Minisplits for each bedroom?
I need to replace my oil boiler/tankless hot water system. My preference would be to use a minisplit, and get rid of oil altogether and use by wood stove as backup. I read somewhere on the site that if a contractor recommends putting a minisplit in each bedroom, you should find a new contractor. Three different contractors, including the guy from MassSave, have now told me I would need a minisplit in either two or all three of the bedrooms. My house is a 1960 ranch, only 912 square feet not counting the basement. Four minisplits for 912 square feet seems excessive.
The contractors have also said that my pipes might freeze if the basement is unheated, which would mean an additional minisplit (half of the basement is finished, but we don’t use it except for storage). I was thinking of getting a hybrid heat pump water heater. I know my electrical will need to be upgraded, since I still have mostly fuses. On the other hand, if I replace the oil boiler I will need to replace the chimney liner.
I really wanted to get rid of oil, but everyone so far has told me its not feasible.
Some additional information:
Thanks for all the input. More info: None of the contractors did a load calculation. My oil boiler is supposedly 83.5% efficient. Last winter we used 2.96 gallons of oil a day for a 45 day period for which the HDD was 1423.5. The outdoor design conditions chart says the 99% temperature is 6 for a town near where I live. The walls are 2×4. MassSave’s contractor insulated the walls a few years ago – sort of. There are several places where there are large cavities, or where a fair amount of the insulation ended up in the interior of the house, such as near the baseboard in the bath and in the basement, and they cracked quite a few of the shingles, which are made of whatever they used in 1960 on tract houses – definitely not cedar. The windows are mediocre but double-pane. The smallest bedroom is only about 99 square feet, and the largest only 121.
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I'll put it to you this way. 1024 square feet 2 bedroom , large kitchen/liv rmm in 1st half, 2nd half bed/bath/bed. ONE 12000 BTU has been OVERKILL
Now this is TIGHT construction, on an older leaky house I STILL DO NOT see why you need that many heads. Within 30 minutes ALL ROOMS are with 4 degrees of each other in the one im taking about.
They'll be oversized because they dont make them small enough
Put a low min output head in a bedroom and you always have the option of turning it off. Your choice - a few degrees degrees too cool when the door is open or the very minor downsides of over-sizing when you want it warmer or want to close the door. Ducted mini-split is another option (but it has little ability to deal with non-proportional changes in room loads).
"Source max output matches design load" plays a small role in thermal comfort. Even more so when only looking at whole building load (vs per room).
Well first step is to figure out how much heat you need. This means a manual J or heating use estimate from your oil consumption.
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
A ducted minisplit in your basement is likely a reasonably good option.
If it were my call, I'd opt for a single mini-split to serve the main living area, and install electric baseboard strips in the bedrooms for supplemental heat, perhaps something in the range of 750 or 1,000-watts each. With the bedroom doors left open during the day as Jon suggests, your incremental energy needs should be fairly modest. Although a little more costly, for added comfort and safety, I'd go with an electric hydronic baseboard heater such as Cadet's Soft Heat or the QMark equivalent.
Another option would be to install a single mini-split as described above, and replace your oil-fired boiler with a small electric unit. The advantage here is that you wouldn't have to remove your existing baseboard radiators, and the heat pump would still do the lion's share of the work. On the flip side, unless the bedrooms are on a separate zone, you may find these rooms somewhat starved for heat.
>"If it were my call, I'd opt for a single mini-split to serve the main living area, and install electric baseboard strips in the bedrooms for supplemental heat, perhaps something in the range of 750 or 1,000-watts each."
Perhaps you're unaware of just how expensive electricity is in most of Massachusetts. Only if the room load comes in under 1500 BTU/hr (~450 watts) would that start to make sense. A 150 square foot bedroom with just one exterior wall and one window might come in that low, but most are going to be higher.
I pay 15.603-cents per kWh plus an additional 2-cent rider for green power, so I'm kinda aware of the cost.
A lot of folks in MA are paying 22 cents(delivered) , even without a green-power rider.
The green-up charges from the utilities is usually more expensive than just buying 100% green power under contract from a broker. Try shopping here (but watch out for automatic renewal clauses and cancellation charges- some of these brokers will bump the price by 50% or more in the auto-renewal, then ding you for a cancellation charge when you bail.):
http://www.energyswitchma.gov/#/
I'm currently paying a bit over 20 cents, which is slightly less than the standard mix, in my area, only due to buying 100% wind on contract through a broker for the supply. There are no discounts on the deliver or other charges.
The average residential electricity price (all-in) in MA as of June 2019 was 21.43 cents:
https://www.eia.gov/electricity/monthly/epm_table_grapher.php?t=epmt_5_6_a
With so many unknowns it's difficult to provide proper guidance, but if we were to assume an average run time of eight hour per day and a combined space heating demand of 1.75 kW for all three bedrooms, fourteen kWh per day at 21.43-cents per kWh would run us a little over $90.00 per month. In the dead of winter it might be a little more, and during the shoulder seasons a little less. But, as mentioned, occupant behavior and expectations can greatly skew the results (I try to minimize our usage of electric resistance heat, relying on our two ductless mini-splits to carry the bulk of the load, and I'm willing to accept larger swings in nighttime temperature for this reason).
Be careful with the "supplemental bedroom heat" concept. Heat the bedroom to the same temperature as the hallway and the supplemental heat will provide 100% of the bedroom load. On the other hand, if you only turn on the supplemental heat for 8 hours/day, then it will only supply ~33% of the bedroom load (which is still significant).
That's a valid point. I would only operate these baseboard heaters during the overnight hours and only if the bedroom doors are kept closed during this time. The presumption on my part is that these rooms would be kept several degrees cooler to facilitate sleeping, and that only a relatively modest amount of heat would be needed to maintain overall comfort (a programmable thermostat would help facilitate this). Different usage patterns and expectations could easily result in significantly higher operating costs.
>"Four minisplits for 912 square feet seems excessive."
D'ya think?
>"Three different contractors, including the guy from MassSave, have now told me I would need a minisplit in either two or all three of the bedroom"
Utter crap! It's TRULY insane, unless you sleep with windows open! Did any of them run a formal heat load calculation on it? (If they did, they're either incompetent or con-artists!)
Run an AGGRESSIVE Manual-J (aggressive rather than conservative assumptions whenever there is any doubt), or an I=B=R spreadsheet. The total load for a 912' house is likely to come in under 25,000 BTU/hr even if it's pretty leaky, under 20,000 BTU/hr if it's reasonably tight.
Since you have a heating history on it, it's a simple exercise to MEASURE the whole-house heat load by correlating fuel-use to heating degree day weather data on wintertime-only fuel use (where the error from hot water use and solar gains tend to be low, and cancel each other.) For details on how to go about it, see this:
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
Odds are pretty good you could do the whole shebang with one 1.5 ton or 2 ton ducted Fujitsu (2.5 ton max):
http://portal.fujitsugeneral.com/files/catalog/files/18RLFCD1.pdf
https://ashp.neep.org/#!/product/25350
https://ashp.neep.org/#!/product/25351
>"The contractors have also said that my pipes might freeze if the basement is unheated, which would mean an additional minisplit (half of the basement is finished, but we don’t use it except for storage)."
Spend the money you save on not buying a ridiculous amount of heat pump on insulating and air sealing the foundation walls and your basement will never drop below 60F. MassSave won't subsidise that (only the bandjoist insulation & air sealing), but using reclaimed roofing foam , 1x4 furring and 3" reclaimed roofing foam it's a pretty easy DIY to insulate the unfinished portion. How you go about insulating the already finished portion depends on the current wall stackups and how much gut/rebuild you want to do, but there's usually a way to improve it, if not bring it fully up to current code.
An uninsulated basement is likely to account for 15-20% of the heating energy use of the house. Insulating it to code minimum might even bring the whole house load down to the 15,000 BTU/hr range if the rest of the house is tight and insulated, and you have low-E double-panes (or low-E storm windows.)
BTW: Even the smallest oil boilers are likely to be 3x or more oversized for your actual loads, which has a negative effect on as-used efficiency. Keep that in mind when running the fuel-use based load numbers- it's going to be a fairly firm upper bound. See the 3x oversize factor column in Table 3 of this document:
https://www.bnl.gov/isd/documents/41399.pdf
While 3x oversizing is going to knock something like 10% off the AFUE or steady state efficiciency on an annualized basis, that isn't a valid derating when using wintertime-only fuel use. For the fuel-use based load calc stick with either the nameplate AFUE efficiency or the most recent combustion efficiency from the last tune-up (if the burner tech tagged it) when making the calibration adjustment for efficiency.
>"...if we were to assume an average run time of eight hour per day and a combined space heating demand of 1.75 kW for all three bedrooms, fourteen kWh per day at 21.43-cents per kWh would run us a little over $90.00 per month."
Adding $90/month beyond what the mini-split was using would likely increase the cost of heating a 912 square foot house by something like 35%.
Without the calculated heat load numbers we're just speculating.
But while we're speculating...
...assuming a load of 20,000 BTU/hr @ +5F, a typical non-coastal eastern MA design temp, and a balance point of (65F it's 65F - 5F =)60 heating degrees, for a ratio of 20,000/60= 333.3 BTU/degree hour, or (x 24= ) 8000 BTU/HDD. A typical non-coastal eastern MA town runs about 6000 HDD/year, with the coldest months running 1100-1200 HDD. That means annually it would use (8000 BTU/HDD x 6000HDD/yr=) 48,000,000/year
....and a cold month would use (8000 BTU/HDD x 1200 HDD=) 9,600,000 BTU in that month.
With a cold climate mini-split averaging an HSPF 10 (= 10,000 BTU/kwh) that cold month would use (9,600,000/10,000= ) 960 kwh At 21.43 cents/kwh that's ($0.2143 x 960=) $206.
if it's all being heated with the mini-split. If only 3/4 of the heat is the mini-split and 1/4 of that 9,600,000 BTU is coming from baseboards the baseboards are using (2,400,000/3412=) 703 kwh, at a cost of ($0.2143 x 703=) $151, and the mini-split is using (7,200,000/10,000=) 720 kwh, at a cost of ($0.2143 x 720=) $154. That adds up to $305 total.
That's a ~$100/month cost-adder for the colder months over doing it all with a mini-split.
With careful management of the baseboard use much of that cost can probably be avoided, but the example above is within the realm of what was seen in the NEEA studies where a single ductless mini-split was added to houses previoiusly heated primarily with baseboards. Details:
https://neea.org/img/uploads/ductless-heat-pump-impact-process-evaluation-billing-analysis-report.pdf
(There are also individual sub-metered house usage details for a few particular houses on the NEEA site if you go digging for it.)
Dana just tore those points apart, as usual! :)
>I really wanted to get rid of oil, but everyone so far has told me its not feasible.
We heard the same. None of the HVAC guys that came out to look at sizing our mini-splits were comfortable with my ask that they replace our ducted LP furnance with just mini-splits (for heating). All of them wanted to keep fossil fuel source for backup heat. An architect that we consulted did convince one HVAC company to consider baseboard as the backup, and we went with them for our multi-split install. But another HVAC company didn't even return a quote after I was willing for pay his folks to remove the furnance (instead of DIY).
>in the NEEA studies where a single ductless mini-split was added to houses previoiusly heated primarily with baseboards.
It seems contractors around here (in Wisconsin) heard of this because most of them were aware that mini-splits could replace baseboard heating and get local State incentives. But that was the the limit of it. Seems HVAC folks still consider mini-splits to be (a) new tech and (b) for cooling.
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Does anyone have experience with the Tjernlund AirShare. A small in wall blower that transfers conditioned air from one room (in this case a hall or adjacent loft type room) to another, thermostatically controlled
The largest one is rated at 110 CFM. With a 4F delta-T, that's 475 Btu/hr. An open door moves much more.
If the ducted minisplits had been available when I did my house I would have been able to skip the boiler. In my case the four rooms[plus laundry and bath] downstairs would have been handled by 2 units with virtually no ducting.
Much depends on your layout and relative heatloss room to room. In an older house a drafty room at the end of the hall is problematic when trying to stretch resources
Resurrecting this old thread because it gets at our current issue (and many others' I suspect).
The ol' head in each bedroom issue / oversized systems
Questions:
1) What is the alternative? The idea to use resistance strips seemed to be frowned upon here, so what exactly is the suggested alternative? By the way, ducted units is not much of an option for many rural parts of the country where the (limited) local installers all stick to ductless.
2) Is an oversized system (for heating purposes) really that bad? My take is Jon R doesn't think so.
I don't necessarily mean as grossly oversized as the OP's question here. Contractors around here still want to put in more BTU/h capacity than the actual design load (or don't do a design load calc at all). Spending all this time on GBA has me saying, "NO!" But at the end of the day, it's unclear to me what moderately to mildly oversizing a system really does that is so negative.
3) Question 1 again: what's the alternative to a head in each bedroom?
I'll give you my personal opinion, having researched it a bit myself, but I'm not an expert by any means:
I would avoid multi-splits if you need to put ductless heads in each bedroom. Most multi-splits don't have great minimum capacity outputs so they tend to overshoot setpoints, sometimes significantly, in small rooms with low loads.
I would, if it were the only reasonable option, consider installing something like a Mitsubishi FS06 head in a bedroom if paired 1:1 with the appropriate outdoor unit. The FS06 can turn down to like 1600-1700ish BTU for both heating and cooling. That's its minimum output and is likely to still be oversized for many bedrooms in many climates, but to me it's getting close enough that I wouldn't be too concerned. Plus, since it's not a multi-split, it should be able to stop heating/cooling when the setpoint is reached, so grossly overshooting setpoints shouldn't be as big a concern as with a multi-split.
Best option, IMO, would be a ducted unit that can serve multiple bedrooms, but I realize that's not always an option.
I tried to get a small ducted unit to serve two bedrooms but the local dealer for our small town in Maine would not do it. He wanted a multi-split, but in the end we did two FS06's each paired with its own outdoor unit.
davidsmartin,
Without giving away more info than desired, what part of Maine are you in? I'm capital region. Im not even sure who could install ducted around here.
I'm in Bridgton. In addition to his unwillingness to consider a ducted unit, the dealer wanted to over-size significantly. It is unfortunate that it is so hard to find dealers who really understand mini splits in a state where they are becoming more and more popular. I don't know if there are better informed dealers in the region because our building contractor really likes working with this dealer and didn't want to consider any alternatives.
How are the FS06 working out in the bedrooms for you? Maintaining temps reasonably close to the desired set point? Any noise complaints?
We are thinking about putting FS06 in two bedrooms in our house. Not an emergency b/c there's a ducted unit that serves the entire house, but in two bedrooms in particular, they get almost no airflow and need something supplemental.
Construction is not yet complete, so I won't know the answer until next winter. Sorry!