Tuesday, November 30, 2010

Hot water tank experiment

I've been wondering why my hot water takes an unusually long time to heat up in recently. When I first moved in almost 8 years ago it seemed that it took only about 20 minutes to get piping hot water. Nowadays it takes 40 minutes or more ... and even then it's not that hot.

I was curious about the temperature profiles of my hot water tank as it was being heated by my natural gas powered water heater ("boiler" or "furnace"). As is normal in Ireland, household hot water is provided from hot water storage tank which is heated via heat exchanger by the central heating system.

The following is the temperature at the heat exchanger inlet and outlet of the hot water tank. Temperature was measured with DS18B20 temperature sensors which were taped to the pipe near the tank.

A few things struck me as being interesting about this graph. First there is a brief but noticeable dip in inlet temperature when the system is first switched on. This is clearly the cold water ahead of the pump/heater entering the heat exchanger. There is a quick rise in temperature with the outlet temperature following the input minus a temperature drop (due to heat transferred to the water in the tank) and a time lag (due to the transit time of water in the heat exchanger). However the most striking feature is the temperature cycling.

It seems that my heater cannot transfer heat to the tank fast enough. The water in the heat exchanger circuit reaches the heater's target water flow temperature (user settable from 30°C-85°C) and the flame cuts off. The pump continues to pump until the temperature drops below some threshold and the cycle repeats. The duty cycle is about 50 seconds flame on, and 180 seconds flame off.

There is another interesting effect during the cooling phase: a brief increase in temperature for about 30 seconds before dropping again. I believe this artifact is because the heater flame is on for less time that it takes for the water to circulate around the system. So there is a hot spot which makes a second round through the system.

I re-ran the experiment with a higher heater temperature:

It looked identical (excepted at elevated temperatures). But only when I looked at the numbers more closely while writing this post did I realize that the period between cycles had increased from about 230 seconds to 330 seconds. So perhaps if I set the heater at the max temperature I could increase the period to a sufficiently long time to heat the water in the tank in just one cycle.

The following graph is from a third run. This time the heater was set at the max temperature (supposedly 85°C) and I set the tank thermostat at 40°C.

Unfortunately there was still cycling, 4 in total, but the cycle period was considerable longer than any than the previous runs. More importantly the duty cycle was much better: 500 seconds of flame on, 100 seconds flame off for the first cycle; 220s on/100s off for second, 180s on / 100s off for third, and 140s on for the last cycle. The tank thermostat cut off the heater at about 1800 seconds (30 minutes) into the experiment.


For fast hot water, running the heater as hot as possible works best (at least for me). At some point the heater temperature must have been turned low which explains why in recent years it seems to be taking ages for the water to heat. However all this probably makes no difference to the energy efficiency of the system.


Tim Bentham said...

Joe, I'd have thought that the most likely conclusion here is that your boiler is simply no longer efficient, which if you've not replaced it in 8 years is almost certainly true (depending on how old it was when you moved in). If I recall correctly, a modern boiler is expected to last 10-15 years before the heating efficiency drops off to the extent that the cost of replacement is outweighed by the excess costs generated by the inefficiency if you continue to run it. Maybe it's time to upgrade?

jdesbonnet said...

I'm not sure if the efficiency drops that much with age.

The unit I have is a Baxi 35/60 (non condensing boiler) with a efficiency rating of 79%. I have thought about upgrading to a condensing boiler (which I believe are up to 91% efficient). I burn at most €400 worth of gas through that system per year. 79% to 91% efficiency increase would yield a saving of about €53/year. A new boiler will be in excess of €1000. So 20+ year return-on-investment? I think I'll stick with what I have for the moment.

But you raise an interesting question: is my boiler still at the 79% efficiency rating? Hmmm... I think I know how I can measure this... more later... :)

Tim Bentham said...

I seem to remember (from GCSE physics almost 20 years ago!) that efficiency degrades by 10% per year (ie.7.9% drop first year, 7.11% drop second year, etc.). I think this rate is less for a condensing boiler. I think you may find the efficiency is now much less than it was.