Why is my modcon sooo efficient ?

Mike E_2

According to a chart that Viessmann put out, as well as what you just found out, is that in addition to the efficiency going up when water temperature is lowered, the efficiency also goes up when the burner modulates down.

Condensing Canuck

Why is TT boiler so good ?

OK folks. Here is a little mystery for you all.

First some background details.
House is a 2005 Energystar rated ICF house, 2400 sq ft main, 2400 sq. ft. in basement. Blower door tested as the 2nd tightest house ever built in Canada. South facing windows, garage on NW side of house for sheltering the rest of the place, R50 in the attic, etc. etc. For a -20C design day, the measured BTU requirement is 15,347. Yup, that's right.

Replaced the 85% efficient Buderus cast iron boiler (69k BTU if I recall) with a Triangle Tube Prestige Excellence Combi unit this summer. Yes I did this by myself, but I was a gasfitter in a former life). Rated at 110k BTU, modulates down to 25% of that. It's great. Never run out of hot water even with the small flash tank (we have solar preheat, nice in the summer, but under a foot of snow now). But I digress...

So obviously even the lowest modulation is too high for this house. The boiler supplies a Lifebreath Airhandler, running at 630 cfm when the thermostat calls for heat, 300 cfm in circulation mode otherwise. I originally set the outdoor reset curve so that I had 86F water on a 64F day (never comes on of course) and 140F water on a 0F day. Under these conditions, the TT fires on and off trying to keep the water temps so low. In an attempt to reduce the wear and ear of the ignitor, I raised the lower water temp to 110F. This keeps the boiler modulating at idle speed, and typically it might run for 20 mins to keep one of our zones warm at 10F outdoor temp.

Now of course, conventional wisdom would say that's not as efficient as letting the water temps be even lower, so I measure the efficiency with my UEI C75 for 7 different supply temps. The UEI was calibrated in June of this year, and the O2 numbers coming out are 4.9% and the CO2 is 9%.
So here are the measurements

Supply Temp(F) Return Temp(F) efficiency(%)
110 98 98.5
120 106 98.2
130 112 98.1
140 120 97.9
150 128 97.7
160 136 97.6
170 144 97.5

The Flue temps are just 2-4F higher than the return water temps. I've set the TT controller to circulate water for 12 minutes after it stops firing. That brings the supply side water down to ambient, so no energy is left in the pipes. So far, so good.

I want the 140F temp to speed up recovery in the mornings, and the 110F min temp to keep cold air from blowing out of the registers on milder days. Yet I seem to be paying no real penalty for this. The 1% drop in efficiency is more than offset by not running the Lifebreath bump and ECM blower motor for hours on end in pseudo-continuous circ mode.

So here's my question. With a 144F return water temp, I should be out of the condensing range, but the boiler is still only 1% less efficient than with a 98F return water temp. That seems pretty amazing, and goes against some of the valued wisdom I read here and elsewhere. Is the TT heat exchanger just that much better than anything else ? Anybody else make such measurements ?

Condensing Canuck

Why is TT boiler so good ?

OK folks. Here is a little mystery for you all.

First some background details.
House is a 2005 Energystar rated ICF house, 2400 sq ft main, 2400 sq. ft. in basement. Blower door tested as the 2nd tightest house ever built in Canada. South facing windows, garage on NW side of house for sheltering the rest of the place, R50 in the attic, etc. etc. For a -20C design day, the measured BTU requirement is 15,347. Yup, that's right.

Replaced the 85% efficient Buderus cast iron boiler (69k BTU if I recall) with a Triangle Tube Prestige Excellence Combi unit this summer. Yes I did this by myself, but I was a gasfitter in a former life). Rated at 110k BTU, modulates down to 25% of that. It's great. Never run out of hot water even with the small flash tank (we have solar preheat, nice in the summer, but under a foot of snow now). But I digress...

So obviously even the lowest modulation is too high for this house. The boiler supplies a Lifebreath Airhandler, running at 630 cfm when the thermostat calls for heat, 300 cfm in circulation mode otherwise. I originally set the outdoor reset curve so that I had 86F water on a 64F day (never comes on of course) and 140F water on a 0F day. Under these conditions, the TT fires on and off trying to keep the water temps so low. In an attempt to reduce the wear and ear of the ignitor, I raised the lower water temp to 110F. This keeps the boiler modulating at idle speed, and typically it might run for 20 mins to keep one of our zones warm at 10F outdoor temp.

Now of course, conventional wisdom would say that's not as efficient as letting the water temps be even lower, so I measure the efficiency with my UEI C75 for 7 different supply temps. The UEI was calibrated in June of this year, and the O2 numbers coming out are 4.9% and the CO2 is 9%.
So here are the measurements

Supply Temp(F) Return Temp(F) efficiency(%)
110 98 98.5,
120 106 98.2,
130 112 98.1,
140 120 97.9,
150 128 97.7,
160 136 97.6,
170 144 97.5,

The Flue temps are just 2-4F higher than the return water temps. I've set the TT controller to circulate water for 12 minutes after it stops firing. That brings the supply side water down to ambient, so no energy is left in the pipes. So far, so good.

I want the 140F temp to speed up recovery in the mornings, and the 110F min temp to keep cold air from blowing out of the registers on milder days. Yet I seem to be paying no real penalty for this. The 1% drop in efficiency is more than offset by not running the Lifebreath bump and ECM blower motor for hours on end in pseudo-continuous circ mode.

So here's my question. With a 144F return water temp, I should be out of the condensing range, but the boiler is still only 1% less efficient than with a 98F return water temp. That seems pretty amazing, and goes against some of the valued wisdom I read here and elsewhere. Is the TT heat exchanger just that much better than anything else ? Anybody else make such measurements ?

Condensing Canuck

Interesting. Well, knowing I have 6 gpm and using the deltaTs I reported and the 499 conversion factor, the TT Excellence is about 3 percentage points better than the Vitodens 200 at any significant load. My 140/120 supply/return number is 97.9% efficient and that translates into a 6*499*20/110000 = 54% of full load. Good old Yankee know how. But yes, the Vitodens 200 does very well with a 140 deg F return temp at low outputs. Both do much better than I would have thought at those levels. Anyone know the physics of that ?