Heat Loss Perception vs Reality

[jebatty]

Last weekend, 1/9 - 1/11, my perception was that for some reason my shop was losing more heat than it did in the past because storage was pretty much fully exhausted as it went from 190F to 100F over a 48 hour period. Last weekend had daytime high temp of +3 and low of -24F. This weekend, 1/16 - 1/18, a repeat with daytime high temp of -6 low of -23F. Then I did the math: btu loss from 1000 gal of storage from 190 to 100 = 8340 lbs x 90 = 754,200 btus / 48 hours = 15,712 btu/hr. Right in the ballpark of past experience for temps in this range, and not too bad for a 1500 sq ft shop with 12' ceiling.

This also means that I'm headed to burn about 152 lbs of wood to recharge the storage: 754,200 btus / 6050 btu/lb of wood = 125 lbs of wood / 81% approx burn efficiency boiler btu to water = 154 lbs of wood. Monday again will be a busy day of burning.

The backup heat in the shop is resistance electric. If I had used electric heat vs wood for this space heating, the cost would have been: 15,712 btu/hr x 3412 btu/kWh = 4.6 kW/hr x $0.115 kWh = $0.53/hr x 48 = $25.42 for two days.

Currently I'm burning almost 100% red pine, about 2900 lbs/cord. 154 lbs of wood is 0.05 cords. At a market rate of about $150/cord of red pine, 0.05 cords = $7.50, less than 1/3 the cost of electric.

My faulty perception has been corrected. Shop heat loss is about right where past experience placed it. Conclusion: my creeping aging process also is creeping out my perception. Return to logic: it's all about the math.

 

[jebatty]

Since it's all about the math, please ignore my reference to 152 lbs of wood and 154 lbs of wood. Creeping aging process again in evidence.

 

[KenLockett]

Last weekend, 1/9 - 1/11, my perception was that for some reason my shop was losing more heat than it did in the past because storage was pretty much fully exhausted as it went from 190F to 100F over a 48 hour period. Last weekend had daytime high temp of +3 and low of -24F. This weekend, 1/16 - 1/18, a repeat with daytime high temp of -6 low of -23F. Then I did the math: btu loss from 1000 gal of storage from 190 to 100 = 8340 lbs x 90 = 754,200 btus / 48 hours = 15,712 btu/hr. Right in the ballpark of past experience for temps in this range, and not too bad for a 1500 sq ft shop with 12' ceiling.

This also means that I'm headed to burn about 152 lbs of wood to recharge the storage: 754,200 btus / 6050 btu/lb of wood = 125 lbs of wood / 81% approx burn efficiency boiler btu to water = 154 lbs of wood. Monday again will be a busy day of burning.

The backup heat in the shop is resistance electric. If I had used electric heat vs wood for this space heating, the cost would have been: 15,712 btu/hr x 3412 btu/kWh = 4.6 kW/hr x $0.115 kWh = $0.53/hr x 48 = $25.42 for two days.

Currently I'm burning almost 100% red pine, about 2900 lbs/cord. 154 lbs of wood is 0.05 cords. At a market rate of about $150/cord of red pine, 0.05 cords = $7.50, less than 1/3 the cost of electric.

My faulty perception has been corrected. Shop heat loss is about right where past experience placed it. Conclusion: my creeping aging process also is creeping out my perception. Return to logic: it's all about the math.

Jim,

Just a thought, if your Tarm is like mine with the Termovar mixing valve and hydronic baseboard I can only extract usable heat down to about 160 DegF. Would this not have any bearing on your overall calculations or does your setup/arrangement allow for you to extract heat down below 160 DegF (i.e. radiant, etc.). Your thoughts?

 

[AroostookDave]

Jim,

Just a thought, if your Tarm is like mine with the Termovar mixing valve and hydronic baseboard I can only extract usable heat down to about 160 DegF. Would this not have any bearing on your overall calculations or does your setup/arrangement allow for you to extract heat down below 160 DegF (i.e. radiant, etc.). Your thoughts?

I have a termovar and standard baseboard and I can extract usable heat (raising the thermostat raises the room temperature) down to 150, and maintain heat down to the upper 130s.
I have 2200 sq feet with 8 foot ceilings. Perhaps this size heating area along with storage makes a difference?

 

[jebatty]

I don't think that the amount of usable heat you can extract from storage has any relationship to the Termovar. I have radiant in-floor which I supply at 100F from storage through a mixing valve. The lower the temp of water you can use, the more btus you effectively can store and then extract from your storage.

 

[Bob Rohr]

Last weekend, 1/9 - 1/11, my perception was that for some reason my shop was losing more heat than it did in the past because storage was pretty much fully exhausted as it went from 190F to 100F over a 48 hour period. Last weekend had daytime high temp of +3 and low of -24F. This weekend, 1/16 - 1/18, a repeat with daytime high temp of -6 low of -23F. Then I did the math: btu loss from 1000 gal of storage from 190 to 100 = 8340 lbs x 90 = 754,200 btus / 48 hours = 15,712 btu/hr. Right in the ballpark of past experience for temps in this range, and not too bad for a 1500 sq ft shop with 12' ceiling.

This also means that I'm headed to burn about 152 lbs of wood to recharge the storage: 754,200 btus / 6050 btu/lb of wood = 125 lbs of wood / 81% approx burn efficiency boiler btu to water = 154 lbs of wood. Monday again will be a busy day of burning.

The backup heat in the shop is resistance electric. If I had used electric heat vs wood for this space heating, the cost would have been: 15,712 btu/hr x 3412 btu/kWh = 4.6 kW/hr x $0.115 kWh = $0.53/hr x 48 = $25.42 for two days.

Currently I'm burning almost 100% red pine, about 2900 lbs/cord. 154 lbs of wood is 0.05 cords. At a market rate of about $150/cord of red pine, 0.05 cords = $7.50, less than 1/3 the cost of electric.

My faulty perception has been corrected. Shop heat loss is about right where past experience placed it. Conclusion: my creeping aging process also is creeping out my perception. Return to logic: it's all about the math.

It's always nice to see a reality check, thanks for crunching the numbers and sharing.

 

[maple1]

Jim,

Just a thought, if your Tarm is like mine with the Termovar mixing valve and hydronic baseboard I can only extract usable heat down to about 160 DegF. Would this not have any bearing on your overall calculations or does your setup/arrangement allow for you to extract heat down below 160 DegF (i.e. radiant, etc.). Your thoughts?

Was your baseboard sized for another heating source? Maybe oil? Or was it designed for your existing source from scratch?

If oil, it was likely designed for a 180 supply temp - so adding more baseboard would help. Maybe with a smattering of cast iron too.

 

[KenLockett]

I don't think that the amount of usable heat you can extract from storage has any relationship to the Termovar. I have radiant in-floor which I supply at 100F from storage through a mixing valve. The lower the temp of water you can use, the more btus you effectively can store and then extract from your storage.

What I meant is that the three way mixing valve recirculates

Was your baseboard sized for another heating source? Maybe oil? Or was it designed for your existing source from scratch?

If oil, it was likely designed for a 180 supply temp - so adding more baseboard would help. Maybe with a smattering of cast iron too.

Yeah the baseboard was sized for oil. I'm not suggesting that my baseboard is not sufficient I was only commenting that my Termovar mixing valve will will not open the return port until the temperature sensed at the return is roughly 160+. This is a design characteristic of the system as suggested by Tarm. My point simply is that until I get the boiler temp up to 165 I cannot extract any btus from the boiler 'storage' as I get no circulation out to the zones until then nor does not my main circulator energize until about 160 Deg. If I had a storage tank and the temp dropped below 165 (100 Deg F per se as in Jim's example) before I recharged the system via losses of course in the boiler system/tank itself, I would need to add more btus to the system to balance the loss before I could extract heat again. The sweet spot in my baseboard system is between 175 - 195 DegF. My boiler fan control is set to turn off at 192 DegF and via deaband turns on again at approximately 182 DegF. Because of hysteresis in the system my temperature range between cycles is actually about 179 to 202.

 

[maple1]

What I meant is that the three way mixing valve recirculates

Yeah the baseboard was sized for oil. I'm not suggesting that my baseboard is not sufficient I was only commenting that my Termovar mixing valve will will not open the return port until the temperature sensed at the return is roughly 160+. This is a design characteristic of the system as suggested by Tarm. My point simply is that until I get the boiler temp up to 165 I cannot extract any btus from the boiler 'storage' as I get no circulation out to the zones until then nor does not my main circulator energize until about 160 Deg. If I had a storage tank and the temp dropped below 165 (100 Deg F per se as in Jim's example) before I recharged the system via losses of course in the boiler system/tank itself, I would need to add more btus to the system to balance the loss before I could extract heat again. The sweet spot in my baseboard system is between 175 - 195 DegF. My boiler fan control is set to turn off at 192 DegF and via deaband turns on again at approximately 182 DegF. Because of hysteresis in the system my temperature range between cycles is actually about 179 to 202.

OK, gothcya now.

Once the return port opens (at 160), do the return temps settle down any below that or do they stay at 160? Return temps with my loading unit go up to about 150-155 when the pump kicks on, then settle back to 140.

 

[KenLockett]

OK, gothcya now.

Once the return port opens (at 160), do the return temps settle down any below that or do they stay at 160? Return temps with my loading unit go up to about 150-155 when the pump kicks on, then settle back to 140.

They stay around 160 on return. There is actually a 160 Deg Termovar and a 140 Deg Termovar. I have the 160 Deg Termovar.

 

[jebatty]

I too now understand what you were asking. I have a manual balancing valve for the Termovar, recommended by the Tarm design, which splits the amount of water flowing from the boiler between the Termovar and the system. I have this valve set to not allow very much water to the Termovar. I think the result is that the Termovar senses 160Fwater (with remaining 160F water going to the system) and partially opens, then mixes that 160F water with return water from system, ending up with boiler return after the Termovar to about 140F. System will roughly stay that way until system return rises enough to result in the mixed water from the Termovar to rise above 140F.