Heating an Old Stone Farmhouse Around Unionville
Two feet of stone is slow to warm, slow to cool, and has its own ideas about moisture.
Stone farmhouses and village homes around Unionville were built for fireplaces and stoves, not ducts. This guide explains how thick masonry walls behave, which heating systems fit them, why chimney liners and moisture matter, what to know inside the Unionville Village Historic District, and what to ask a heating company before you change anything.
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Why is an old stone farmhouse hard to heat?
Stone farmhouses have thick, uninsulated masonry walls, deep window openings, few spaces for ducts and chimneys built for wood or coal. They also tend to leak air at windows, doors and floors. The walls themselves hold heat well but take a long time to warm, so systems that cycle on and off often feel uneven.
Around Unionville, many farmhouses date from the 1700s and 1800s, and most grew by additions. A kitchen wing from one century, a bedroom over the porch from another, a family room from the 1970s. Each part loses heat differently, and the heating system was often extended room by room.
The good news: these houses were built to breathe and dry. A heating plan that respects that tends to work. One that fights it, by trapping moisture or forcing big ducts through walls, tends to cause new problems.
What is thermal mass, and how does it affect heating?
Thermal mass is a material's ability to soak up heat and release it slowly. Thick stone walls flatten daily temperature swings. The National Park Service notes a heavy masonry wall can give back stored heat over many hours, while a light wall with the same R-value passes it through in about two.
That's why a stone house often feels cool in summer and steady once it's warm in winter. It also means a cold stone house takes a long time to come up to temperature. Deep night setbacks can backfire: the walls cool, and the system then runs for hours re-warming them.
What thermal mass means in practice
- Steady, low-temperature heat suits stone better than short, hot bursts
- Keep setbacks modest; ask your installer what fits your system
- Radiators and radiant floors pair naturally with heavy walls
- Rooms against an earth bank or north wall may stay cooler and damper
- After a long outage, expect a slow recovery even once the heat is back
Which heating systems work best in stone houses?
Hot water boilers with radiators or baseboard are the traditional fit. Ductless mini splits add efficient heat and cooling without ducts. Small-duct high-velocity systems snake 2-inch ducts through existing cavities. Radiant floors suit additions and renovated rooms. Conventional ducted forced air is the hardest to fit without opening walls.
| System | How it fits a stone house | Cools too? | Disruption to install | Main caution |
|---|---|---|---|---|
| Hot water boiler and radiators | Pipes are small and already in place in many homes | No | Low if replacing a boiler | Chimney liner or new venting with a new boiler |
| Steam boiler | Found in some older village homes | No | Low | Water level, venting and pipe pitch need regular attention |
| Ductless mini split heat pump | A small refrigerant line set through the wall; heads on walls or ceilings | Yes | Low to moderate | Head placement and line routing on visible walls |
| Small-duct high-velocity system | Flexible ducts about 2 inches across, routed through walls and ceilings | Yes, with a coil | Moderate | Needs an air handler space in an attic, basement or closet |
| Air-to-water heat pump | Heats water for radiators or radiant floors | Not usually | Moderate | Old radiators may need higher water temperatures |
| Radiant floor | Tubing under floors in additions or gut renovations | No | High in existing rooms | Slow to respond; floor height changes |
| Conventional ducted forced air | Large ducts through closets, walls and joists | Yes | High | Lost space, cut joists, leaky panned runs |
The Department of Energy describes air-to-water heat pumps as ductless units that heat water for radiators or radiant floors. Small-duct figures come from Unico, one maker of these systems.
Should you keep the radiators and boiler?
Usually, yes. Cast iron radiators hold heat and suit thick-walled houses. If the boiler is old or inefficient, replacing it while keeping the radiators and piping is often the least disruptive upgrade. ENERGY STAR gas boilers are rated 90% AFUE or higher and oil boilers 87% or higher.
Before replacing a boiler, a careful technician checks the whole loop: radiator valves, air vents, expansion tank, circulator pumps and any zones added over the years. A new boiler on a sludgy, air-bound system won't heat any better than the old one.
A high-efficiency condensing boiler changes the venting, usually to a sealed PVC or stainless vent through a wall. That can leave a water heater alone in an old chimney, which is covered below. For repairs and service, see boiler repair.
Can you add mini splits to a stone farmhouse?
Yes. Ductless mini splits are one of the simplest ways to add efficient heat and air conditioning to a house with no ducts. The Department of Energy says they're the most common choice for radiator-heated homes in the Mid-Atlantic. Each indoor head serves a room or zone, fed by a small line set through the wall.
In a stone house, plan the line routing carefully. Drilling through two feet of rubble stone is slower than drilling frame walls, and visible line covers on a front facade can matter in a historic setting. Many owners run lines up a rear wall or through a basement and closets.
Good places for a mini split
- A kitchen or family room addition the boiler never reached
- Upstairs bedrooms that overheat in summer
- A converted attic, barn studio or office
- Main rooms for fall and spring, so the boiler runs less
Look for ENERGY STAR Cold Climate models, which must keep at least 70% of their capacity at 5°F. More in heat pumps and mini splits.
Does an old chimney need a liner for an oil or gas boiler?
Often. Old masonry flues were sized for wood or coal and are usually too big or too damaged for a modern oil or gas appliance. The Chimney Safety Institute of America says a more thorough Level 2 inspection is required whenever the system changes, including a new appliance, a fuel switch or relining.
CSIA cites NFPA 211, the national chimney standard, in calling for chimneys to be inspected at least once a year. Its Level 2 inspection adds a camera scan of the flue interior and checks attics, crawl spaces and basements. It's also the level CSIA lists for a property sale.
- A stainless steel liner sized to the appliance is the common fix for oil and gas.
- CSIA says gas and oil liners generally don't need insulation, while liners serving wood stoves do.
- When a furnace or boiler moves to PVC venting, an old gas or oil water heater can be left alone in a big flue. InterNACHI calls this an orphaned water heater; poor draft can spill exhaust, including carbon monoxide, into the house.
- Fixes include a properly sized liner, a new vent, or replacing the water heater with an electric or high-efficiency model.
Signs of trouble include crumbling mortar at the chimney base, rust at the vent connector and scorch marks near a water heater's draft hood. See the carbon monoxide guide for the safety side.
Is it safe to insulate stone walls from the inside?
Not without a careful assessment. Interior insulation keeps masonry colder and wetter and slows its drying, which raises the risk of freeze-thaw damage and rot in joist ends set into the wall. Building Science Corporation advises against interior insulation where rain control can't be fixed. The National Park Service urges professional evaluation first.
Building Science Corporation's guideline on insulating masonry walls lays out the order of work: inspect for water problems, test the masonry, upgrade flashing and sills, then add a continuous air barrier and mechanical ventilation. The National Park Service adds that vapor retarders are generally not recommended in existing buildings, and that spray foam's long-term effect on old masonry isn't well documented.
Before any wall insulation: fix gutters, grading and failing pointing so the walls stay as dry as possible. Many stone walls with damp problems need water management, not insulation.
A heating company is not the right expert for this question. Ask a building scientist, an energy auditor experienced with masonry, or a preservation professional.
What should you do before changing the heating system?
Seal air leaks and add storm windows first. The National Park Service calls reducing air leakage the first priority in a preservation retrofit and says it can account for 5% to 40% of heating and cooling costs. Energy.gov says low-e storm windows can cut those costs by about 10% to 30%.
- Get a blower door test. It measures leakage and shows where air comes in.
- Seal the top and bottom. Attic hatches, chimney chases, plumbing holes and the sill where walls meet the foundation.
- Insulate the attic and basement rim before thinking about walls.
- Weatherstrip doors and windows. Leave drainage paths at sills and clapboards open.
- Add storm windows. Interior or exterior, with low-e glass where you can.
- Seal and insulate any ducts in unheated spaces.
- Add ventilation if needed. A tighter house may need mechanical fresh air.
Why it matters for sizing: once leaks are sealed, the house needs less heat. Do the air sealing first and the new boiler or heat pump can often be smaller and less expensive.
The National Park Service says a well-fitted storm on an old wood window can approach the performance of a double-glazed replacement, and the interior sash should be the tighter of the two to avoid condensation between them.
Heating changes inside the Unionville Village Historic District
The Unionville Village Historic District joined the National Register in 1979 and covers about 120 acres around PA 82 and PA 162, with 69 contributing buildings. East Marlborough Township has both a Historical Commission and a Historic Architectural Review Board. Ask the township before adding visible outdoor units, wall vents or line sets.
The district includes buildings like the Cross Keys inn (1751), the Union Hotel and Unionville Academy (1834), the Grange Hall (1845), Unionville Hall (1849 to 1850) and the saddle shop (1887). Homes around them are brick, stone and frame, many from the 1800s.
For heating work, the visible items are what tend to matter: an outdoor condenser by the street, a PVC vent through a front wall, a stainless liner cap on a prominent chimney. Placing equipment at the rear or side often avoids conflict. The township office is at 721 Unionville Road, (610) 444-0725.
Farmhouses outside the village face the same technical issues. For nearby horse country, see West Marlborough, and for the Brandywine valley, Chadds Ford.
Get connected with local heating companies that work on boilers, chimneys and mini splits. The referral is free.
How do you heat additions and cold rooms in an old house?
Treat each addition as its own zone. A cold kitchen wing or sunroom usually needs its own heat source, such as a mini split head, a separate boiler zone or radiant floor, rather than one more radiator on an already stretched loop. A thermostat in each zone keeps the old core from overheating.
The Department of Energy notes that heating rooms separately with zoned electric heat can save more than 20% compared with heating the whole house, and the same logic applies to boiler zones and mini split heads. Zone the rooms you live in, and let guest rooms run cooler.
- Add a boiler zone with its own circulator or zone valve for a large addition
- Use a mini split where you also want cooling
- Consider radiant floor when an addition's floor is being rebuilt anyway
- Keep unheated rooms with plumbing above about 55°F, the level the Red Cross recommends against frozen pipes
What to ask a heating company about an old stone house
Ask whether the company regularly services boilers and radiators, how it will size equipment for thick walls and air leaks, what the plan is for the chimney and any orphaned water heater, and how lines, vents and outdoor units will be routed. Ask for its Pennsylvania HICPA registration number too.
- How many pre-1900 homes have you worked on this year?
- Will you do a room-by-room heat loss calculation, accounting for stone walls?
- Does the chimney need a Level 2 inspection or a liner with this change?
- What happens to the water heater's venting if the boiler moves to PVC?
- Where will the outdoor unit, line sets and vents go, and can they avoid the front?
- Who handles the masonry drilling, and how is the hole sealed?
See Pennsylvania contractor rules and look up registrations at hic.attorneygeneral.gov. Comparing fuels? Read oil vs propane vs heat pump.
Stone farmhouse heating questions from Unionville homeowners
Do stone walls insulate a house?
Not in the way modern insulation does. The strength of a thick stone wall is thermal mass: the National Park Service notes heavy masonry slows heat transfer and evens out daily temperature swings. Air sealing and storm windows usually help more than anything done to the walls.
Why is my stone house damp in the basement?
Stone foundations let ground moisture through, and old mortar can wick water upward. Gutters, grading and a dehumidifier often help. Damp basements also shorten the life of furnaces and boilers, so a raised pad is worth asking about.
Can I install central air in a house with radiators?
Yes, with ductless mini splits or a small-duct high-velocity system. Both avoid large ducts. Mini splits add heat too, which can cut how often the boiler runs in spring and fall.
Should I insulate my stone walls with spray foam?
Be cautious. The National Park Service says spray foam's long-term effects on historic masonry aren't well documented and that it should be avoided where masonry is poor or rising damp is present. Get a moisture assessment first.
How often should an old chimney be inspected?
At least once a year, according to NFPA 211 as cited by CSIA. That includes chimneys serving gas and oil appliances. A more thorough Level 2 inspection applies whenever the heating appliance or fuel changes.
Do I need permission to add a heat pump in the historic district?
Check with East Marlborough Township before you start. The township has a Historical Commission and a Historic Architectural Review Board. Equipment that can't be seen from the street usually raises fewer questions.
Is it worth replacing an old oil boiler in a stone farmhouse?
Often, if it's inefficient or failing. Energy.gov says older oil burners below 75% steady state efficiency may be worth replacing. Price a new oil boiler, a fuel switch and a boiler-plus-mini-split setup side by side.
Is this site a heating contractor?
No. UnionvillePAFurnaceRepair is a free referral service that connects you with independent local HVAC companies. They set their own prices and terms. This site doesn't perform or guarantee work and may be paid for referrals.
Sources
- National Park Service: Preservation Brief 3, Improving Energy Efficiency in Historic Buildings.
- Building Science Corporation: BA-1505, Measure Guideline for interior insulation of masonry walls.
- Chimney Safety Institute of America: the three levels of chimney inspection.
- Chimney Safety Institute of America: homeowner resources (NFPA 211 annual inspection; liner insulation).
- InterNACHI: Orphaned water heaters.
- Energy.gov: Heat pump systems (ductless and air-to-water).
- ENERGY STAR: heat pump key product criteria and ENERGY STAR: Boilers.
- Energy.gov: Storm windows.
- Energy.gov: Electric resistance heating (zoning savings) and Oil-fired boilers and furnaces.
- Unico: small-duct systems for older homes.
- Wikipedia: Unionville Village Historic District.
- East Marlborough Township (Historical Commission, HARB, contact details).
- American Red Cross: Frozen pipes.
Written by Lawrence. Updated October 25, 2026. Facts on this page were checked against the public sources listed above.
This is general information from a free referral service, not a heating contractor. For gas smells or carbon monoxide alarms, leave the house and call 911.
Related furnace help
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Mention the age of the house, the walls, what heats it now and which rooms stay cold. Older homes need a company that's comfortable with boilers, chimneys and odd layouts.
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