Why these still belong on the binder
Radiant floor is the dominant emitter in our houses, but it is not the only emitter. There are four common cases where the warm floor on its own falls short of the comfort the architecture is calling for: (1) primary and guest baths where a warm towel is the difference between an okay morning and a good one; (2) guest rooms that will be unoccupied 320 nights of the year and need rapid wake-up time when they are occupied; (3) stair landings and vestibules with concentrated infiltration where the slab below cannot quite supply the local load; and (4) rooms with extreme glazing — south-facing window walls in particular — where the local heat loss exceeds the floor's output ceiling. Hydronic supplements solve all four cases cleanly.
Because these emitters share the same boiler plant as the radiant floor, the homeowner gets a single system to maintain, a single set of warranties, and a single set of failure modes to learn. We do not install separate electric towel warmers if we can avoid it; we do not install standalone wall furnaces in bonus rooms; we do not specify ductless mini-splits as a heating supplement because they belong to a different trade with a different commissioning protocol.
Towel warmers
We install three towel-warmer lines almost exclusively. Runtal R-series — Swiss-designed, flat-panel, painted or polished, with hydronic ports rated to 150 psi at 220°F. Myson Avonmore and Multirail — UK-designed, classic ladder-style, low BTUH-per-rung but plenty of rungs. Mr. Steam Marathon — for the spec that calls for a heavier, statement-grade towel warmer with a built-in timer integration. All three lines hold a hot towel through a morning rotation; all three perform within a tight BTUH band, between 1,500 and 2,200 BTUH per unit depending on size and supply temperature.
Towel warmers are wall-hung, fed from above or below with ½″ copper or PEX-a, with a Thermostatic Radiator Valve (TRV) at one port so the homeowner can set the warmer's setpoint independently of the bath's radiant slab. A bleed valve at the high end of every towel warmer makes annual de-airing trivial. The warmer is on the same loop as the bath's radiant slab on most installs, dropping in series from the manifold; on some installs with very long pulls, we run a dedicated mini-loop from a hydronic separator.
Low-temperature baseboard
For guest rooms and stair landings, we run hydronic baseboard at a lower supply temperature than a traditional fin-tube installation. Slant/Fin Multi-Pak 80 with the high-output aluminum fin tube produces approximately 580 BTU/h per linear foot at 180°F supply, dropping to about 320 BTU/h per ft at 140°F supply, and 220 BTU/h per ft at 120°F. We size for the lower supply, run the baseboard a bit longer, and gain the comfort advantage of warmer surface temperatures along the perimeter walls without compromising the boiler's condensing operation.
Panel radiators
When an architect specifies a panel radiator into the elevation — most often a north-facing window wall in a primary bedroom or a kitchen nook — we install one. Our default is Runtal R-Series for the contemporary clients, Buderus Logatrend for the more traditional installs, and on rare occasion a vintage cast-iron radiator pulled from the original house and restored if the spec is a renovation. Sized per the manufacturer's output table at our chosen supply temperature, with a TRV on the supply and a lockshield on the return, fed from the manifold like any other emitter.
What it costs
A single hydronic towel warmer installed in a house we are already plumbing for radiant runs $1,800 to $3,200, including the unit, the pipe runs from the manifold, the TRV, the bleed, and the commissioning addition to the binder. A panel radiator run is in the same band depending on the unit cost. A 12-foot baseboard run lands around $2,400 to $3,800. As an addition to an existing radiant install (mid-build), these are small line items; as a retrofit to a house already finished, they are substantially more expensive because of the pipe-pulling work through finished walls — typically $4,000–$7,000 per emitter.
Read across
Read the radiant floor scope for the dominant emitter, and the legacy rehabilitation scope for what we do with cast-iron radiators on retrofit jobs. The manifold component sheet shows how supplemental emitters drop off the same manifold as the floor loops.