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Discussion Starter · #1 ·
Hello from a confused DIY renovator, thanks in advance for looking at this. I am renovating a 1800 SF house, Seattle area. Everything will be as air tight as I can get it, R30 ceiling, R30 floors, variable walls (re-insulating the old 2x4 stud walls). New windows, doors. There will be two direct vent propane fireplace inserts for emergency heat and Cadet wall electric heaters in each bathroom.

I ripped out an old electric forced air system including the leaky ducts in crawlspace. I am talking to vendors and getting quotes to install four ductless mini splits in the house. Ran the Loadcalc.net and came up with roughly 22,000 BTU heat required. (I'm not worried about cooling, whatever cooling I get will be fine with me). But then on Loadcalc.net, under Manual S it calcs "emergency maximum" of about 32,000 BTU.

These vendors first of all seem to be trying to sell me oversized heat pumps without doing a load calc, which is why I ran it myself. I am about ready to get quotes for the mini split system. Should I go with a 24,000 BTU heat pump and ignore the "emergency maximum" 32,000 BTU? I have the propane and the wall heaters for emergencies already.
 

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I’m just now going through the same process for my house, so thanks for that link to Loadcalc.net. I’m going to compare the result from that to the tool that I’ve been using. I’m in a similar climate just north of you.

I had originally been looking at a “multi-split” (one outdoor unit with multiple indoor), but soon realized that the ability for it to turn down (run at lower capacity) would be problematic when only one indoor unit was calling for heat. That is a problem that I have with the existing heating technology for the house. I’m now looking at three separate mini-split systems for our 2400 sq. ft. house. My goal is not so much to have the house comfortable in unusually cold (anything below freezing) conditions, but rather for the systems to run within their operating range for the normal outdoor temperatures during the heating season (about 0-12C, 32-54F). We can heat the house with wood fired units when the power is out, or provide auxiliary heat when it is unusually cold, so I don’t see the value in sizing the mini-splits for “emergency maximum” when there is a downside in oversizing them.
 

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Discussion Starter · #3 · (Edited by Moderator)
I had originally been looking at a “multi-split” (one outdoor unit with multiple indoor), but soon realized that the ability for it to turn down (run at lower capacity) would be problematic when only one indoor unit was calling for heat. That is a problem that I have with the existing heating technology for the house. I’m now looking at three separate mini-split systems for our 2400 sq. ft. house.
that’s an interesting idea, had not thought about it until I saw your post. For a while I have been looking at 4 mini split wall units fed by one heat pump. Splitting the system up would require two heat pumps but each would be smaller. One heat pump would feed 2 bedrooms and the other would feed living/dining/kitchen.
 

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You pick an outdoor unit that matches the capacity that you need according to your load calc. The indoor units can add up to more capacity than the outdoor unit. As long as you don't go for the cheapest mini splits out there, it'll run fine. With that said, dual and single zones are the most cost effective. 4 zones units are generally more expensive than 2 dual zones.

Look up the performance data on the unit and look for it's heating performance at 0°C. Usually they are rated at 2 or 3 outdoor temps, sometimes presented as a chart. You have to match your heating requirement to the heating capacity of the units at that outdoor temperature. With the wall heaters you don't have to worry about emergency heat.

Screenshot the results of the loadcalc and post them.
 

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I’m just now going through the same process for my house, so thanks for that link to Loadcalc.net. I’m going to compare the result from that to the tool that I’ve been using. I’m in a similar climate just north of you.
I used the LoadCalc website and came up with a heating load that was within 5% of the other tools that I’ve used. The cooling calculation requirement was way off though (10% of the load calculated by two other programs). Maybe it was user error, but like you I’m not really concerned about sizing for cooling. The heating load will define the size and there is no issue with oversizing the cooling in the very non-humid summer conditions where I live.

If you’re looking for another data point and don’t mind spending a tiny amount compared to the cost of the equipment in question, you could take a look at the software available at this website. It’s easy to use. The cost was US$49 for a 2 month license.

I hope that you won’t think that I’m hijacking your thread, but I’ll post some data and ask @supers05 for guidance on sizing for my house. It will probably be useful info for you, too.
 

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Look up the performance data on the unit and look for it's heating performance at 0°C. Usually they are rated at 2 or 3 outdoor temps, sometimes presented as a chart.
Do you know where that data could be found for Mitsubishi mini-splits? I've looked on the Canadian and US sites, but can't find it. This catalog gives the heating performance at -8C and +8C, but 0C would be a better value to use and I'm not sure that interpolating between the two would be valid, since the relationship between performance and temperature might not be linear. Thanks.
 

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Here’s an example of the turndown issue that I described above when using one outdoor unit with multiple indoor units. The spreadsheet snippet here shows the heating load for our house at -8, 0 and +8 Celcius (17, 32 and 46F) for the three mostly separate areas that it makes sense to have separate indoor units servicing.
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The graphic below (from page 32 of this) shows the smallest Mitsubishi outdoor unit that meets the 43K BTU/hr whole house load at -8C. The capacity range at +8C is 12,400-45,000, though, the lower end of which is higher than the heating load from any of the three zones in the house at that temperature. If my understanding is correct, if just one of the three indoor units was calling for heat, in order for it to maintain a constant indoor temperature the outdoor unit would be cycling on and off because it can’t turn down low enough to supply that small an amount of energy on a continuous basis. As I understand it, that “short cycling” is one of the things to avoid with HVAC equipment since it leads to premature failure. Is my understanding of this correct?
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At plus 8°C your heat loss is still greater than the minimum modulation. It would run continually. It won't short cycle as its control system prevents that. 43MBTU sounds high for Seattle. (-8°C.) You would have to have a fairly large house. That's why I asked for screenshots of your results. The capacity logs between 8°C and - 8°C is fairly linear. You have to look up the extended performance data for the model.
 

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At plus 8°C your heat loss is still greater than the minimum modulation. It would run continually. It won't short cycle as its control system prevents that.
In a multi-split system (one outdoor unit and three indoor) the maximum turndown of the outdoor unit is 12,400 BTU/hr, but any of the three indoor units, when they are the only ones calling for heat, use less than that. Are you saying that the system controls prevent independent operation of the indoor units?

43MBTU sounds high for Seattle. (-8°C.) You would have to have a fairly large house. That's why I asked for screenshots of your results.
I’m confident in the multiple Manual J calculations that I’ve done. 40% of the heat loss in the house is through the unusually large area of windows.

You have to look up the extended performance data for the model.
Do you know an online source for that for Mitsubishi units?
 

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The indoor units only shutoff when over the setpoint and the minimum throttling is still too much, or you turn them off. You don't save a lot and cause lots of problems if you try to keep rooms at radically different temperatures. They shouldn't be more than a few degrees at most, for comfort and to protect building materials. Generally if one room is cold, they all are. If you wanted to get one of the heat reclaim units then you could have south rooms with windows in ac with north rooms in heat, if you think that'll be a problem.

Are those the model line that you want? I'll look them up.
 

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The indoor units only shutoff when over the setup and the minimum throttling is still too much, or you turn them off. You don't save a lot and cause lots of problems if you try to keep rooms at radically different temperatures. They shouldn't be more than a few degrees at most, for comfort and to protect building materials. Generally if one room is cold, they all are. If you wanted to get one of the heat reclaim units then you could have south rooms with windows in ac with north rooms in heat, if you think that'll be a problem.
I’m afraid that I don’t understand your comment. Here’s a situation that happens occasionally during spring and summer that we can use as an example. The outdoor temperature is low but the sun is shining in the west facing windows present in two of the three zones of the house. The interior temperature goes over the setpoint for the thermostats in those zones and the heating shuts down. The temperature in the third zone is such that it still requires heat because there is no solar gain. If those three zones had independent indoor units tied to a single outdoor unit, and the cold zone only requested heat, I believe that the outdoor unit would cycle on and off because it could not turn down low enough to supply just one indoor unit. Is that not true?

Using the wood stoves also produces the same effect of heat required in only one zone or if we want the temperature in the bedrooms cooler at night, etc.

Are those the model line that you want? I'll look them up.
Thanks, but no. Right now I’m thinking that I’ll need three independent systems each with their own outdoor and indoor units, because of the issue outlined above.
 

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I’m afraid that I don’t understand your comment. Here’s a situation that happens occasionally during spring and summer that we can use as an example. The outdoor temperature is low but the sun is shining in the west facing windows present in two of the three zones of the house. The interior temperature goes over the setpoint for the thermostats in those zones and the heating shuts down. The temperature in the third zone is such that it still requires heat because there is no solar gain. If those three zones had independent indoor units tied to a single outdoor unit, and the cold zone only requested heat, I believe that the outdoor unit would cycle on and off because it could not turn down low enough to supply just one indoor unit. Is that not true?

Using the wood stoves also produces the same effect of heat required in only one zone or if we want the temperature in the bedrooms cooler at night, etc.


Thanks, but no. Right now I’m thinking that I’ll need three independent systems each with their own outdoor and indoor units, because of the issue outlined above.
Cycling on and off isn't a terrible thing, it's just something we aim to avoid when the demand is high enough to do so. Verses completely oversized and always cycling. Give me the model numbers that you're looking at and I'll look them all up.

Heat reclaim units can run heating and cooling at the same time. It transfers heat around your house rather then one heat outside and another pulling heat from cooler air. If it's a bigger concern in your house go with the slightly smaller unit and rely on the wall heaters for supplemental heating.

The downside of one unit is that if it breaks you lose all heating /cooling, and in the fairly narrow case during the shoulder seasons. Since you don't actually get very cold there, I'd go with a smaller unit. The downside is multiple units is that it takes up more space in the yard /wall, more parts to break, a worse turndown ratio, meaning that minimum capacity is higher as a percentage.

Like I said, I'll look it all up and see what I can find.
 

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Discussion Starter · #13 ·
4 zones units are generally more expensive than 2 dual zones.

Look up the performance data on the unit and look for it's heating performance at 0°C. Usually they are rated at 2 or 3 outdoor temps, sometimes presented as a chart. You have to match your heating requirement to the heating capacity of the units at that outdoor temperature. With the wall heaters you don't have to worry about emergency heat.

Screenshot the results of the loadcalc and post them.
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I ran the loadcalc again and got about 21,000 BTU needed for heating. About 7,000 BTU heating needed for the two bedrooms combined and the rest goes to living/dining/kitchen/family room which is all open and can be heated with two wall mounts. (Open floor plan with two propane fireplace inserts). The specs I found show heating capacity at 17 degrees F, did not find the spec for 0 degrees C.

If I split this into two outdoor heat pumps, one for the bedrooms and one for the rest of the house, I am looking at a ducted unit for the bedrooms, with a duct to each room:

9,000 BTU 17.3 SEER Concealed Duct Mitsubishi Mini-Split Single Zone H2i Hyper Heat Heat Pump - SEZ-KA09NAHZ By Mitsubishi Model SEZ-KD09NA4R1.TH / SUZ-KA09NAHZ, Heating capacity at 17 F: 7,600 BTU/hr.

Then I would end up with something like this with two wall mounted units for the rest of the house:
20,000 BTU 20 SEER Mitsubishi Dual Zone Heat Pump System 9+9 By Mitsubishi Model MXZ-2C20NA2 / 2-MSZ-FS09. Heat capacity at 17 F: 12,500 BTU/hr.

Wondering if that purchase makes sense for heating performance/comfort, compared to something like a four zone outdoor heat pump with 4 mini splits. The smallest outdoor unit for four zones I've found punching on the internet is 30,000 BTU.
 

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Discussion Starter · #14 ·
Do you know an online source for that for Mitsubishi units?
I've been looking on hvacdirect.com, engineering manuals are linked to the various models including Mitsubishi. I also have been looking initially at Mitsubishi, mainly because I'm told they offer the best warranty. Have not verified the warranty rumor yet though.
 

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I've been looking on hvacdirect.com, engineering manuals are linked to the various models including Mitsubishi. I also have been looking initially at Mitsubishi, mainly because I'm told they offer the best warranty. Have not verified the warranty rumor yet though.
Mitsubishi and fujitsu are king in this arena. Daikin being a no frills but still decent quality alternative.
 

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I've been looking on hvacdirect.com, engineering manuals are linked to the various models including Mitsubishi. I also have been looking initially at Mitsubishi, mainly because I'm told they offer the best warranty. Have not verified the warranty rumor yet though.
Thanks for that. The 500+ page Mitsubishi “Technical Data Book” available there is something that I couldn’t find on the manufacturer’s website.

Another resource that I found which allows a good apples to apples comparison between equipment from different vendors, because they have to submit standardized data, is here. The screenshot below is for the second Mitsubishi unit that you mentioned in post #13. The graphs allow you to see the range between minimum and maximum output at temps from 5 to 47F.

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