How to heat a renovated small cottage that has wood stove, but prefer minisplits

Caleb

Have a 740 sq ft cottage in southern new england. Getting a total renovation as it was a 1950s pre-fab w/o insullation. Currently has an oil boiler that doesn't work and baseboard radiators. Also has a wood stove in the basement.

Trying to decide heating for year round comfort for my family.. I like mini splits, but the rooms are pretty small, with 2 bedrooms, both small, what is value based way to heat the house, and maybe use the wood stove occasionally in to basement, but not rely on it? (The wood is free, have acres of woods to cut from, just takes time and energy)

Thinking I could use a 2-way mini split for the open LR/KIT area, and the bigger bedroom, then maybe put resistant electric heating in small bedroom (8x10) and bathroom? That would save from getting a 4-way mini split that is overkill. Heat loss calc with 1 ACH is 20k BTU

Also feel like I should install radiant and get a wood boiler in case I need to heat during power outage or go off grid some day with solar..

Lastly, have a large 10 acre pond/stream 75 ft from house I could draw water from if it was worth using some geo-heat solution.

Thanks for reading and any thoughts

hvacfreak2

A wood boiler is as dependent on electricity as an electric clothes dryer. The only ( practical to consider ) way to heat without power that I know of is the woodstove that you mentioned or a millivolt gas or lp steam system.

As for the minisplit all of the manufacturers offer multi - head single outdoor systems and single phase VRV-VRF. As for the pond there will be 100 different opinions , but for me I'd vote no on the pond.

SWEI

Water-source heatpumps are spendy, not even close to economical for a small space.

Multi-split heatpumps are going to be massively oversized for you (and also have low heating capacities.)

A single split mini-duct would be my choice for that application. Fujitsu and Daikin both have 9k-12k models which produce ~1.5x their nominal ratings in heat at 5°F ambient. Mitsubishi probably does as well -- I'm not familiar with their current lineup. The current LG offerings have much lower heating capacities, and the no-name Chinese stuff is even worse.

Proper sizing and duct design are critical.

Steamhead

What's wrong with keeping the hot-water system? It will deliver far better comfort than any blown-air method. Radiant is nice, but quite expensive so may not fit your budget.

Do you have natural gas at the house? Are the current baseboards cast-iron or fin-tube?

Jamie Hall

Where in southern New England? Makes a difference.

As for options -- I agree with @Steamhead . What is wrong with keeping the baseboards and putting in a nice little mod/con LP boiler to heat them? It isn't quite as cutting edge as a mini-split, and I'll grant that it isn't, perhaps, as nice as radiant -- but it's a lot easier or cheaper or both than either one!

The wood stove in the basement, if you have a big enough one for the space, will also work well for when you want to do the work, particularly if you have adjustable floor grates to let the heat up into the various ground floor rooms. That will have the significant advantage that it will work very happily when the power goes out -- which it will -- or at some future time if you want to go off grid.

Going off grid with solar is a very green idea. However, you will need a pretty decent size PV array to power any heating system other than a wood stove (a wood boiler takes electricity!) -- and a very big battery system. For reference and back of the envelope calculation purposes, you would need to store enough energy to keep things running for -- at a minimum -- three days (72 hours) without any solar input to speak of, and taken as a year round average you would get about 3 hours per day of usable power output from the array; less in the winter -- so the array will have to have the capacity to provide the energy required from that short time, and the battery storage, as i said, to keep things going when the sun doesn't shine.

Robert_25

I would fix/replace the oil boiler and use the woodstove when convenient.

SWEI

I fully agree on keeping the radiant if all you need is heat.

Caleb

Thanks for the input. Yea skip the pond use for now. I tore out the baseboard copper fin/tube, was so gross and nasty I coundn't stand to look at it. The boiler wasn't working either as I was told. And it was in the same single flue chimney as the wood stove. So plan to do mini split..

Here is my heat calc:

Heat loss calculator, BTU / hour
Heating loads, worst case
◊ Infiltration = cu. Ft. * R * air changes/hr * temp. diff

Inside temp = 70

House » Floor, w carpet
740 sq ft
8.5 R value
A/R = 87
delta F 15

BTU Loss 1,306
† Lighting Watts 100 341
† People 2 272
Total insides 740 87 1,919

Outside temp = 15
desc sq ft R A/R *F Loss
Floor 740 19 39 55 2,142 Dependent
Ceiling 750 19 39 55 2,171 on outside temp
N wall 261 15 17 55 957 minus winds
S wall 261 15 17 55 957 minus winds
W wall 310 15 21 55 1,137 minus winds
E wall 310 15 21 55 1,137 minus winds
Windows 84 3 28 55 1,540 7 3 x 4
doors 42 1.8 23 55 1,283 2 2 x 4
TOTAL Outside loss = 11324

◊ Infiltration 7,400 0.018 1.00 55 7,326

Total Insides 1919
Total Outsides 11,324
Infiltration at 1.0 ACH 7326

TOTAL 20,569 BTU heat loss

So this house that is 740 sq ft, currently down to studs, will get spray foam sealed and batt insulation, should be around 20K BTU on a 15 deg night. Would be easy with a mini split, but I have a small 8x10 bedroom and 10x12 bedroom to get heated.. trying to find best way to get heat in them when doors closed.

Interesting comment on Fujutsu able to output 1.5x rating, any source for that? I have Mitsu hyper heat 18K mini split currently for my 480 sq ft apartment with old insulation and it does nice even when it was 8 deg last winter.

This is 10 miles from CT coastline..

Dreaming up a solar install.. I have full south orientation, 30 x 35 ft south roof area at 4x12 pitch. (shingle-Covered porch). Tracking my electric use last 3 years in the area, I average 12 KWH a day, not including hot water which is gas currently (10 therms a month). If I take 25% of my daily average, that is a 3.2 KW solar size needed to cover all electricity less hot water.

Thinking a small instant propane water heater for showers/dishes/laundry. I bought an eccotemp unit to try out..

NYplumber

Get the smallest mod con you can find on LP, pipe it through a reverse indirect as a buffer and use pannel rads. I dont see a use for the splits in the summer as the cooling season in that area is less then four weeks at max.
If you dont want the complexity of a mod conn get a cast iron boiler.

Jamie Hall

On the solar -- just don't forget that the sun doesn't shine in southern Connecticut for days at a time. Either plan on BIG batteries, or keep the grid connection.

hot_rod

See what solar incentives apply. The Sun bandit is a PV powered DHW if you want to stay all solar PV

if you have gas, a small combi tank unit like the Bradford White CombiCor could provide DHW and power some panel rads. unless you need AC, then the mini split may be the better option.

http://programs.dsireusa.org/system/program?state=CT

Caleb

Jamie Hall said:

On the solar -- just don't forget that the sun doesn't shine in southern Connecticut for days at a time. Either plan on BIG batteries, or keep the grid connection.

Understand.. so my 3.3 KW system needs to be a 10K if needed for 3 days.. thats about $20K from what I found

I

NYplumber said:

Get the smallest mod con you can find on LP, pipe it through a reverse indirect as a buffer and use pannel rads. I dont see a use for the splits in the summer as the cooling season in that area is less then four weeks at max.

If you dont want the complexity of a mod conn get a cast iron boiler.

Interesting.. Will look at reverse indirect, its just hard to fathom a whole boiler system for my 21x29 ft foundation with 10x13 addition. I'm opening the walls between LR/DR/Kit, so its one 15x30 ft room, then 2 small bedrooms, and a 5ft x 8 ft bath, thats it. Maybe I can route some hot water into the bathroom or make a towel warmer with DHW from HPHW heater tank.

Leaning towards 1 mini split at 15K BTU (the 14 HSPF Fujitsu) and electric heat in the bedrooms for now while grid connected. And wood stove in basement. Maybe can route the wood heat from basement to rooms.

Once I get moved in, would like to look more seriously about solar, tap into 10 ft waterfalls nearby for hydro power, and use the wood stove to step away from grid power with generator as needed.

hot_rod

You probably already know that RE is expensive and not usually cost effective at today's energy cost.

Unless you want to be off grid or just cover a fraction of your energy cost, stick with the mini split idea.

If you can do the work yourself and watch for deals on scratch and dents, RE is a fun hobby.

You will be surprised how you modify your structure and living routine to maximize the solar available.

I agree a wood stove is a nice, simple, non-electric means to provide comfortable heat. Great for power outage season.

On my wish list is one of these high efficiency, gasification parlor boilers.
Marc can add a HX to supply DHW or some hydronic. He had one of the stainless steel topped Hydronic cook stoves at the RE show a few years back.

Jamie Hall

It's the storage (batteries). As well as the array. The array needs to be big enough -- assuming you are trying to go off grid -- to generate enough watts, based on an average of 3 hours per day operation, to cover your usage. The batteries need to be able to store enough watts to cover your usage for an absolute minimum of three days with no recharge.

So... figure the number of watts you use in an average day. The array needs to be able to provide that many watts in three hours time. The batteries need to be able to store three times that average use.

As a minimum...

SWEI

Don't forget that you want to have 30-50% capacity remaining in those batteries at the end of three days. Also that you lose 25-30% of the energy produced by your PV array just charging and discharging.