where all the ducts are passing (in the floor space).


But how would it be connected to all the ducts. Would the ducted unit fan provide the ventilation needed for the whole house like a furnace blower. I would guess not. So I would need to add in-line fan(s) to help it reach the required CFM (1241 CFM if I use 400 CFM per 12,000 BTUs?) to propagate all those BTUs throughout the house. Hopefully it would not get in the way of the ducted unit efficiency.

Last edited by virtualpaul; 03-13-2019 at 09:50 AM.
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Old 03-13-2019, 09:48 AM   #8
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Re: Can ducts/fans be added to use a Mini split for the whole house


The ducted indoor unit is just like an air handler. They can be had in various sizes. They can also be had in various static ranges. Most are rated for low static.
No inline fans are needed.
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Old 03-13-2019, 05:21 PM   #9
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Re: Can ducts/fans be added to use a Mini split for the whole house


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Originally Posted by virtualpaul View Post
I am puzzled why a ducted unit is less efficient than a wall unit. Unless the ducts are passing though the attic or a crawlspace where they would loose energy. Or if the air is not evacuated optimally because of a lack of CFM perhaps..
Any heat loss / gained is part of the efficiency loss.

Creating pressure (inside the ductwork in this case) requires energy. Even if no airflow occurs, just maintaining that pressure using a blower requires energy. This is Also part of the efficiency loss.

If people use ductwork that's too small or otherwise restrictive, it can cause further degradation in performance.

The wall units don't need to make much air pressure at all, and have no duct losses. They are inherently more efficient, but only marginally when the ductwork is done well.

PS. For the calculations, you have to be very careful in the numbers you enter. Everything from the R valve of the walls, surface area, direction, roof R valve, windows, etc all make a difference. That particular site generally uses design temps that are a bit too mild for my taste. I always adjust them to match the location better.

Cheers!
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Old 03-15-2019, 09:38 AM   #10
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Re: Can ducts/fans be added to use a Mini split for the whole house


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Originally Posted by roughneck View Post
The ducted indoor unit is just like an air handler. They can be had in various sizes. They can also be had in various static ranges. Most are rated for low static.
No inline fans are needed.

The ones I found mentioned that they are 'compact duct'. I am wondering if this is what you meant. Some say these don't have much air moving power so I am not sure they would move enough air through all the ducts.
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Old 03-15-2019, 09:42 AM   #11
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Re: Can ducts/fans be added to use a Mini split for the whole house


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Originally Posted by supers05 View Post
The wall units don't need to make much air pressure at all, and have no duct losses...

This is why I thought that putting an in-line fan on a ducted unit would help it. Also from what I saw, the ducted ones are 'compact duct' so that would restrict air flow I imagine.


Quote:
Originally Posted by supers05 View Post

PS. For the calculations, you have to be very careful in the numbers you enter...

The calculation is not easy. I tried a few different tools and they all give me different numbers.
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Old 03-15-2019, 10:00 AM   #12
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Re: Can ducts/fans be added to use a Mini split for the whole house


The slim duct units arenít designed to have a lot of duct attached to them. Most are designed to be zoned and service the area theyíre installed in. Not push air through the whole house. Various manufacturers offer high static models, this may be better for your application.
Creating some experiment with inline fans isnít recommended and I think youíll find it isnít going to work well, if at all.
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Old 03-15-2019, 10:15 AM   #13
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Re: Can ducts/fans be added to use a Mini split for the whole house


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Originally Posted by roughneck View Post
The slim duct units arenít designed to have a lot of duct attached to them. Most are designed to be zoned and service the area theyíre installed in. Not push air through the whole house. Various manufacturers offer high static models, this may be better for your application.
Creating some experiment with inline fans isnít recommended and I think youíll find it isnít going to work well, if at all.

Ok so I guess I have to search for something else than 'slim duct' or 'compact duct'. I have not found any 'normal ducted unit' yet. Not sure how these are called. The central heat pump don't have blowers since they are meant to be connected in a furnace and the mini-split heat pumps seem to be either wall units or 'slim duct'. I'll try to search a bit more, perhaps I will find something.
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Old 03-15-2019, 10:20 AM   #14
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Re: Can ducts/fans be added to use a Mini split for the whole house


Central heat pumps have blower motors mounted in the air handler.
To decide what you need you must first figure out how much air you need you push.
The small ducted units are only intended to condition one or two rooms, not a whole house.
Mitsubishi and Samsung make high static air handlers. But they donít offer a warranty to DIY work.
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Old 03-15-2019, 10:46 AM   #15
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Re: Can ducts/fans be added to use a Mini split for the whole house


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Originally Posted by virtualpaul View Post
This is why I thought that putting an in-line fan on a ducted unit would help it. Also from what I saw, the ducted ones are 'compact duct' so that would restrict air flow I imagine.





The calculation is not easy. I tried a few different tools and they all give me different numbers.
The ductwork looks compact but there's an actual science to it. With proper calculations, and sizing it really isn't a problem.

The same goes with the heat load calculators. It's normal for them to have up to 10% difference between them as they use slightly different assumptions to make the interface easier. If you make sure that the outdoor design temps and other inputs are the same, the results should be close.

Cheers!
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