WE'VE BEEN TALKING for the last three months about how to properly size a heat source when replacing oversized cast-iron boilers that once used solid fuel. They come with an existing castiron radiation distribution system.
At then end of last month's column (pg. 46), I promised to tell you how to evaluate the data you will have obtained from monitoring one of these systems. As a good test case, I'll use my "Live-in Laboratory" that I call home.
As you are aware, I have been monitoring my own home and have exceeded the million points of data mark in digital information. In my three previous columns, I explained how to retrieve runtime information from a bang-bang type of operation, and the methodology I have provided works well with that type of scenario.
The good news is that most of the older types of heating appliances have a two-position gas valve on them. They are either on all the way or off all the way. Most do not have the ability to modulate their flame based on real-time operations.
With that said, however, as you are probably already aware, the heat source in my living lab can modulate. In order to accurately track this consumption, I had to install a standalone gas meter on the heating appliance with a points pulse output for each cubic foot of gas that passes through the meter. Simply monitoring the run time of the gas valve would not give me an accurate picture of my real-time energy consumption.
Many years ago, I performed a heatloss calculation on my home. This calculation is typical of the ones that we as contractors perform on a regular basis. It takes into consideration all of the theoretical load factors, such as thermal conductive losses, infiltration losses and exfiltration losses from mechanical equipment.
What these calculations don't take into consideration are the real-time things that influence the internal temperature of a given structure. Things like the flywheel effect of mass, the lack of either infiltration or exfiltration, solar gain, internal gains and all the things that actually happen in real life. The theoretical heat loss calculation stated that my home needed 40,148 Btuh. Not an unreasonable number for a home of my size.
Real fuel-use numbers don't lie.
Now, here comes the interesting part. During this last heating season, our local temperatures dipped down to "design conditions" for a few days in a row. I figured that would exhaust any "banked" energy in the flywheel mass equation and would put the house into a true fully loaded condition. In other words, an ideal opportunity to check the " realtime" energy consumption of my home.
I connected all of the necessary recording devices and began collecting the data. After the cold snap moved on, I downloaded all the information I had acquired, and to my shock and disbelief, I found that my home was using 15,770 Btuh between the hours of 4 a.m. and 6 a.m. That's less than half of the calculated energy per hour that I should be using. When I accounted for the electrical consumption, gains from body heat of the occupants and so forth, it only reduced the load by 1,311 Btuh. That's still less than half of my calculated hourly energy demand.
My hourly domestic hot water load would total 33,320 Btuh. My DHW gets priority over my space heating calls, so it is not necessary to compound these two peak loads. It is important that the highest hourly load be covered when examining these scenarios. If prioritization is not a part of the picture, then the two loads would be added together. In my case, 33,320 plus 17,081 would total 50,401 Btuh.
If I had installed a bang-bang type of appliance, the duty cycle during design condition would have been about 40%. That's a lot of turning on and off. It would cause what is known in the field as short-cycle efficiency loss. I am thoroughly convinced that boilers that have the ability to modulate around real-time loads are the best bang for the buck, and as I watch my extremely efficient Munchkin T-50 modulating boiler running continuously while modulating around the load, it convinces me even more.
In closing this series of articles, I want to cover some items that may not have been covered that could influence the outcome of a given survey.
If consumers have programmable thermostats, have them set them for continuous temperature operation. Set back conditions will reduce the fuel consumption but throw off your readings. Also ask that customers avoid using aesthetic heating devices during the data acquisition period (gas log fireplaces and fireplaces in general).
If you're dealing with an older appliance that uses millivolt gas valves, install a pressure-actuated switch between the burner and the gas valve to monitor when the gas valve is open. When this switch closes, the event recorder will record an ON event. When the burner shuts down, the pressure will drop and the recorder will record an OFF event.
Whenever possible, start testing after the weather has been moderately cold and cloudy outside for a few days prior to beginning data acquisition. The amount of energy that can be charged into the thermal flywheel mass of the house during a short period of warm and sunny weather has much more impact then we give it credit for. Also, pay attention to potential wind loading during the acquisition period.
If you're dealing with an oil burner, check with the manufacturer to determine where the best point of monitoring burner on-time should occur, and follow their recommendations as it pertains to setting the equipment necessary to perform the tests.
Remember that this information can generate some interesting numbers that will cause you to look at heat loss and load numbers with a pretty jaundiced eye. If in doubt, consult a mechanical engineer prior to making your final recommendations, but remember this, real fuel-use numbers don't lie. They are just unduly influenced by factors you may not have taken into consideration.
Tune in next month as we look into the thermal flywheel mass effect that influences real-life situations in the heating and cooling world. Until then, Happy Hard to Believe Hydronicing!
Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at [email protected] or by phone at 303/778-7772.