HVAC, DHW & Ventilation

Table 1 maps the parts to the three subsystems. Subsections that cover one subsystem carry an Applies to tag. An Informs tag marks the subsections behind each decision above.

1 · Loads: what the system has to deliver

Bottom line. The design heating load is the envelope UA at the design temperature difference plus the ventilation air after heat recovery; in a mild climate it is small per square foot and falls sharply with envelope work.

Water carries the heat; a separate small air system ventilates

Water carries about 3,500 times the heat of the same volume of air per degree, so a radiant system moves the whole load through ½–⅝ in tubing at a few gallons per minute, and the only ductwork is for ventilation air, about 106 cfm in the reference house (Figure 1). The heat pump and the ERV share no air; they meet only in the rooms, which is why humidity in a radiant-cooled house has to be handled on its own (Part 3).

Design heating load is UA times the design temperature difference

Heating and coolingHot waterVentilation

Informs D2

UA_envelope from the Insulation guide, including design infiltration · ε_sensible ERV sensible effectiveness

heat loss at design is the envelope's heat loss per degree, times the degrees, plus whatever the ventilation air carries off after the core recovers part of it.

The envelope UA by component is built in Insulation · Envelope UA by component; for load it uses design infiltration (ACH₅₀ ÷ 15, 123 cfm in the reference house) rather than the annual average. Title 24 requires a load calculation (§150.0(h), ACCA Manual J or equivalent) using the JA2 design conditions for the location; for heating the design temperature may not be lower than the JA2 Winter Median of Extremes, a colder value than the ASHRAE 99% dry bulb. In climate zones 3–4 the 99% dry bulb runs from the low 30s °F inland to about 40 °F beside the bay (34 °F at San Jose, 39 °F at Moffett Field). This guide uses 35 °F, a 35 °F difference, as its reference.

Table 2 and Figure 2 carry the reference house through three envelope states.

A ΔT of 25 °F understates the design load by about a quarter against the 35 °F reference (Table 3).

Load per square foot by floor sets the emitter question

Heating and coolingHot waterVentilation

Informs D1

Splitting the components by floor (garage walls, slab edge, doors and most infiltration to the ground floor; ceiling and floor-over-garage to the second; walls and windows evenly) gives about 17 Btu/h·sf on a 900 sf ground floor and 9 on a 1,400 sf second floor with U-0.27 windows (Figure 3). Individual rooms with large windows or two exterior walls run higher than their floor average, and cabinets and fixed furniture cover 20–30% of a floor, so the output per square foot of active floor runs about a third above the gross figure; the room-by-room calculation is where emitter layout is checked.

Window SHGC decides whether sun or conduction sets the cooling peak

Heating and coolingHot waterVentilation

Informs D3

I incident solar flux, Btu/h·sf · SHGC solar heat gain coefficient

summer gain is the small conduction term plus sun through the windows plus people and appliances.

At 84 °F outside and 76 °F inside the walls conduct little, but the ceiling sees attic air near 120 °F under a tile roof (Insulation · Summer gain). With an average 60 Btu/h·sf incident across faces at the peak hour, the reference house needs about 22,200 Btu/h of sensible cooling with its original glass (SHGC 0.65), of which sun is 11,700 and conduction 8,700; with SHGC 0.23 it needs 13,400, and conduction (7,500) leads sun (4,100). A west room of 180 sf with 40 sf of glass at 180 Btu/h·sf late afternoon still needs about 13 Btu/h·sf, at the edge of what a radiant floor can cool (Part 3; glass by face in Windows & Doors · SHGC by face).

2 · Radiant heating: output, water temperature and emitters

Bottom line. Output per square foot is the surface-to-room temperature difference times a combined coefficient; comfort caps the surface temperature, and everything between the tube and the room sets the water temperature needed.

A radiant surface's output is a coefficient times its surface excess

Heating and coolingHot waterVentilation

Informs D1

h combined radiant and convective coefficient, Btu/h·ft²·°F (ISO 11855): floor heating 1.94, ceiling heating 1.06, floor cooling 1.23, ceiling cooling 1.94

warm air rises off a warm floor and pools at a warm ceiling, so floors heat better and ceilings cool better per degree of surface temperature.

Table 4 collects the coefficients and limits.

Comfort limits set the surface temperature

Heating and coolingHot waterVentilation

Informs D1

ASHRAE 55 places a floor for shod occupants between about 66 and 84 °F and limits radiant asymmetry from a warm ceiling to about 9 °F (5 °C); a cool ceiling tolerates far more. Because a radiant room has no air stream, cold glass is felt directly as asymmetry, so window U and the placement of emitters near large glazing matter more than in a forced-air house (Windows & Doors · Condensation and cold glass). Operative temperature, the average of air and mean radiant temperature, is what occupants feel, so a radiant room is comfortable at a lower air temperature.

Floor covering and emitter build-up set the water temperature

Heating and coolingHot waterVentilation

Informs D1 D2

R_above tube concrete or plate over the tubing (≈ 0.15) · R_cover finish flooring

every layer between the tube and the room adds degrees the water must supply for the same output.

Table 5 and Figure 4 show that covering does not cap the output; it raises the water temperature needed to reach it. The cost appears at the heat pump: about 10% of COP per 10 °F of supply temperature (Part 4). Dry plate systems and ceiling panels add their own resistance and typically run 10–20 °F warmer than a slab for the same output.

Radiant delivery by floor: four constructions

Heating and coolingHot waterVentilation

Informs D1

Figure 5 sketches the four constructions and Table 6 compares them.

A heated slab is also a heat store, which works only to the extent the slab is insulated from the ground below and at its edge; the electricity it shifts is the stored heat divided by COP.

3 · Cooling and humidity: the dew-point limit

Bottom line. A radiant surface can only be as cold as the room's dew point allows, and it removes no moisture, so the latent load needs a separate path.

Radiant cooling is capped by the room dew point

Heating and coolingHot waterVentilation

Informs D3

A surface below the room dew point condenses, inside a floor or behind a ceiling where the water is not seen. Controls hold the supply water a margin (here 2 °F) above the measured dew point; the surface then runs a few degrees warmer than the water (about 3 °F for a ceiling panel, 5 °F for a slab), and comfort sets a minimum of about 66 °F for floors and 62 °F for ceilings (Figure 6). At 76 °F and 50% RH the dew point is 56 °F; at 60% RH it is 61 °F.

Against the cooling loads in Part 1, a radiant floor covers an average room and falls short in a west room with large glass; a ceiling covers both while the room dew point stays under about 58 °F (Table 7).

With no air coil, latent load needs its own remover

Heating and coolingHot waterVentilation

Informs D3 D5

Δgrains humidity ratio difference, grains per lb of dry air

moisture brought in by air is its flow times how much wetter it is than the room.

A forced-air coil dehumidifies as a side effect of cooling; a radiant system does not. At a 1% dehumidification design dew point of about 61 °F outdoors (Moffett Field, typical of the South Bay) and a 55 °F indoor target, the reference house takes on about 3,000 Btu/h of latent load: occupants 1,000, infiltration 1,000, ventilation 600 after an enthalpy core recovers about half, and cooking and bathing about 400. With a sensible-only (HRV) core the ventilation term doubles and the total is about 3,500. A ducted or stand-alone dehumidifier, an enthalpy core and a room dew-point sensor at each cooled zone are the means available.

4 · Heat pump: capacity, efficiency, buffer and backup

Bottom line. An air-to-water heat pump loses capacity and efficiency as outdoor air cools and as the water it makes gets hotter, so it is sized at design conditions and run at the lowest water temperature the emitters allow.

COP falls with water temperature and with outdoor temperature

Informs D2

T in kelvin, at the condenser (water + 3 K) and evaporator (air − 6 K) · η ≈ 0.5 for current units

efficiency depends on the temperature lift, so lowering the water temperature or raising the air temperature both raise it.

Table 8 and Figure 7 apply the model across emitter types and outdoor temperatures.

Capacity is read at the design temperature, not the rating point

Heating and coolingHot waterVentilation

Informs D2

Air-to-water units are rated near 47 °F outdoor air. Below that capacity typically falls 1.5–2% per °F, and between about 25 and 42 °F frost and defrost cycles take another 5–10%. With 1.7% per °F and an 8% defrost allowance, a unit delivers about 73% of its rated capacity at 35 °F, so the reference load of 27,600 Btu/h needs about 37,700 Btu/h of rated capacity (Figure 8).

At a 35 °F design, derating and defrost are the largest reason rated capacity exceeds the load; hot water recovery during a heating call and distribution losses add to it. Oversizing has costs of its own: a unit far larger than the load reaches its minimum modulation at mild temperatures and cycles.

Table 9 compares the configuration choices that sit alongside size.

Outdoor units emit about 55–65 dBA of sound power at full output, with fan and compressor tones; siting away from bedroom windows and property lines and mounting on isolators of adequate deflection follow the machine rules in Soundproofing · Isolation.

A buffer tank sets the minimum run time

Heating and coolingHot waterVentilation

Informs D2

q_min minimum compressor output, Btu/h · t_run minimum run time, min · ΔT_diff control differential, °F

the tank must absorb the compressor's smallest output for its minimum run without overshooting the control band; water flow is load divided by temperature drop.

A 3-ton unit modulating down to 30% (10,800 Btu/h) with a 10 minute minimum run and a 10 °F differential needs about 22 gal of buffer or equivalent loop volume. The reference design load at a 10 °F supply–return drop needs about 5.5 gpm. Many small zones on a large heat pump without a buffer short-cycle the compressor.

Backup heat and the electrical service

Heating and coolingHot waterVentilation

Informs D2

Title 24 (2025) requires heat pump supplementary heat to run only below 35 °F outdoors except during defrost or emergency (§150.0(h)7), caps resistance capacity at 2.7 kW per nominal ton (§150.0(h)8), requires defrost delay timers set to at least 90 minutes where adjustable (§150.0(h)6), and requires thermostats to lock it out above 35 °F (§150.0(i)). Whether the heat pump's controls prevent the compressor and the resistance element from running together decides whether the electrical load calculation may treat them as non-coincident (NEC 220.60), which can move a service calculation by tens of amperes (Electrical · Sensitivity of the calculated load).

5 · Hot water: draw, storage and recovery

Bottom line. A household's peak hour sets the stored volume, storage temperature stretches it, and a shared heat pump makes hot water at its least efficient water temperature.

Peak-hour draw sets the storage

Heating and coolingHot waterVentilation

Informs D4

T_cold incoming water, about 58 °F in coastal California

water stored hotter and mixed down at a valve delivers more usable volume from the same tank.

Table 10 builds the peak hour. Storing at 140 °F with a mixing valve makes a 50 gal tank deliver about 66 gal at 120 °F, and holds the tank above the range where Legionella grows (ASHRAE 188 guidance). An indirect coil needs water about 10 °F above the tank setpoint, so 140 °F storage needs about 150 °F leaving water at a model COP near 2.6 at 47 °F; units that cannot reach it use a resistance top-up or a periodic high-temperature cycle. Recovery from 20,000 Btu/h of heat pump output is about 29 gal/h (Figure 9). A third shower in the hour raises the peak to about 50 gal.

Shared heat pump or a dedicated water heater

Heating and coolingHot waterVentilation

Informs D4

Table 11 compares the two. With a shared unit, priority logic decides which load waits: a hot water call during a heating call pauses the emitters, which a slab rides through and light panels do not.

6 · Ventilation: rate, recovery and duct layout

Bottom line. The whole-dwelling rate follows floor area and bedrooms; an ERV recovers most of its heat, and the duct layout decides the acoustic and routing consequences.

The whole-dwelling rate follows floor area and bedrooms

Heating and coolingHot waterVentilation

Informs D5

cfm; A_floor conditioned floor area, sf

the house needs a base flow per square foot plus a share per likely occupant.

Table 12 gives the rate. Local exhaust is added on top: kitchens and bathrooms each need a fan, with ASHRAE 62.2 minimums of 100 cfm intermittent or 5 ACH continuous for kitchens and 50 cfm intermittent or 20 cfm continuous for baths, and Title 24 adds range-hood capture and sound requirements for new or replaced hoods with ducting. A balanced ERV can provide the bath exhaust continuously; a range hood stays separate because cooking grease does not belong in a recovery core.

An ERV recovers most of the ventilation heat

Heating and coolingHot waterVentilation

Informs D3 D5

At design, 106 cfm with no recovery would cost 4,000 Btu/h; at 70% sensible effectiveness the core returns about 2,800 of it (Figure 10). An enthalpy core also moves moisture, which halves the summer latent load from ventilation (Part 3) and, in a dry winter, keeps indoor humidity from falling as far. Supply goes to bedrooms and living rooms, exhaust comes from baths, laundry and kitchen, so air crosses the house through door undercuts or transfer paths, which are also sound paths.

Duct topology decides crosstalk between rooms

Heating and coolingHot waterVentilation

Informs D5

Any two terminals joined by a short duct path pass sound around the wall between them (Soundproofing · Ducts). Home runs from a manifold put the full length of both ducts between any two rooms; a trunk with tees joins neighbors by a short path that a downstream silencer cannot reach (Figure 11). Lining, attenuators and terminal velocity are treated in the Soundproofing guide.

7 · Distribution: where pipes and ducts can run

Bottom line. A two-story house under trusses has three routing zones, each bounded by a different rule, and all distribution has to fit them.

Routing zones are set by the foundation and the roof framing

Informs D1 D5

In the attic, ducts and pipes pass through open truss webs or below the bottom chords; no member is cut (Framing · No truss member is cut), and ducts there are outside the thermal boundary and need insulation and sealing. In the floor framing between stories, runs follow the joist bays, and holes across joists stay within the notching and boring limits (Framing · Notching and boring). In a slab, tubing is cast in or laid in a topping; nothing runs below it, so ground-floor ducts come down from the floor framing above or through chases and soffits (Figure 12). Over a raised crawlspace the ground floor is framed: tubing goes under the subfloor in plates from below, and ducts and pipes can run in the crawlspace, outside the boundary unless the crawlspace is conditioned.

Pipe and duct insulation, and the hydronic loop

Title 24 requires insulation on space-conditioning and hot water piping (§150.0(j)) and on ducts outside conditioned space (R-6 in CZ 3, R-8 in CZ 4 for altered systems), with duct leakage tested where a space-conditioning duct system is new or extended past 25 ft; whether that test applies to ventilation-only ducts is a plan-check question. A chilled-water pipe also needs a vapor-tight jacket, because a cold pipe in warm air condenses. The plumbing and mechanical codes govern the loop: oxygen-barrier tubing where ferrous parts share the loop, an expansion tank sized to the water volume, backflow protection at the fill, temperature and pressure relief on the hot water tank, and pressure relief on the closed loop and its buffer.

8 · Code, permits and documentation

Which code applies

The 2025 Energy Code (Title 24 Part 6) applies to permits filed from 1 January 2026, with the California Mechanical Code for hydronics and ventilation equipment and the California Plumbing Code for water piping and hot water. AB 130 (2025) holds residential standards at this cycle until 1 June 2031.

Figure 13 draws which provisions a project engages.

Table 13 lists the provisions. Documents follow the Insulation pattern: CF1R at plan check, CF2R from the installer, and CF3R where an ECC rater verifies a measure such as duct leakage, refrigerant charge or ventilation airflow.

Where it goes wrong

1. Sizing at the rating point or at a mild design temperature. A unit rated at 47 °F delivers about three quarters of that at 35 °F with defrost (Part 4, Part 1).
2. High-resistance coverings chosen without a water temperature check. The floor still reaches its output, at a supply temperature that costs COP (Part 2).
3. Air left in the loop and zones unbalanced. Without air elimination at the high point and balancing at the manifold, distant loops run cold.
4. Dew-point sensors in the wrong place. A sensor in a hall misses a bathroom or kitchen zone that cools; each cooled zone reads its own dew point (Part 3).
5. Assuming the ERV dries the house. An HRV core moves no moisture; an enthalpy core reduces the ventilation latent load but removes none of the rest (Part 3).
6. A zoned system with no buffer or loop volume (Part 4).
7. The resistance interlock treated as a setting. It is a product capability that the electrical calculation depends on (Part 4).
8. Mixing valve missing or set high. Storage at 140 °F without a tempering valve at the heater is a scald risk at the fixtures (Part 5).
9. Ventilation airflow fails verification. Long or crushed flex and added silencers raise static pressure, and the measured airflow on the CF3R falls short of the design rate (Part 6).
10. ERV core frost and filters. Cores frost in cold snaps without a defrost strategy, and clogged filters cut airflow without a symptom (Part 6).
11. A trunk-and-tee ERV layout that joins rooms acoustically (Part 6).
12. A truss web cut for a duct (Part 7).

Reference

Glossary

AWHP

Air-to-water heat pump.

Balance point

Outdoor temperature at which heat pump capacity equals the house load.

Buffer tank

Water volume between the heat pump and the zones that lengthens compressor run time.

Enthalpy core

ERV core that transfers moisture as well as heat.

First-hour rating

Hot water a heater can deliver in one hour starting full.

LWT

Leaving water temperature from the heat pump.

Operative temperature

Average of air and mean radiant temperature; what occupants feel.

Radiant asymmetry

Difference in radiant temperature between opposite directions, such as a warm ceiling over a cool floor.

Sensible, latent

Heat that changes temperature, and heat carried by moisture.

Winter Median of Extremes

The JA2 heating design temperature column used by Title 24.

Formula sheet

Figure color legend

Red: deficient, or fails the stated criterionAmber: marginal or conditionalGreen: meets the criterionGray: existing or baselineTeal: neutral quantity, reference line or annotation; never a grade

Water and tubing are drawn in blue, as a material; the house load line in the capacity figure is dark gray.

Code points and inputs to confirm

• JA2 Winter Median of Extremes and 1% cooling values for the parcel
• How §150.0(h) and §150.0(i) apply to an air-to-water hydronic heat pump
• §150.0(o) infiltration credit for single-family dwellings and the ventilation triggers in §150.2(b)1L–M
• Pipe insulation (§150.0(j)) and duct insulation values for altered systems
• Product data: capacity and COP curves at the chosen water temperatures, minimum modulation, defrost behavior and resistance interlock

Sources

• California Energy Commission, What's New for 2025: Single-Family — §150.0(h)5–9, §150.0(i), §110.3(c)7, §150.0(o) filter and HRV/ERV provisions.
• Energy Code Ace, Building Location Data — heating design at the Winter Median of Extremes, cooling at 1% (JA2).
• 3C-REN, 2025 Energy Code in Practice: Single Family Additions and Alterations (March 2026) — §150.2(b)1C, ventilation in alterations, duct insulation R-6 / R-8.
• CEC-CF1R-ALT-02-E (2025) — duct insulation, leakage and airflow for altered systems.
• NOAA Engineering Weather Data, Mountain View (Moffett Field) — 99% heating 39 °F, 99.6% 37 °F, 1% cooling 84 °F with 65 °F mean coincident wet bulb, 1% dehumidification 61 °F dew point; PG&E CZ 4 summary — San Jose 99% 34 °F.
• CEC, 2025 Residential Mandatory Measures Summary — §150.0(h)5–8, §150.0(i), §150.0(m)1 and (m)12, §110.3(c)7.
• ASHRAE 62.2 (ventilation and local exhaust); ASHRAE 55 (thermal comfort); ASHRAE 188 (Legionella); ISO 11855 (radiant system coefficients).
• Companion guides: Insulation, Framing, Soundproofing, Electrical Panel Strategy, Windows & Exterior Doors (set v2).

Set v2 draft 1, 2026-10-23: rewritten as a generic theory guide to spec set v2 from the v1 guide (archived). Design temperature corrected from 45 °F to a 35 °F reference with the Title 24 JA2 rule; ceiling emission coefficient taken from ISO 11855; slab storage stated in electricity shifted; floor-over-garage load assigned to the second floor; latent load, COP, capacity, buffer, configuration and cooling added. After review: latent load on the 61 °F dehumidification dew point, cooling capacity with water-to-surface rise and minimum surfaces, hot water at 150 °F leaving water. Values from one script. 2026-10-23: backup-heat interlock links to the Electrical guide (set v2).

↑ Back to top