[ktkarch]

Ok. Got your attention yet?

Seems everyone either wants to replace existing HVAC with same, or just use mini-splits. Then the argument is more heads for the distant rooms. But in a 'retrofit' we have the existing ductwork. I want to put it to good use.

I want to rip out the guts of the 18 year old furnace, heat exchanger has been putting out 'sand' for years.

No AC in this unit, this is Seattle not New Mexico...:wink2:

My plan is to replace the HV system with 3 tons of Mini-split into 1000 sf open area, put simple low speed fan in old furnace's place so it takes tempered air from open area (return grill in this space) pulling it through existing/remaining electronic filter and out through half the remaining ducts to the outer area rooms (additional 1000 sf).

Seems that since I have the airflow system already installed by way of the old ductwork why not use it. If the fan can also turn on the electronic filter I save this simple appliance and retain its benefits for the distant rooms (and control the excessive cat hair from two pets).

So it all comes down to downsizing the blower and building the distribution box/channel to replace the existing furnace box. I'm thinking that since I'm not trying to get massively heated or cooled air to all rooms of the house immediately, or blow it past an obstacle like the heat exchanger, that a lower volume 'always on' fan would do the job. Much like the low level blower on a variable speed furnace or whole house/bath fan does. And since half the house is already 'supercooled' by the 3 ton mini-split (2 well spaced 18k heads) I only need to push enough air to get some movement into the other rooms. All are just one doorway away. Maybe 300 CFM for 6 registers?

So how does one size & find a low volume blower? Where does one get a furnace box replacement? I have to be sure the slower blower pushes the air through the ducts and not back out the 'return air' register. Any ideas? I'm sure I'm not the first one to think of this. Anyone else try it and have stories to tell?

 

[user_12345a]

What's the furnace's model number?

Generally the heat exchanger isn't overly restrictive.

This could really be as simple as cutting the fan speed if direct drive.

Even a belt drive blower can be slowed down.

I wouldn't try to modify the furnace other than cutting the fan speed, possibly using a ecm retrofit motor which can run at a very low speed.



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To be honest I don't know why on earth you're trying to replace a gas furnace with mini-split heatpump when you have ductwork.

Presumably it's to save energy and pull the plug on natural gas and/or add some cooling.

In a mild climate like seattle's you could get a air source central heatpump with a little bit of electric backup. Not as energy efficient as the modern mini-splits, but practical and provides good distribution.

Heck, there are variable capacity central heatpumps now that work like the mini-splits, super efficient, maintain capacity as the outdoor temperature drops.

 

[user_12345a]

Maybe 300 CFM for 6 registers?

Does the system only have 6 registers? what are the pipe sizes feeding them.

 

[supers05]

I'd personally just use the existing furnace and lower the fan speed. (or get a lower speed motor / ecm retrofit) The heat exchanger is generally easy enough to remove if you really don't want it. I'm failing to see the reasoning behind this though.

Cheers!

 

[beenthere]

A standard conventional A/C cools the house by moving roughly 400 CFM per ton of A/C. Generally the air being discharged from the A/C is 16 to 20 degrees cooler then the air entering it. If the air is only 8 to 10 degrees cooler, it will take twice as much air flow to do the same amount of cooling. So slowing the blower will do more to hinder your cooling then help it.

 

[bernie963]

Good Idea:

30 plus years ago (when I was just a young first time homeowner) I installed a Tempwood II top loading downdraft wood stove in my basement. I wanted to distribute the heat evenly through out the house. I removed the end of one return trunk line to suck in the heated air for distribution. Worked great with out any modification. All i did was run the fan whenever I was burning wood.The oil heat cycled normally.

during the cooling season I ran a large window ac upstairs and a dehumidifier in the basement. When doing that I removed the fan access cover and ran the fan 24/7. Worked ok, but not as good as the heat set up.

bernie

 

[user_12345a]

A standard conventional A/C cools the house by moving roughly 400 CFM per ton of A/C. Generally the air being discharged from the A/C is 16 to 20 degrees cooler then the air entering it. If the air is only 8 to 10 degrees cooler, it will take twice as much air flow to do the same amount of cooling. So slowing the blower will do more to hinder your cooling then help it.

all the more reason to install a conventional ac or better yet, heatpump.

 

[ktkarch]

Thank you all for the quick and thoughtful answers.

While status quo and an AC add on is an option, it is not as simple as that. The ideas about reusing the furnace's shell and down sizing the blower are sounding effective. Balancing the heat to the outer rooms is more important (twice the season length) in this climate than is the cooling so max air through ducts is also not necessary. So I'm leaning toward finding/costing a proper (read quiet) blower as my next step. I would appreciate more info on 'ecm' blowers. Stepping down the existing blower is not an option. Installed before addition and noisy even at its lower speed.

My reasons for switching out are many including personal:

-Original single stage 3 ton HV unit is at Front & Center of house and very noisy. Return air is 1' from blower. No way to change this or I would have as it echos throughout the entire house.
-Central Air unit won't come close to efficiencies available for mini-splits.
-Heat pumps w/ electrical resistance are more expensive to run in Seattle than the moderate climate leads one to expect. Many of my clients are choosing to replace them during their remodels.
-I'm not pushing 3 tons of cooling to the entire house through the ducts in the mini-split model, just enhancing convection cycle to max of 50% of house (1000 sf) at cooler evening to morning hours. Except on weekends and holidays.:wink2:
-Cost is the same to replace with same single stage (noisy) furnace and adding central air unit, as it is for the mini-splits, including a garage head.
-Central Air does not reach 'garage' home office (additional 570 sf).
-3rd head (9k) very efficient and replaces resistance heaters and window unit. Included in mini-split price so mini-splits come out ahead in cost and efficiency.
-Zoning available as the 'outer rooms' are bedrooms and pantry/laundry (100-160 sf ea) so the in-duct fan may be seldom used unless family and guests are about.
-Converting existing ducts to simply move air means no need for additional heads in each of 5 out lying rooms. Major cost savings and reduces the overkill of multiple heads in such small rooms.
-Size & location of compressor is a limiting item. The 'big' cube compressors of central systems are not a good fit for the more public location available. Simple wall mounting of a mini-split's compressor will work very well in the same location.
-Note my username: ktkarch -(ITECT) : my house is my business's showroom and it helps to show clients what is available and that it will work in their design too. And I certainly don't need the noise of the central HVAC when entertaining clients, designing or trying to sleep.

e.g. 11 total registers - 2 in baths, plus 4 already in the open area, these would be 'closed/sealed' as there is no reason to push air to them. They are directly serviced by the mini-splits. Bath vents can quickly change their room's air temp if necessary. Outer rooms (5-6) fed by 6" max. ducts, each on their own duct.

Since outer rooms are just 'a door away' (longest hall is 5') there is no reason to shove "5" air exchanges per hour to outer rooms via the ducts. Simply want to get a small convection movement to the far reaches of these rooms. The old style ducts put the vents on the far side of the rooms from the door where the mini-splits would have trouble reaching by simple whole house convection. Slow air from ducts should make it more effective to 'balance/enhance' the temps throughout the rooms without detrimentally affecting the natural movement of Mini-split's air flow. Each room already has a ceiling fan to assist mixing.

Some of the home's air will still pass through the electronic air filter reducing the cleaning load on the Mini-splits' filters. Especially essential for bedrooms.

This is a 60 year old rambler w/ a 15 year old addition that doubled it's size and connected it to the formerly detached garage. The remodel took it from no insulation up to basic 2001 code level R13/R19 4/6" walls, R19 floors, R40 ceiling, .35 U windows.

The remodel & garage is sized and positioned for up to 12 kw of PV solar, so moving to electrical based demands has its advantages... once the neighbor to the south can be convinced to cut down the overgrown, property line hugging ornamental plum tree her dad planted 45+ years ago.:vs_no_no_no:

Again, thank you all and feel free to continue commenting. I will let you know what system I eventually pull the trigger on in the next two-four weeks.

 

[beenthere]

How hot is the air coming out of the registers in heating season. You may not be able to slow the blower much and still use the furnace.

Lots of people with sun rooms thought they wouldn't need much air to keep it cool because its open to the rest of the house. They found out the hard way, they needed far more air then they thought.

Slow convention like your trying to achieve, simply causes the cooler air you did put in the room to have more time to warm back up.

 

[user_12345a]

This may not work as well as you'd think without heat sources in every room. In fact it may well be against code.

The idea of removing the heat exchanger from the furnace is silly. If anything, you should keep the furnace in case the mini-split system fails.

OR you could install a central heatpump and use gas for backup.

OR put in a small, quiet 2-stage or modulating furnace - very quiet especially in a mild climate. Existing furnace is likely oversized.

Heatpumps aren't all they're cracked up to be when compared to gas (they're fantastic when replacing resistance heat) The power plants producing the electricity are 30 to 40% efficient so you would need a COP of 3 to break even with a condensing furnace.

Most new power plants are gas.

Electric makes great sense if all your electricity comes from hydro though.

Using PV for heating is a very, very expensive proposition. Silly subsidies aside, it's not suitable for heating and without batteries you're just drawing from the grid a lot of the time especially in the winter.

If you want to heat with solar get a large solar-thermal system with lots of hot water storage, put a hydronic coil in the plenum.

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If you insist on carrying this out.

Do a search for ecm ecotech retrofit motor, it wires in an identical way to a regular psc minus the capacitor.

It has a 5th speed, very low which can be wired with a relay for low speed continuous fan.

 

[supers05]

Have you thought about one of these? It's the same tech as the ductless but is easier to retrofit. They use ecm blowers and modulate them just like the mini splits. (You would barely hear the fan unless the system was at full capacity.)

http://www.mitsubishielectric.ca/en/hvac/zuba-central/specifications.html

Or at least a ducted version for the few rooms that you don't need fine control over.

http://www.mitsubishipro.com/en/pro...es-heat-pumps---indoor-units-(multi-zone)/sez

Cheers!