Bigger is not Better
Bigger isn't better
How is a boiler sized to a home?
It consists of 2 major considerations; the heat loss of the home and the domestic hot water demand. Heat loss is how much heat (in BTUs) will be lost from your house and would need to be replaced in order to maintain a consistent temperature. Domestic hot water is the amount of hot water you will be using in showers, sinks, dishwashers, etc.
How is a heating system sized?
A contractor would come into the house and determine the heat loss. Let's say it figures out to be 80,000 BTUs. Most contractors would want to “play it safe” and up the BTUs to make sure the boiler will have sufficient output for the home. Now he is up to, let's say, 100K BTUs. The next step would be to choose the boiler. The one with the nearest output without going lower is 120K BTUs. The house requires 80K, the contractor wants 100K and the closest boiler is 120K. This sort of stepping process takes place where the size of the boiler just kind of creeps upward and can end up being grossly over sized.
So why is bigger not better? It's all about the BTUs. If the house has 133 feet of baseboard it can only deliver 80K BTUs (133 feet X 600 BTU/ft) into the space and no more. A boiler producing an output of 120K when all that can be utilized is 80K is wasteful; all that energy is used to create heat that is never used.
Another major point is that most homes consist of at least 2 - 4 zones. Zones are where certain areas of a house can be heated independently of one another. In our example house all zones would have to be calling for heat at the same time to require all 80K BTUs at once. Only on the coldest of cold days would every zone be calling at once for any extended period. Most of the time only 1 or 2 zones would be calling at once. So even though a system should be sized to handle all 80K BTUs, it will only be asked to do so a small percentage of the time.
The argument can then be made that when a heating system is more accurately sized for the home the hot water will suffer. Let's say our example house has a 40 gallon indirect water heater with a 175 gallon/hour ratting that works off the system. If the boiler is sized more accurately to the heat loss of the house then this water heater might not be able to deliver 175 gallons, but it could put out 155 gallons/hr. Now as long as you aren't showering, running the dishwasher, taking a bath and washing clothes all at the same time you would most likely never know the difference. If you do, there is a multitude of auxiliary water heating and storage products in the market.
What can we do to fix this problem?
First of all, as heat loss calculations become more precise so should the system design. We need to get away from the bigger is better mindset and focus on accuracy of design.
One option being used more and more are outdoor reset controls. What this tool does is allows the system to monitor the outdoor temperature and adjust the boiler water temperature accordingly. The colder the outdoor temperature the hotter the boiler water will get (within the programmed parameters). This is a great feature that can increase fuel efficiency dramatically. Also, it can be set so that the system will not heat if the outdoor temperature rises above a predetermined setting. This is perfect for those spring days when you want to open the windows but you forget to turn down the thermostat.
Another option is to install multiple boilers on what is called a lead-lag system. This is where at least 2 smaller boilers are used to heat a home. One will fire when heat is called for; if it can't handle the load then the other will fire up. This setup has a higher initial expense but could ultimately save money by having lower operating costs.
Bigger means what?
Bigger means more energy, more waste and more money.