Respect the radiator

0

By Roy Collver

“Now is prime opportunity time to interest people in retrofit hydronic radiator systems to replace their electric baseboards.”

When most people around the world think of hot water heating systems (if they think of them at all), they think of radiators. Nice and cozy, quiet and comfortable – radiators used to rule the roost in North America too. From the massive cast-iron column radiator of yore to the skinny little high temperature baseboard convectors of the 50’s and 60’s – radiators were the heat terminal units of choice for good reason. Radiant floor heating has held our attention for the last three decades, but nothing beats the “lowly” radiator for flexibility, ease of installation combined with affordability and, yes, comfort.

Radiators got a bad rap from the days where economy designers and builders went for the most inexpensive hydronic baseboard systems for tract housing and apartment buildings. They saved money and wall space by operating the radiators at the highest practical design temperatures, reducing the length of each baseboard.

To make matters worse, they would run a whole house or apartment from a single thermostat, often with no balancing valves to trim heat output in individual rooms. The boilers ran at design temperature all heating season, meaning occupants suffered from expansion noise, air temperature swings, over heating in some zones, and under heating in other zones.

But then we developed and fine-tuned what I call the modern triumvirate of hydronic heating comfort, making it easy for designers to put superior systems together using radiators:

  • Detailed and accurate computer programs for easy room-by-room heat load calculation
  • Variable flow control devices (pumps, thermostatic radiator valves, modulating valves)
  • Full outdoor reset control (utilizing condensing boilers and mixing devices)

This Stelrad bathroom radiator doubles as a towel rack/warmer.

Better design

Balance is the key for comfort when using radiators. First and foremost, they can be sized precisely to the calculated design load of each heating zone. The flow and temperature through the radiator can then be trimmed constantly to match the heat output to the actual zone conditions.

The best news is that you can have it all without having it cost a fortune. Most condensing boilers come already loaded for outdoor reset. Add a variable speed circulator for your system flow and some thermostatic radiator valves to trim the flow in each radiator and you have provided your customer with a gold standard system.

The sky is the limit from there, of course. I will leave it to each reader to explore all of the sophisticated stuff being provided by your local hydronics suppliers – I can hardly keep up with it anymore as new players come into the market – but I really don’t think there is much advantage to running the average heating system from “The Cloud” or similar such razzle-dazzle. If your customer likes gizmos – knock yourself out – just make sure it’s what THEY want rather than because of YOUR fascination with some new doo-dad.

Radiator fundamentals

The basics of radiators couldn’t be simpler. For any given radiator the heat output is dependent on just a few basic factors. Here are the two sides of the equation:

Water-side performance is based on heat transfer material and surface area + fluid temperature + fluid flow rate

Air-side performance is based on heat transfer material and surface area + air temperature + air flow rate + average unheated surface temperature

Of course each side of the equation is totally dependent on the other. The air temperature affects the temperature of the heat transfer surface that the water-side sees, and visa-versa.

Fortunately, the manufacturers of radiators do most of the math for you, based on their testing data, making it easy to select the right size and type of unit. Typically, they will have a very intuitive way of choosing a radiator model using a chart listing the various sizes of radiator, along with the number of Btu’s or Kilowatts per hour each size will deliver – but you have to really pay attention because the charts can get complicated really fast. Before you start, sit down and assemble the following information:

  • Btu/h output required for each zone at heating design conditions (coldest weather)
  • Desired indoor air temperature for each zone.
  • Entering air temperature (EAT) expected at the intake side of a convector-type radiator if applicable – usually a few degrees cooler than the desired air temperature
  • The supply water temperature (EWT) you will be feeding the radiator at design conditions (cooler is better for efficiency, but the rads get bigger as the supply water temperature drops)
  • Desired temperature drop (Delta-T) of the water as it passes through the radiator

Correction factors may be required if your system is significantly different from “normal” conditions. The charts should have those factors, sometimes as a footnote.

Two Slant/Fin baseboard radiators – a residential model on the left and commercial/institutional version on the right.

Sizing considerations

Look at your plans to confirm the wall space available. One-piece radiators such as panel rads come in fixed sizes and fixed output ratings per model size. The charts for baseboard convectors typically will give you a Btu/h per linear foot rating and you size them as needed for the heat loss. You will normally have to work back and forth and try different sizes and configurations, maybe even split the load in larger zones between multiple radiators. Work with different charts from different manufacturers until you see the similarities between them all – it really is quite simple once you fully understand the basics.

If you find the radiator you like does not have enough output, you can modify the following parameters:

  • Up-size the radiator (space permitting)
  • Supply hotter water (energy penalty)
  • Give it more water flow (reduces Delta-T but raises average water temperature)
  • Change the radiator to a style that provides more air convection (same as above but with an air side modification of conditions). You may even look at forced-flow (fan assist) radiators or air handlers for difficult zones.

Seen at a trade show, Eco-Con radiators are designed for low temperature systems.

Recent innovations

So what is new in the radiator universe? There have been some important innovations in the design and availability of radiators that will deliver more Btu/h at cooler water temperatures. These radiators can take full advantage of condensing boiler efficiencies and can simplify piping and control work by allowing the radiator to operate at the same temperature as radiant floor panels – avoiding the addition of expensive controls or mixing devices.

Look at the manufacturer’s charts and start looking at the outputs with 120°F(49°C) to 140°F(60°C) supply water temperatures. Also start designing with higher Delta-T. Look at 30°F(17°C) and even 40°F(23°C) Delta-T in order to send cooler return water back to the boiler. Designing with home run systems also promotes lower return water temperatures, but make sure you maintain minimum boiler flow rates with some sort of bypass arrangement or buffer tank.

It is interesting to speculate about the future market for hydronic radiators. I chatted with Robert Moroney of Express Radiant in Barrie, Ont. recently and he told me that it is pretty much business as usual – a healthy renovation and replacement market, with minimal new construction, but considerable interest in the replacement of electric baseboard heaters with a boiler and hydronic radiators. The last was no surprise to me after hearing tales of woe from homeowners with electric baseboard heating here on Vancouver Island. Hair-raising electricity bills in a coastal climate (Zone 8) translate into breath-taking electricity bills in the rest of Canada.

Now is prime opportunity time to interest people in retrofit hydronic radiator systems to replace their electric baseboards. Using a high efficiency gas or oil-fired boiler in combination with low temperature radiators will provide outstanding savings on the heating bill and improve their comfort to boot.

Flexible PEX distribution home run piping can be easily snaked through most existing structures, and full-line radiator suppliers have all the bits and pieces to finish the job nicely. Talk to them and they will give you many valuable tips and tricks to help you close the deals and get the systems installed at a tidy profit. Go to some trade shows and look around, go online and look around – the choices are mind boggling in styles that will suit any décor. Get some fancy brochures from your suppliers and get your customers excited about the possibilities.

Roy Collver is an author
and consultant on hydronic
heating based in Qualicum
Beach, B.C. He can be
reached at hoth2o@shaw.ca

Share.

Comments are closed.