Radiant Heat & Mechanicals

Question 1

We just bought an Eichler in Green Gables and the radiant heat hasn't been used in years. The previous owner switched to forced air. Is it worth reactivating the in-slab system, and what should we realistically expect to spend?

Accepted Answer

Great question, and congratulations on Green Gables — that's one of the most intact Anshen & Allen tracts in the peninsula. Reactivating a dormant in-slab radiant system is absolutely worth evaluating, but you need a specialist who has actually worked on copper-in-slab systems, not a general HVAC contractor. The original Eichler systems ran copper tubing embedded in the concrete slab, and after decades of dormancy the first thing you need is a pressure test to check for leaks. A good radiant specialist will pressurize the loop with nitrogen and hold it for 24 hours. If it holds, you're in excellent shape. If it doesn't, you're looking at either a targeted repair — which is possible if the leak is near a manifold — or a full overlay system using PEX tubing run above the slab. Reactivation on a clean system typically runs $3,000–$6,000 including a new boiler. A PEX overlay on a compromised slab can run $15,000–$25,000 depending on square footage. The comfort payoff is real — radiant heat is genuinely superior for these open-plan post-and-beam homes because it heats the mass of the slab and radiates evenly. No cold spots, no drafts, no noise.

Question 2

I've read that some Eichler tracts used glycol in the radiant loops and others used plain water. Does it matter for a 1960s Sunnyvale home, and how do I find out what's in mine?

Accepted Answer

It matters a lot, actually. The original Eichler systems were filled with plain water, sometimes with a basic inhibitor. Over the decades, many homeowners or service techs added glycol — either ethylene or propylene — to prevent freeze damage, which is largely unnecessary in the South Bay but became common practice anyway. The problem is that glycol degrades over time into glycolic acid, which is corrosive to copper. If you have an older glycol fill that hasn't been refreshed, you may have acidic fluid actively eating your copper tubing from the inside. The test is simple: a radiant specialist can pull a small fluid sample and test the pH on-site. You want a pH between 7 and 9. Below 7 and you have an acidic condition that needs immediate flushing and inhibitor treatment. I've seen Sunnyvale homes where the fluid was pH 4.5 — that's vinegar-level acidity sitting in copper tubing embedded in concrete. Flush it, refill with treated water and a proper inhibitor like Fernox or Sentinel, and test annually.

Question 3

How do homeowners handle heating and cooling without ruining the flat roofline?

Accepted Answer

Options include ductless mini-split heat pumps, small-duct central systems (like Unico), plastic tubing radiant upgrades, or baseboard units. Standard rooftop ducts are discouraged unless painted to match the roof/house.

Question 4

How does radiant floor heating work in an Eichler, and what happens if it fails?

Accepted Answer

Original Eichlers feature slab-on-grade concrete floors with embedded radiant heating pipes. If original copper or steel pipes leak or fail, owners often transition to overhead/mini-split HVAC systems or install modern hydronic tubing during floor renovations.

Question 5

Is the original radiant floor heating system still functional, and has it been pressure-tested recently?

Accepted Answer

Active, leak-free hydronic radiant heating systems are a major selling point for Eichler purists. However, because these systems utilize copper or iron pipes embedded directly within the concrete slab-on-grade, they are susceptible to corrosion, leaks, and failure over time. A professional pressure test is required to verify system integrity before purchase.

Question 6

Has the piping in the slab suffered leaks, or has the home been retrofitted with copper or PEX overhead?

Accepted Answer

If the original slab pipes failed, past owners typically bypassed the slab entirely by running new water lines through the ceiling or roof structure using copper or PEX tubing. While overhead rerouting restores water distribution, it eliminates the original floor heating unless a new above-slab hydronic panel or subfloor system was installed.

Question 7

What boiler system powers the radiant heat, how old is it, and when was it last serviced?

Accepted Answer

Modern high-efficiency condensing boilers (such as Navien or Lochinvar) have largely replaced original, inefficient cast-iron boilers. A modern boiler unit improves fuel efficiency, maintains consistent water temperature, and provides reliable zoning controls, though the underlying slab pipe network remains the primary variable for longevity.

Question 8

Has a ductless mini-split system or small-duct high-velocity HVAC been installed for cooling and supplemental heat?

Accepted Answer

Because Eichlers feature flat or low-pitch roofs without traditional attic space, standard ductwork is impractical. Modern retrofits frequently utilize multi-zone ductless mini-split heat pumps or small-duct high-velocity systems (e.g., Unico) to deliver effective air conditioning and backup heating without disrupting post-and-beam sightlines.

Question 9

What is the condition and age of the main sewer line underneath the concrete slab?

Accepted Answer

Original cast-iron sewer lines beneath 1950s and 1960s concrete slabs are prone to cracking, root intrusion, and channel erosion. Conducting a video camera sewer scope during pre-purchase inspections is essential to evaluate pipe integrity, as replacing under-slab main lines requires expensive trenching or trenchless pipe-bursting and epoxy lining.

Question 10

Has an electrical panel upgrade (e.g., 200-amp service) been completed to replace obsolete panels like Zinsco or Federal Pacific?

Accepted Answer

Mid-century homes were built with 60-amp to 100-amp electrical service, often using legacy panels (such as Zinsco or Federal Pacific Stab-Lok) that pose known circuit breaker safety risks. Upgrading to a modern 200-amp panel is required to support modern appliances, EV chargers, mini-split HVAC, and solar installations.

Question 11

What permits and building code requirements apply when replacing an original in-slab radiant heating system with modern hydronic lines or mini-split HVAC heat pumps?

Accepted Answer

Mechanical and electrical permits are required for modern heat pump mini-splits, with strict noise attenuation rules applied to exterior condenser placement near side setbacks. Replacing or abandoning radiant loops requires plumbing permits and careful coordination to avoid damaging post-tensioned slab cables or structural pads.

Question 12

Is the original in-slab copper radiant heating operational?

Accepted Answer

Original boiler systems offer even, silent heat, but aging copper pipes embedded in concrete slabs can fail. Always request a pressure test on the radiant loop before closing.

Question 13

If the radiant heat failed, what heating and cooling system replaced it?

Accepted Answer

Ductless multi-split heat pump systems are the standard solution because they preserve the exposed beam ceilings. Overhead exterior ductwork can look unsightly if not carefully hidden.

Question 14

If ductwork was added, how was it routed relative to the roofline?

Accepted Answer

Rooftop ductwork can violate neighborhood design guidelines and alter low-profile silhouettes. Look for low-profile interior runs or well-integrated ductless mini-splits instead.

Question 15

What is the capacity of the main electrical panel?

Accepted Answer

Original 60-amp or 100-amp Zinsco or Federal Pacific panels require replacement. Upgrading to a 200-amp panel ensures capacity for heat pumps, EV charging, and induction cooking.

Question 16

Has the under-slab galvanized plumbing been repiped?

Accepted Answer

Galvanized pipes cast into concrete eventually corrode. Verify whether the home has been re-piped through attic or roof cavities, or rerouted with PEX or copper above the slab.

Question 17

What perimeter slab insulation or air sealing has been completed?

Accepted Answer

Uninsulated concrete slabs radiate cold during winter. Knowing if perimeter insulation was added during past landscaping or foundation work helps gauge thermal comfort.

Question 18

Is the concrete slab level and free of major structural cracks?

Accepted Answer

Minor hairline cracks are typical for aging slabs, but significant shifting can indicate drainage issues or historical movement around radiant lines.

Question 19

How does the original radiant heating system work in these homes, and how do I inspect it?

Accepted Answer

Original Eichler radiant heating relies on copper or steel pipes embedded directly inside the concrete slab floor, circulating hot water heated by a boiler. When inspecting, always perform a thermal imaging sweep and a system pressure test. If the pressure drops, there is an active slab leak. Many original systems have exceeded their lifespan, so evaluating whether the loop is intact or if a previous owner converted to an overhead mini-split system is step one during due diligence.

Question 20

What are my options if the original radiant heating in the slab has failed?

Accepted Answer

You have two primary paths: abandon the slab loops and install high-efficiency multi-zone ductless mini-split heat pumps, which provides zone-controlled heating and modern air conditioning without compromising vaulted wood ceilings; or re-pipe radiant manifolds or install thin-profile hydronic floor panels over the slab if you want to keep radiant heat under engineered hardwood or tile.

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