Heating Separate Rooms With Ducted Minisplit Heads
Looking for suggestions, limitations, and cautions for installing a mini split to heat two or three adjacent bedrooms. OR a better alternative. And maybe a way to justify suggestions to someone lacking an intuitive understanding of heat loss, heating system alternatives or quantitative analysis. (If you don’t want to read all the details, just read the summary.)
SUMMARY:
Judging from related GBA articles etc., I doubt a ducted mini split head would work very well for heating all three unequally-sized bedrooms differing greatly in heat load. But that seems the best alternative, given client’s preferences. Even worse, separate heads per bedroom are overkill and won’t work comfortably. The worst alternative is replacing the 41-year-old oil boiler combo central heating and domestic hot water system. Get a hybrid heat pump water heater. Each bedroom with a thermostat and aesthetically pleasing electric heating unit seems the best practical and cost-effective solution, but is rejected by the client. Your recommendations??
DETAILS:
GF’s house is located on Cape Cod, MA, climate zone 5A. (Attached is a floor plan with approximate dimensions and conductive heat loss estimates.) Single family detached three bedroom ranch circa 1979, single panes with storms, upgraded R-50 attic cellulose, likely R-11 fiberglass in walls, and existing R-19 fiberglass in floor joists above full basement (really!).
· Existing 41-year-old oil-fired boiler and single zone hydronic heat distribution system has space-hogging radiators plaguing all rooms including both bathrooms.
· New mini split replaced dead central AC dinosaur last summer. Sized for heating (minimal AC needed on coastal New England, Cape Cod location.) Minisplit head located in open living room area, with head located directly opposite a corridor to bedrooms (to direct heat down the corridor). Mini split head located within 4” of ceiling and 4” of adjacent wall, with no remote thermostat (GF won’t upgrade). The plan was to add a second mini split for bedrooms later, if needed.
· Kitchen/dining stays adequately heated and cooled, located semi-open to the adjacent living room and mini split head. Even the further distant converted breezeway room is okay, especially since its rarely used.
· No additional heating or AC is desired for kitchen/dining or breezeway rooms, just bedrooms.
When bedroom doors are open, with mini split fins directing heat down the corridor to bedrooms, heating and cooling the bedrooms works marginally. BF had used small, quiet fan to improve heating airflow. When bedroom doors are closed, insufficient heating of course. Typically, doors are left open for two bedrooms daily (and door usually remains closed mid-winter for unused small bedroom). Sometimes the larger, more remote bedrooms #1 or #2 are not heated well enough during the day, even with doors remaining open, when doing office work there. Last couple months, GF has been using oil-fired boiler for whole house heating, rather than relying on the mini split.
If ever needed, there is:
— a relatively new, small, efficient window AC unit;
— an old, less efficient, deafening window AC unit;
— and a few cheapo plug-in freestanding resistance heaters (which BF uses sometimes).
GF does not consider these crude appliances acceptable, except in emergencies.
There also is a brick fireplace in the living room that is never used (and air-sealed temporarily).
ALTERNATIVE SOLUTIONS:
1. GF wants to replace the failing oil-fired boiler for $6,500. Then during the summer, purchase a second mini-split with heads in each of three bedrooms for occasional air-conditioning (and heating as well). GF perennially worries about lack of money, but likes having multiple heating systems. GF doesn’t believe that oil heating is more expensive to operate then heat pump electric heating.
Insulation and air-sealing extremist BF, who despises fossil fuel central heating systems, has suggested:
2. Ditch (don’t replace) the global warming oil-fired boiler and pricey oil bills, along with its quaint single-zone distribution system (uninsulated heat distribution pipes and all);
3. Get a hybrid heat pump water heater with rebate; or
4. Get an electric resistance tank heater;
5. Continue to upgrade insulation and air-sealing improvements to reduce heating load;
6. Install individually controlled thermostats and wall, ceiling, cove radiant, or attractive resistance heating units in each of the three bedrooms (which didn’t go over very well with GF);
7. Consider one or two through-the-wall individual wall-hung heat-pump units ~$2,000 each (e.g.,
https://innova-usa.com/wp-content/uploads/2020/01/Brochure.pdf
8. Or add the planned second mini-split with single head ducted to two bedrooms. (Smaller bedroom rarely used by visitors a few times/year. Note smaller bedroom is in the center of the home, and likely needs minimal winter heating; but occasionally AC during summer with door closed and guest(s) sleeping there. GF says window AC unit not nice enough. GF insists any additional mini split must heat and AC small bedroom.
WHAT’S YOUR SUGGESTIONS, OPINIONS, CAUTIONS, AND RELATED EXPERIENCES? WHAT ALTERNATIVES WOULD YOU SUGGEST? AND HOW MIGHT YOU JUSTIFY TO A RESISTANT NON-ENGINEER? Thanks in advance for your replies.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
It's relatively easy to show the cost differences to operate the heat pumps vs the oil fired boiler, so I'd do that first -- get at least that argument out of the way with some hard numbers. Pick a median cold day in the winter, calculate the heating cost for the heat pump to operate at that temperature. Calculate the oil fired boiler operating cost for that same day for comparison purposes. Oil prices are low at the moment, so I'd use a weighted average price over the past decade or so which is probably more accurate in the short term. The heat pump should end up significantly cheaper to operate in comparison to the oil fired boiler, so you now have that point settled in favor of the heat pump system.
You can do a similar operating cost comparison for anything using electric resistance. My guess is that electric resistance will range from close in cost to the oil fired boiler in the best case, and will likely cost more. This should be a point against the electric resistance systems, which helps your "use a heat pump" argument. It might be useful to use a radiant panel in a few spots to avoid any cold spots though, so they do have their uses, but I'd try to limit them to niche applications like filling in cold corners and the like.
There are some arguments to be made in favor of redundant systems. If their concern is "what do we do when the power goes out?", point out that the boiler won't function without electricity either, so "power failure" isn't really a negative against a heat pump -- power issues affect all of these systems pretty much equally. If they want backup power and don't have natural gas available, a small diesel generator can often be connected to a heating oil tank to provide long runtimes in case of a power outage. The generator can then run the heat pump (or boiler), and solve the "what do we do when the power goes out?" problem. If they want to consider a generator, I'd go with natural gas fueled as a first choice, propane second (note that bigger tanks are usually needed than you might expect though), and diesel last. This is assuming a permanently installed backup generator.
If their primary concern is global warming and/or emissions in general, point out that most new electrical generation has been natural gas fired, so heat pumps aren't "emission free". This would be a point in favor of additional insulating/air sealing, which will limit energy use in general regardless of their heat source. If this is where they want to go, I'd attack the walls first, since R11 isn't much by today's standards. If they have a need for new siding, exterior continuous insulation would gain them a lot of insulating performance in the walls. I'd check the rim joist area too, you might have some low hanging fruit where a relatively small and simple project might have a big gain in energy efficiency for them. With the recent cold snap, an IR camera would be a good way to show them if their rim joist area is leaky and could benefit from some insulating and air sealing work.
A zoned system with occupancy sensors would help reduce overall energy use, but that would be very complex to implement with a hydronic system so I'm not sure if that's really an option here.
I would consider the water heater as a seperate project. If the existing unit isn't failing, replacing it at this time is probably a net loss no matter how you look at it, except potentially in terms of operational energy efficiency.
Bill
Bill,
Yes I should quantify the cost of oil fuel vs. electricity used by a minisplit per year. Or per decades, really. Showing the historical higher cost and variability in fuel oil prices would be good point to make. Another plus for electrical heating choices is this property has south-facing solar access (located at the corner of two streets to the south and west of the home). So installing solar panels would work well, and provide a stable electricity cost. A friend of mine said buying and paying a loan on solar panels would cut electrical bills by about 25% like in his case. High electricity rates here in New England (and high oil and gas prices too).
I suspect that redundancy point is just GF looking for any justification imaginable. GF already has the plug-in electric heaters and AC window units if the main system fails, and even that fireplace if there’s no electricity due to an extended outage.
The old oil heat and hot water boiler is failing, with one call for service when it was leaking water last month from a gasket failing. The oil service guy said he’s rarely seen equipment this old, and not repairable. Either it needs to be replaced with his bid $6,500, or another type of water heating installed. Space heating is needed less with Spring arriving, hopefully sooner than later. The existing mini split and portable electric heaters would provide enough heating until next Fall.
Thx--Bob
The wall mount per bedroom only is a no go from the start.
The smallest 3 zone multi split is 24000BTU. This is so oversized that chances are the efficiency will be barely above resistence heaters. Lot of times this also creates comfrot issues as the unit needs to bypass refrigerant through the zones that are not calling for heat, overheating those. Pretty much defeats the individual temperature control. Wall mounts also make a fair bit of noise when they go into defrost, not something you want to happen in the middle of the night.
With the hallway and closet right by the outside wall a slim ducted unit mounted there makes the most sense. You can do a bit of zoning with these without any smarts. The idea is to over provision a zone and install a zone damper that doesn't close all the way. For example set the flow rate to the zone to 120% of load but only deliver 75% when the damper is closed. Most inverter based units have no issues modulating around this bit of flow/pressure change.
In this case without much need of AC, probably your best bet is to install a wall mount in the hallway with a resistance panel heater in the rooms. This lets the wall mount do the bulk of the heating and the panel heaters running mostly when the doors are closed.
Akos,
Researching slim ducted products now. Will talk to dealers.
The unit might fit at the top of the 5'+ closet between bedrooms #1 and #2. I’m guessing the basement ceiling would be a good place to mount the unit, and ducts could be short. The three bedrooms are near each other at the end of the hallway. Smallest bedroom 3 might need a longer duct but needs less much less heating.
Thanks--Bob
Consider an air-to-water heat pump plus fan coils.
For example:
https://www.pmengineer.com/articles/94860-john-siegenthaler-details-for-small-scale-hydronic-cooling
https://www.spacepak.com/hydronic-heating-and-cooling