Well, it’s that time of the year again.  The weather is cooling off and we’ll have to begin the heating season once again.  The performance of the wood boiler has been very good, although with last winter being colder than normal we ended up using about 6 cords of wood between October 2008 and June 2009.  Operationally we’ve made very few tweaks to the control system software, and the variable speed circulator that controls the storage tank loop has proven to be magic.  PID tuning of its control loop was pretty easy, and if I wasn’t doing more monitoring and research, the control system could be greatly simplified, reducing it to something any HVAC technician or homeowner could maintain. Using  just thermostats and a couple relays, and one of those little PID controller 1/4 DIN modules from Automation Direct would be enough.  There have been no issues with reliability, everything has worked as designed.  Also, when I took a water sample from the boiler drain at the end of the heating season, it came out crystal clear, thanks to maintaining a system fluid pH of about 8.5.

I can’t emphasize enough the importance of maintaining the proper pH level of the water in the system, since this is responsible for ensuring long system life and minimizing any corrosion problems.  Many hydronic systems are severely neglected regarding this.  A friend of mine who installs geothermal systems reports that since he started raising the pH of the water in the ground loops and heating loops of the systems he installs this has virtually eliminated problems with rust and corrosion.

-Jeff

A friend of mine recently asked me how I made all the heat transfer calculations on the new FOM heating system. He’s also in the HVAC business but does forced air and geothermal mostly, and has not dealt much with hydronic heating systems. We’ve both seen a number of “hack jobs” where outdoor wood boilers have been lashed up to existing forced air or new radiant systems. I asked one homeowner who was having some problems with his new radiant system to show me the heat loss and heat flow calculations for the job. The ensuing blank stare told me this was not going to be a fun visit. With all the computer programs out there these days that simply plug and chug and give you numbers close enough to work with, there’s no excuse for not doing things properly. But I digress, we are supposed to be talking about heat transfer!

Heat transfer calculations in hydronic systems are dead easy, so there’s no excuse for not running the numbers. In the USA we still use BTU for our units for heat. Recall that 1 BTU is equal to the amount of heat that causes a temperature rise of 1^oF of 1 pound of water. Let’s say we have something like a geothermal system that uses a ground loop. A fluid, usually water or water mixed with glycol to prevent freezing, is circulated through the ground loop outside and a heat exchanger inside, either transferring heat from the refrigerant circuit (summer cooling) or transferring heat from the ground to the refrigerant circuit (winter heating).

1. A geothermal system is operating at steady state in cooling mode. If the flow rate in the ground loop is equal to 10 gallons per minute, the temperature of the water entering the loop from the heat exchanger is 110^oF and the water returning from the ground loop is 90^oF, what is the amount of heat, in BTU per hour, that is being conducted into the ground?

Answer:

heat transfer(BTU) = ΔT * Flow rate(gpm) * 8.3 * 60, where ΔT is the temperature drop around the ground loop in degrees F from input to output.

We simply multiply ΔT times the flow rate times weight of 1 gallon of water times 60 minutes. We have to multiply by 60 to rationalize our units since flow is in gpm but we want to know the number of BTU’s per hour. Since 8.3 * 60 = 498 it’s common to write the heat transfer equation simply as ΔT times flow times 500, to make it easier to do ‘rule of thumb’ calculations in one’s head. So the answer to our question is then:

20 * 10 * 500 = 100,000 BTU per hour.

So remember, it’s simply delta T times flow times 500. Not so hard, is it?

I just got the December electric bill in the email this evening.  This should be the first bill where the new wood gasification boiler system has been completely operational.  The November billing period was from October 15 to November 14.  Initial system startup was the first week of November.  The bill for that period was $149.60, but the pro-rated savings are uncertain since the previous month’s bill was unusually high (about $182) because it was estimated.  For the billing period November 15 to December 12, the first full month of operation, the electric bill came in at $89.16.  Since there were no other changes to our usage patterns the little over $60 savings appears to be pretty much due to the DHW tank no longer being electrically heated.  Most likely the savings are even more since the $149.60 figure is probably lower than it would have been had we not started firing the boiler when we did.  That sounds about right, because from past experiences when we’ve been away for extended periods I estimated the DHW tank consumed about $60 per month.  Now if I can find some vacuum tube solar collectors cheap I have some numbers to use for cost justification analysis…