By Ron Beck, Outside Technical Advisor and Manager of Training for U.S. Boiler Company
What??? I hear this often. I also hear, “It gets too cold here for outdoor reset” or “I love OD reset, but I don’t understand how to adjust it”. I even hear “I always get complaints from my customers that the radiation is not hot enough since the new boiler went in” and “OD reset does not work with this type of system.”
So, let me ask you. What kind of hot water system do you know of where outdoor reset will not work? I’ll answer this with one word… NONE.
OD reset works with all types of hot water systems. What benefit does OD reset provide? It saves fuel, creates higher comfort levels in the building, and reduces short cycles. It does so by changing the system’s supply water temperature depending on the current outdoor conditions. A few things come into play here; heat loss of the home (size and level of insulation), radiation (type and quantity), and of course your climate.
Those of you who have attended one of my seminars know that I’m a big proponent of reducing fuel bills with by optimizing boiler and system efficiency. How do we do that? Proper boiler sizing and OD reset.
I believe that not using outdoor reset at every opportunity is doing a dis-service to your customers.
U.S. Boiler includes outdoor reset on our high efficiency modulation/condensing boilers. In fact, to achieve maximum efficiency of a condensing boiler, you must use OD reset. We also offer optional OD reset cards that simply plug into most of our cast iron boiler control systems. Remember, the lower the water temperature, the higher the boiler efficiency.
But, back to the misconception that there are applications where outdoor reset doesn’t work. Let’s touch on the different system types and utilizing OD reset. I’ll discuss four basic types of systems: radiant floors, cast iron radiation, copper fin-tube baseboard, and hydro-air.
Before we get into it, let’s take a quick look at the default factory ODR settings on our boilers:
1. Low OD air temp – 0°F
2. High OD air temp – 70°F
3. Low boiler temp – 110°F
4. High boiler temp – 180°F
5. Min water temp – 130°F
Radiant floors are installed in numerous ways. There’s staple-up, where the tubing is fastened to the underside of the sub-floor with or without heat transfer plates. Sometimes tubing is installed above the subfloor, with a lightweight gypcrete pour overtop the tubing. It can also be installed above the subfloor in grooves, with flooring installed on top of it without a gypcrete pour. And of course, tubing can be installed under a concrete slab.
To maintain setpoint temperature, all of these systems will require different temperatures. The more mass and insulation below the tubing, the lower the required water temperature. And, with outdoor reset, all of these applications will provide efficient boiler operation. We could easily see water temperatures as low as 80°F or as high as 160°F. You may have to change the low boiler temperature, minimum water temperature, and possibly the high boiler temperature depending on the specific application.
The folks that say they don’t need ODR on a radiant system claim that the slab’s thermal mass is enough to flywheel the system through outdoor temperature swings, and that the slab doesn’t respond quickly enough to make a difference, even with fluctuating supply water temperatures. It’s true that a radiant system doesn’t respond as quickly as copper fin-tube and the thermal mass of the slab will hold a more constant room temperature.
Those are both comfort considerations, however, and while they’re important, they completely overlook the fact that even with an in-slab system, if the water temperature is too high, the boiler will be short cycling and burning more fuel than needed.
Keep in mind, because radiant systems are lower temperature by nature, you might need to change the default ODR settings
Cast iron radiation, contrary to popular belief, is low-temperature heating, especially in modern homes. Sure, back in the day of standing cast iron radiators, they might have needed 170 degree water. With single pane, wooden frame windows and zero insulation in the walls and ceilings, homes had HUGE heat loads. Now, insulate that same house and install new windows. What do you have? About half the heat load, at most. Nonetheless, the cast iron radiators haven’t shrunk, so you’ll be overheating the home or short-cycling the boiler. Probably both… Unless of course there was a way to drop the water temperature. Ahhhh. Yes. Outdoor reset control.
I’ve been asked many times about what water temperature cast iron systems require. My answer is always the same, “it depends.” There is a lot of mass and high water volume in cast iron systems. Therefore, if it’s an insulated home, the water temperature doesn’t need to be very high. I’ve seen plenty of cast iron systems where none of the boiler settings were changed from factory settings.
In cast iron applications where the home has some insulation, I like to see the low boiler temperature, minimum water temperature, and possibly the high boiler temperature reduced. Do the homework. Do a heat loss, calculate the water temperature required at design temperature with the amount of installed radiation. Once you have your design water temp, change the OD design temperature to 60°F on the heat loss and calculate the minimum water temperature. You may be really surprised.
Copper fin-tube baseboard, when connected to a high-efficiency boiler, means that OD reset is a must. Not only is copper baseboard a very responsive system, but the supply temperature needed will vary greatly depending on the ratio of baseboard to heat loss. Some systems can actually operate with supply temperatures under 180°F when it’s below 0°F outside.
I know of a baseboard job in Andover, MA, with a high limit of 163°F. Despite the bitter cold spells we’ve seen in the past two years, the temperature has never needed to be raised. That application is running a minimum boiler temperature of 120°F instead of 130°F, and a low boiler water temperature of 100°F. Outdoor reset changes the supply temperature accordingly, and the home is always comfortable.
There are still folks out there who think condensing boilers should never be used with fin-tube baseboard, because these systems require high water temperatures. It’s true that fin-tube does require, on average, higher water temps than other systems. But it’s really only at design temperatures where a mod/con boiler wouldn’t condense. For the other 99% percent of the heating season, with an OD reset installed, the boiler will run at maximum efficiency. Fin-tube and condensing boilers go together like peas and carrots, but only with outdoor reset.
Hydro-air might just have the most misconceptions surrounding it. And that’s understandable, because bringing air movement into the equation does tend to make it a bit more difficult for wet-heads. One thing is for certain, you can lower the supply water temperature to your coils when the outdoor temperature gets milder.
Don’t believe me? We’ve kept tabs on a few hydro-air jobs. We’ll talk all about it in a future Beck Tips article.