The Facts of the Matter
When sizing and selecting modulating-condensing boilers we, as heating professionals, size the equipment based on the Btu load of the home. Most homes in the New England area have a heat loss of around 25-35 Btu/h per square foot. Most mod-cons have a reset controller built in to vary the supply temperature based on what is happening outdoors. The colder it is outdoors the warmer the water in the boiler; the warmer it is outdoors the cooler the water in the boiler. Boilers also have thermistors on the supply and return which adapt the firing rate of the boiler based on the load i.e. the boiler control will increase or decrease the firing rate based on the difference between the supply and return temperatures. Today, most mod-con boilers have a turndown ratio of at least 5:1.
The DHW Conundrum
When sizing the boiler for say, a 2,500 sq ft home with a heat loss of 75,000 Btu/h, we find that the DHW load (as much as 150MBH) is significantly higher than the heating load. Unlike tankless type water heaters, this shortfall in Btus can be overcome with the addition of an indirect water heater. But the boiler is still oversized for the heating requirements.
To further complicate matters, many of today’s larger homes will have a combination of heat emitters which require different temperatures. It is not uncommon for a 2,500 sq ft house to have as many as 6 zones with varying temperature requirements: high temp for the DHW; moderately high for the baseboard and/or air handlers; moderately low for the staple or dry fit radiant and low for in slab radiant.
To better understand the challenge, let’s take a look at one of these zones.
500 sq ft zone heated with 25 feet of baseboard with a rated output of 610 Btuh @ 180°F per linear foot.
Assuming that the heat loss for this zone requires 30 Btuh per sq ft on the coldest day, the calculation is simple: 500 sq ft x 30 Btus = 15,000Btuh.
This represents the total required Btu/h for this zone. So what effect will outdoor reset have on these numbers?
Short Cycling
What does short cycling do to these low mass condensing units? You will burn more gas; you will create wear and tear on all components; you will shorten the life expectancy of the heat exchanger and yes, your customer will not be happy as you recommended the more expensive super efficient boiler that was supposed to save them money! So what can be done?
The Buffer Zone
A simple way to overcome the short cycling is to add a buffer tank; a 4 port, well insulated vessel designed to act as a Btu reservoir. The buffer tank is installed between the boiler and the system, much like a hydraulic separator (see illustration below). This also has the added benefit of eliminating the need for primary/secondary piping.
Sizing is important and easy as the manufacturer provides a calculator on their website. Or you can use a simple formula: Lowest Firing Rate - Lowest Load x Run Time ÷ 10,000 (ΔT x 500). Or in our case: 30,000 Btus – 10,000 Btus x 10 minutes ÷ 10,000 = 20 gallons. So, by installing a 20 gallon (or next nearest size) buffer tank, you get the following benefits:
- Elimination of short cycling
- Easy piping for mixed temperatures as primary secondary is no longer needed
- Prevention of wear and tear of your boiler components
- Hydraulic separator for better mixing
- ½ °F loss per hour of your heated water
- Increased water content for heating loads
- Ability to micro zone
