Fan convector hydronic unit in NYC apartment  (C) Daniel Friedman Guide to Water-to-Air Heating Fan Convectors: Inspection, Repair, Maintenance

  • FAN CONVECTOR HEATERS - HYDRONIC COILS - CONTENTS: How to identify, inspect, diagnose, and repair problems with hot water to air heating convectors - hydronic heating fan convectors & hydronic heating coils. Guide to convector fan heaters and similar heat sources in buildings. Freeze Proofing by adding water to air or electric fan convector heating units - convector fan heating. Backup heat for heat pump systems using water to air hydronic heating coils. Hydronic heating coils for backup heat or water to warm air heat conversion
  • POST a QUESTION or READ FAQs about heating convectors and water to air fan convector heating units, problem diagnosis & repair

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Water-to-air fan-convector heating systems:

Here we explain the identification, selection, diagnosis and repair of hydronic or steam heated convector fan heaters and similar water-to-air or steam-to-air heating devices such as ceiling, wall or floor mounted heating convectors and heating coils in heat pump air handlers and ductwork. This article focuses on heating convectors heated by hot water from a heating boiler or heat pump. Hydronic heating convectors use hot water or steam from a separate boiler to heat a water-to-air heat exchanger coil mounted in the heating convector unit. A thermostat or manual switch operates a fan to blow room air across the heat exchanger coil to deliver heat where it is wanted.

Our photo above (at page top) shows a hydronic (hot water) heated wall-mounted fan convector heater in a New York City apartment. You can see dual blower fans at the bottom of the unit (the air intake port); Hot water piping from the building boiler is visible at the left edge of this convector heater.

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How Ceiling, Wall, or Floor Fan Convector Units or Hydronic Heating Coils Convert Hot Water Heat to Warm Air Heat

Hydronic coil converts hot water to warm air heat (C) Daniel Friedman

Fan convector heaters combine a heat source such as hot water piped through a heat exchanger coil and a fan to blow warm air into HVAC ductwork of a warm air heating system or to push warm air into a chilly spot such as a basement, crawl space or into a bathroom or kitchen where there may not be wall space for a conventional heating register radiator.

Fan Convector Heaters as Backup Heat for Heat Pumps

Fan convector heater units as a main building heat source or as a back up heat source where a heat pump is installed (combining air conditioning and heating).

As our photo of a hydronic heating coil (photo at left) shows, hot water heated by a hydronic heating boiler is used to warm air via a heat exchanger that is mounted in an air handler.

Pipes conduct hot water from the boiler to the heat exchanger and back to the boiler. The air handler blows building air from return registers and ducts into the air handler, across the heating coil, and on to supply ducts and registers.

We illustrate and discuss conventional heating convectors (units that work by "gravity" or warm air convection without the added output of an internal fan) at RADIATORS.

Fan Convector Heaters as Add-On Heat for Crawl Spaces, Basements, Garages

Used in crawl spaces, garages, basements, or other locations where additional heat is required or where we want to add heat to a previously un-heated area, ceiling, wall, or floor mounted fan convector heating units are a convenient way to push warm air heat into a chilly or un-heated area by converting hot water heat produced by a heating boiler to warm air heat provided by a fan convector unit.

Fan Convector Heaters as Add-On Heat for Freeze Protection in buildings

We often find an electric or hot water-heated convector unit mounted in crawl spaces or other building areas in freezing climates as a step to protect building plumbing from freeze-ups.

Plate warming home heating radiator (C) Daniel Friedman

Notice that the convector heater suspended from the crawl space ceiling (photo at left) has nothing combustible too close to the heater? Make sure that your added heat source does not create a fire risk

This is an electric ceiling mounted fan convector unit.

Similar fan convector heaters obtain their heat from hot water piped as a separate heating zone or as a loop off of an existing heating zone that is otherwise heated by a conventional hot water heating boiler or hydronic heating system fueled by heating oil, gas, a heat pump, or even solar heating.

You can see an example of a ceiling mounted fan convector used to avoid freezing pipes in a crawl area at Heat tapes, Heat, Insulation prevent Freeze-Up.

Reader Question: standard temperature ranges for heating water at the boiler, at the baseboard or at the convector heater unit of a PTAC (Packaged Terminal Air Conditioning / Heating Unit)

Trane PTAC Models PTEE or PTHE 7000 to 15000 BTUH

[Click to enlarge any image] Above: A Trane packagted terminal air conditioner & heat pump unit model PTEE or PTHE, cited below.

2016/02/14 Rich said:

I own a condominium unit in a 7 story building in NYC. The heat is provided by a PTAC unit with a heat convector below the unit and a fan in the PTAC that draws air through the convector and blows the warm air through the vent at the top of the PTAC unit.

I've owned the unit for 15 years and, until recently, the heat dispersed by the convector was always sufficient to heat the unit without the blower, unless temperatures were below 20 degrees. Then we would have to turn on the blower. The last two years, it seems building management has lowered the water temperature of the hot water that flows through the system and the convector is never hot to the touch. It is only warm.

I had the super test the temperature of the pipe feeding hot water to the unit and the temperature of the metal side of the unit. The temperature of the pipe was 137 degrees and the unit was 106 degrees. I'd like to know if there is a standard temperature, or temperature range, of the water entering the unit and a standard temperature, or temperature range, of the convector surface.


Interesting question, Rich. I don't know an authoritative answer but here is some data along with some arm-waving.

There are some conflicting objectives in building heat management - I don't think the problem is peculiar to PTAC units (Packaged terminal air conditioning / heating).

1. Lower boiler temperature to save heating cost?

The super lowers the boiler temperature thinking that they are saving on heating cost. And indeed there are some (more sophisticated) heating controls that adjust the boiler set temperature as outdoor temperatures vary, using lower temperatures when the outdoor temperature is higher. I know there are claimed heating savings but I mistrust them a bit as I'll explain. I will agree that it wastes energy to overheat a room in response to a call for heat, particularly if there is one thermostat controlling heat in multiple rooms whose individual rate of heat loss will vary.

2. Higher boiler temperatures give faster & more efficient heat transfer into the occupied space

Because the thermal conductivity of water is exponentially greater at higher temperatures, we actually get more efficient heat transfer from the heating convector into the room air when the temperature difference between the convector water and the room air is greatest. Or re-stating, we get not only faster but more efficient transfer of heat out of the boiler water, through the finned copper tubing into the room air when the boiler water is hotter.

3. Get objective heating data by measuring actual water temperatures

Because heating baseboard runs vary in length, diameter, and in radiating device designs (some convectors or baseboards are designed to output far more heat per linear foot of radiating device than others), and because heating water velocity (typically 4 gpm) is also variable depending on system design and controls, I can't predict what the water temperature will be at a specific point in a hot water heating piping loop for an unknown building. Some HVAC engineers might make a stab at it but IMO for an installed system instead of arm-waving we should just measure temperatures - as you have done. You may want also to measure the water temperature at the INLET end of the heating convector as well as at the OUTLET end. That will give us some important data about the heat transfer rate.

With that data and specs on the convector heater, along with air temperature and air speed, one could predict what the heating capacity of the convector is.

You can see how challenging this becomes when we want true objective data. Because variations in outdoor temperature as well as variations in the heat loss rate for a given heated building or apartment or room all must be considered in making a statement about the required input BTUh to give adequate heat.

4. Recommended boiler operating temperatures

With all of the apology above, I prefer to run a boiler as hot as possible - close to 200F.


5. Hot water heat design temperatures & BTU output rates:

Typically the heating system is designed so that by the end of the heating loop the temperature drop will be about 20 deg. F. between the temperature of water leaving the boiler to the temperature of water at the end of the heating loop.

I guesstimate that in a tall building with many occupying units, if you go to the most-distant point in the heating water loop - where the temperature would be expected to be at its lowest, you will probably find that if the water is under 150 °F the heat output is going to be rather modest. Or from some sources, a linear foot of baseboard (that's less than at a convector) will output about 600 btuh at 180 °F and a flow rate of 4 gpm.

The specs for your convector will be different (ask the manufacturer). But using typical 3/4" finned copper baseboard tubing, at 4 gpm flow, at 200 °F we get about 750 BTUh per foot, dropping to 340 BTUh at 140 °F. Here are some more example numbers courtesy of Slant/Fin:

Rated Heat Output for Finned Copper Baseboard in Residential Installations

Heater Element Type Water Flow Rate GPM  Pressure Drop1 BTUs / Hour / Linear Foot at 65 °F entering air2
110°F4 120°F4 130°F4 140°F 150°F 160°F 170°F 180°F 190°F 200°F3
NO. 30-75
Baseboard with
3/4" E-75 element
1 gpm 47 160 210 260 320 380 450 510 580 640 710
gpm4 525 170 220 270 340 400 480 540 610 680 750


Source: Slant/Fin see citation given below.

1. Pressure drop given in milli-inches per foot.

2. Ratings at 140°F determined by multiplying 150°F rating by the I=B=R conversion multiplier of .84.

Ratings are for element installed with damper open, with expansion cradles. Ratings are based on active finned length (5" to 6" less than overall length)
and include 15% heating effect factor. Use 4 gpm ratings only when flow is known to be equal to or greater than 4 gpm; otherwise, 1 gpm ratings must be used

3. The original source gives added data up to 220°F

4. The three lowest temperatures, 110°F, 120°F 130°F are ratings given for condensing boilers.

If the temperature drops from 200 °F at the boiler to 140 °F at the input to your convector, the heat output at that convector is cut in half. The heated space will need enough convector capacity (or for baseboards, linear feet of baseboard) to provide enough BTUh to handle the room's heat loss on the coldest days of the year.

6. Recommended input water temperatures for PTAC units

Trane PTAC air conditioner heater installed, original source

Using as an example Trane PTAC units such as the Trane PTAC shown installed above (source cited below), the BTUh heating capacity ranges (depending on model and number of heaters employed) between 5,800 BTUh and 15,000 BTUh. The heat output varies by water flow rate, system pressure drop, and the fan speed. But only two water temperatures were cited by Trane for these PTAC Units

PTAC Unit Heating Capacity using Hot Water using Trane PTAC unit data

  7000 & 9000 Btuh PTAC Units 12000 Btuh PTAC Units 15000 Btuh PTAC Units
GPM Pressure Drop (psig) 200 °F EWT1 180 °F EWT 200 °F EWT1 180 °F EWT 200 °F EWT1 180 °F EWT
Fan Speed Fan Speed Fan Speed Fan Speed Fan Speed Fan Speed
Coil 2-way valve Hi Lo Hi Lo Hi Lo Hi Lo Hi Lo Hi Lo
1.00 0.93 0.16 14.9 13.4 12.6 11.2 16.9 14.4 14.3 12.2 17.9 15.5 15.0 13.3
2.00 2.10 0.64 16.8 15.2 14.2 12.7 19.3 16.1 16.2 13.6 20.4 17.8 17.2 15.3
2.75 3.85 1.21 17.2 15.5 14.6 13.0 20.0 16.6 16.8 14.0 21.1 18.1 17.7 15.5


Source: Trane, cited below. The data in this table is excerpted from Trane's Table 9, Heating Capacity - water. "Trane Packaged Terminal Air Conditioners & Heat Pumps, Catalog", (March 2007), retrieved 2016/02/14, for PTAC models PTEE and PTHE, PTAC-PRC002-EN, original source:

The Trane units cited below also are offered in steam heated models for steam pressures between 2 and 5 psig. Depending on steam pressure and fan speed and model, the BTUh output for heating ranges from about 17,800 BTUh to 26,300 BTUh.

Based on ARI Rating Conditions of 70°F Entering Air Temp., 200°F Entering Water Temp and 180°F Leaving Water Temp., Max. Water Temperature 200°F Max. Water Pressure - 200 psi

1. EWT = Entering Water Temperature

Check with the manufacturer of your PTAC units to find the design input water temperatures for your unit by brand and model.

7. Other stuff to check:

Finally - well for now anyway - check that the convector is not dust clogged so we can avoid THAT embarrassment.

Question: what do you call the heating system that pumps hot water to "radiators" inside of a forced air system?

2016/05/29 Pat said:
The house I just put an offer on has a boiler, however, it does not have traditional radiators. It pumps hot water to radiators inside the forced air system. I have never seen a system like this. What isint called and is it efficient? Also, I am planning on doing a renovation to the home and was wondering should I replace the system with a traditional forced air system?

This question was posted originally at BOILERS, HEATING

Reply: fan coil unit, hydronic coil, fan convector heater properties, efficiency, references

Thanks for the question, Pat. You are describing a water-to-air heating system. Synonyms include "hydronic coil" and "fan convector" or "fan coil unit" heating system. Hot water heated by a heating boiler (a hydronic heating system) is circulated through a finned coil placed in an air handler or in some cases right in an air duct. Air is blow through the duct (or through the air handler) across the heated coil, thus heating the air as it passes by.

Your system may be a "blow through" (fan pushes air through the heating coil) or a more thermally-efficient "draw through" (fan sucks air thorugh the duct system and through the heating coil).

This approach to heating is often used when a designer wants to use the same duct system for distributing heated air or cooled air; In some designs the use of a a boiler-heated fan coil to warm air can be quite efficient. These systems are also often used as backup heat in a heat pump system.

Take a look at
FAN CONVECTOR HEATERS - HYDRONIC COILS beginning at the top of this page

Water to Air Hydronic Air Heat, Convector & Baseboard Heat Output Resources


Continue reading at WALL CONVECTORS HVAC or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see ELECTRIC HEAT where we show a wide variety of electric-heated wall, floor, and kick space fan convector heating units.

Suggested citation for this web page

FAN CONVECTOR HEATERS - HYDRONIC COILS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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