Minisplit layout
Hello – I’ve recently received several quotes for updating our old gas boiler to minisplits or heat pumps for heating and cooling. I’ve had quotes range from ductless and ducted minisplits to a central ducted heat pump, but there does not seem to be a clear cut winner for what the best option for my house is. Cost is a factor as the central ducted unit is at the very top of our price. The price of all ductless units or a combination ductless and ducted minisplits is more reasonable.
The house is a 1966 center entrance colonial outside Boston that is fairly cut up into several rooms. I’ve attached a floor plan. We have hopes to open up the kitchen in a few years but are not sure what walls will realistically be able to come down. We are trying to keep our kitchen renovation options flexible when considering our current HVAC plan.
Heat load calculations from contractors have been around 50kBTU for the first floor and 30BTU for the second floor. I plan to work on a more aggressive manual J. We moved into the house in August so have limited gas usage history.
If I go with ductless units, do I need 4 heads for the first floor because of all the rooms and partitions? Would ducted be a better option for the first floor?
The only place an air handler can go for the 2nd flor is in my vented attic. Should I avoid putting an air handler up there and stick to ductless units for the 2nd floor with a head in each bedroom? I do not think we would like the comfort of a ceiling unit in a hallway outside the bedrooms.
Anything to consider for basement heating and cooling options? The basement is a finished with a walkout sliding door. Only 2.5 walls are below grade.
Thank you for any guidance you can provide!
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Replies
Hello Mod13, looks like a nice house based on the plans.
Question for you: What is the reason for switching from gas to electricity? You will better understand my question with my comments below.
You mentioned:
We moved into the house in August so have limited gas usage history.
Comment:
Some utility companies will provide historical figure if requested to new owners. Did you ask you gas utility if they would provide more data?
I would be surprised if they don’t. You just have to realize that the previous owners may have kept the place warmer or cooler than you might.
You mentioned
… old gas boiler to minisplits or heat pumps for heating and cooling.
And
Heat load calculations from contractors have been around 50kBTU for the first floor and 30BTU for the second floor.
Comment :
1. What is the design temperature in the manual J calculations to arrive at 80k btu/hr?
2. This page provides some temperature data.
https://weatherspark.com/m/26197/1/Average-Weather-in-January-in-Boston-Massachusetts-United-States
3. Cost of electric heat
Cooling is always going to be electric but heating we have many options. Have you priced out the cost difference to heat with gas vs electric? Our 1700 sf requires 13,000 btu/hr at -13F so going electric was something that we could manage. At 80,000 btu/hr + basement, that is a lot of electricity. I also want to move away from natural gas but it needs to be affordable.
4. Have you costed out the anticipated bills to heat with electricity?
If the gas utility refuse to provide the information, you could calculate an estimate using Heating Degree Days. This webpage explains how it works.
https://help.dudesolutions.com/Content/Documentation/Energy/UtilityDirect/Reporting/Understanding%20Heating%20and%20Cooling%20Degree%20Days.htm
https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/cdus/degree_days/
You will need a few figures to make the calculations
- # of heating degree days
- btu/hr (currently at 80k +basement)x 24hr = btu/day
- number of btu in a kilowatt
- number of btu for the units of gas (we use cubic meters)
- the COP of your unit (this varies based on temperature so you may need to pick a figure based on average outside temperature)
You mentioned
Cost is a factor as the central ducted unit is at the very top of our price.
Comment
See my comments about the air handler below. Because you don’t have space for the air handler, this is not a feasible option. Add that it is the costly option and I would myself absolutely rule it out. Done.
You mentioned
If I go with ductless units, do I need 4 heads for the first floor because of all the rooms and partitions? Would ducted be a better option for the first floor?
Comment
With our own house requirement of a total of 13,000 btu/hr so we could not find individual heads small enough to condition rooms individually.
Individual mini-split heads
With your requirement of about 10k btu/hr per room on the main floor, individual heads might make a lot of sense. You need a btu/hr figure for each room. The sun room is likely the outlier.
You could also position the head in a spot where it would provide airflow to more that one room. This is not always easy and provides less control per room so it may end up providing less comfort.
Ducted mini-split option
These are in the ceiling or wall and can have limited ductwork to 2-3 rooms. You need to be sure ductwork can be installed. Opening walls to install ductwork makes little sense to me. In fact I would do everything to avoid opening walls. This makes much more sense in a new construction or as part of a larger renovation.
For fully ducked, see my comments on air handler below.
You mentioned
The only place an air handler can go for the 2nd floor is in my vented attic. Should I avoid putting an air handler up there and stick to ductless units for the 2nd floor with a head in each bedroom?
Comment
It is not advisable to install HVAC equipment and/or ductwork in « unconditioned » space. There are all kinds of problems that arise, first being significant loss of efficiency but that is just one of many issues.
You can put heads in each room in my view.
Again, you can look at the «ducted mini-split » solutions for the 2nd floor. But opening walls always leads to finding other issues and expanding the scope of the project. Not my first choice personally.
You mentioned
Anything to consider for basement heating and cooling options? The basement is a finished with a walkout sliding door. Only 2.5 walls are below grade.
Comment:
If it is humid in the walkout basement, consider installing a dehumidifier. Other than that, you can use the same system as used for the main and 2nd level.
Of course, if this space has an additional heating load, that again puts a question mark on the affordability of going electric.
***general comments***
With the size of your requirement in terms of BTU/hr, you might consider more than one outdoor heat pump. Having one heat pump outside for the main floor and one for the 2nd floor would allow better control and better efficiency.
This is because all the heads one a given outdoor unit will work at the same time, as soon as even one head/room calls for heat. You may want to keep the main floor warmer during the day and cooler at night. The reverse is likely true for the 2nd floor. If the basement doesn’t have a separate outdoor heat-pump, I would expect it to be on the same one as the main floor.
After all of this, I still think you need to know how much it will cost to heat with electricity. First call the utility company for figures and if that is not an option then use heating degree days. If you have trouble doing the calculations, just post another question on GBA.