There are some old radiator sizing formulas but they will most often produce wildly oversized systems. If you’re good and accurate with measurement, are quite familiar with the construction of your home and like doing “new” things, go to and download the one-time use program for a modest fee.

To know a bit of what you’ll need: surface area of walls exposed to the outside; surface area of glass; surface area of ceilings & floors; surface area of partitions adjoining “unheated” or “less heated” areas; levels of insulation in all these areas; basic type of construction to include sheathing, doors, windows, etc. Much of the surface area can be calculated automatically given dimensions. Since spaces loose most of their heat through the ceiling, it is important to be EXTREMELY thorough with ceilings and a given room will often have MULTIPLE “types” of ceiling; i.e. a room under a finished attic (the eave space will be different even if unheated), a projecting bay window, stairwell or a “bump-out” addition.

An ACCURATE heat loss calculation will tell you how much heat the structure requires as a whole and how much each space requires as well.

Radiators are sized based on their Equivalent Direct Radiation or EDR. This is an OLD throwback to the earliest form of radiator that was essentially a flat panel looking something like a mattress. EDR is measured in square feet and sizing data is readily available for most old rads and all new rads.

Unless you’re REALLY handy and are willing to study quite a lot, installing a good hydronic system isn’t a DIY project. Even if you size everything and run your own piping have a pro install the boiler–or at an absolute minimum pay them well to review your boiler installation.

Once you’ve computed your heat loss, the next step is to determine the general temperature at which you want the system to run.

Remember that radiators DO radiate significantly and that radiation is generally regarded as the most comfortable and efficient method of space heating. In general, the lower the temperature of the radiator the higher its proportion of radiation; the higher the temperature the higher its proportion of convection.

In other terms, you can (within limits) make radiators as small or as large as you like–they just have to ALL be reasonably proportional to the heat loss of each space. There are however some problems if you plan for large radiators with low temperature requirement:

1) heat loss calculations are based on an “average” LOW winter temperature in your area–consequently the system will rarely be loosing the “full” amount of the heat loss.

2) Many boilers do not “like” low temperatures and as manufacturers have wrung the last drop of efficiency out of traditional designs their sensitivity to low temperature has increased greatly.

3) Traditional boilers achieve their best efficiency at higher temperatures around 180 deg F.

Consequently, a system with low temperature requirement teamed with a traditional boiler MUST have measures taken to protect the boiler from low temperature–there are a number of ways but all add complexity, cost and are generally counter-productive fuel-wise.

Condensing boilers are fairly new. They are INTENDED to operate at low temperature and achieve their best efficiency when supplying water BELOW 140 deg F or so. In general, the lower the supply temperature the higher the boilers’ efficiency.

Flue gasses hold water–LOTS of water–POUNDS of it. It is never “seen” in conventional boilers because the temperature of the flue gas is high enough to keep the water in the form of invisible vapor. When the temperature of the flue gas drops below the condensation point (dewpoint) the invisible vapor appears in the form of water, but not quite pure water. It is slightly acid, about like tomato juice. If the condensation occurs inside a conventional boiler (or its flue) these components corrode–sometimes very rapidly.

Most condensing boilers aren’t any more efficient at heating water initially. They gain efficiency by recovering the heat contained in the condensate–LOTS of it as there is so much water and water holds so much heat. The process itself is simple, but the realization is much more difficult.

Like condensing warm-air furnaces, some condensing boilers have had problems dealing with the mildly corrosive nature of condensate and consequently heat exchangers are made from rather exotic and expensive materials. Again, like condensing furnaces, condensing boilers all have “forced” draft–a fan of some type supplies combustion air to the burner instead of plain ambient air in many conventional models. Some forced draft equipment is quite loud–both inside and at their exhaust port outside. Condensing boilers MUST NOT be vented through a typical old house chimney!

Europeans (particularly Germans) are WAY ahead of the U.S. regarding condensing equipment. One of the main reasons for this is the fact that hydronics have a very limited market in the U.S.–thus less money available for research and development.

Even if a U.S. mfgr WOULD make their own condensing boiler the standard by which they are rated (the AFUE “number” you see from ASHRAE) DOES NOT ACCURATELY PORTRAY THEIR TRUE EFFICIENCY AS INSTALLED IN A SYSTEM!!!

While a few condensing boilers may “appear” American, they are really assemblages of “stock” parts from all over Europe. The condensing boiler with the longest and best track record assembled by an American company is most likely the Monitor MZ. Most “simple” condensing boilers can be fueled with natural gas, propane or fuel oil.

Once your heat loss is complete, you have sized your radiators and designed a system appropriate to the temperature at which it will operate, you must consider how the system will be controlled.

The most common way (in the U.S.) is digital–the wall thermostat that everyone knows. The flow of heat is either “on” or “off” NEVER “in-between.” Even if the system is divided into “zones” you can rest assured that the system IS digital if it is controlled by wall thermostats.

The other method to control (VASTLY superior by the way) is proportionally. The amount of heat delivered to a radiator depends on the temperature OUTSIDE and the desired temperature INSIDE–it is not “on” or “off” in most circumstance, but continually and automatically changing based on conditions–both IN AND OUT OF YOUR CONTROL.

While this may sound fantastic and hideously complex, IT IS NOT! Fully self-contained devices called Thermostatic Radiator Valves (TRVs) have existed for DECADES and are known for high reliability, high efficiency and incredible comfort. These devices are REQUIRED in much of Europe. While there are many manufacturers (some even in the U.S.–even if they do actually make them in Mexico) the “best” are likely from a Danish company named Danfoss. As even “plain” radiator valves have become a specialty item in the U.S., the price of TRVs is not particularly high and WELL WORTH THE COST. You wind up with the ability to keep each and every space at whatever temperature you desire with amazing accuracy. While some systems using TRVs will still have a wall thermostat, the better way (in my mind) has NO indoor thermostat save the TRVs installed on EVERY radiator.

Until quite recently ALL boilers available for sale in the U.S. have been digital–i.e. full fire or no fire–nothing in between. In an attempt to reduce fuel use, many systems (both digital and proportional) use outdoor reset. Outdoor reset adjusts the temperature in the system based on the temperature outside–i.e. as it gets colder outside, the water gets hotter. Reset certainly works well and can certainly save energy, but those “problems” with low temperatures in conventional boilers can really bite you in the a## when teamed with reset.

Enter the condensing, modulating boiler. There are not many available in the U.S. at the current time. Of those few that are assembled in the U.S., problems (warranty-required INSIDE heat exchanger cleaning requirement in one and questionable exchanger material/longevity in another) preclude my recommendation. The “best” is likely the Vitodens by Viessmann (Germany)

The Vitodens has an absolutely unique “burner” that does not produce flame in the conventional sense–it transfers the vast majority of its heat to the water via radiation. When installed in a system that has variable flow (TRVs), even the built-in circulating pump modulates! On such a system the burner will almost always be “on” at some level as long as the structure requires heat. Real-world “numbers” coming from this boiler are absolutely extraordinary regarding efficiency. Flue gas temperature is often only a few (less than five) degrees higher than the temperature of the supply water! Of course it is highly complex and very expensive but based on its use in Europe for much longer and stellar reputation for Viessmann reliability it really deserves the highest consideration. Remember that it is designed in Germany where TRVs are REQUIRED in any space larger than a closet and it will achieve its best efficiency when TRVs are used.

Standing water-filled cast iron radiators have been associated with gentle even heat, comfort, efficiency, reliability, cleanliness, adjustability and safety since their inception. While some may view them as clunky and hopelessly old-fashioned, no other space heating method in common use today offers this wonderful mix of qualities. Systems using standing iron radiation have ALWAYS been expensive. They have always required SKILLED labor to work at their best. No other space heating systems has (or likely ever will) existed that IMPROVES with age and whose lifespan is truly indeterminate with CENTURIES actually possible!

How, you ask, can a heating system possibly IMPROVE with age? Much of the system remains reasonably accessible in the event of any problems. Continued improvements in both boiler and control technology will always be able introduced with little or no modification. As homes age they become “leakier” as do forced air systems–hydronic systems MUST stay sealed to operate and they will NOT introduce air pressurization problems as the home proper ages. Any conceivable new and fantastic energy technology is CERTAIN to be able to heat a liquid that in turn heats a physical object like an iron radiator.

As a “heat only” source, you will NEVER have to replace both heat and cooling simultaneously as often happens with combined forced air systems. Combined heat/cool forced air systems by the way ALWAYS have concessions that make them less efficient/comfortable with either heat or cooling. Standing iron will ALWAYS make a bold statement of “permanence” to many home buyers and resale value is maximized for those who demand quality over “flash.”

Mike Thies


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