Navien NCB-240E possible purchase
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DR-DEATH
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Gotcha lol. I have tried to research how that would make things work correctly but haven't found much. I'll wait for an expert opinion!LL header theory is above my pay grade here
One of the pros will have to address that...
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BTUs in has to equal BTUs out to keep it from cycling. Without significant thermal mass to work with the cycles will be short, thus, the only way a low loss header can address short cycling is if it has a lot of water mass, say a 20+ gallon hydraulic separator, not Navien's pre-built closely spaced tees:
(The tall tank next to the boiler with the vent on top)
vs.
(The fatter pipe with the connections to the boiler, which is Navien's pre-made low loss header.)
The napkin math:
It takes 1 BTU to raise 1 lb of water 1 degree F.
The typical high/low differential on a condensing boiler is between 5F and 10F.
At 110F average water temp typical fin-tube puts out about 200 BTU/hr per foot. A 14 foot zone delivers thes 2800 BTU/hr.
The output of the NCB-240 at that temp is about 95% of the 18,000 BTU/hr minimum input, or about 17,000 BTU/hr.
So the excess heat going in is about 14,000 BTU/hr.
With a low-mass low-loss header, 50 feet of distribution plumbing the 14' of fin tube and the internal volume of the boiler adds up to about 2 gallons of water, x 8.34 lbs/gallon is about 17 lbs.
With 14,000 BTU/hr going into 17lbs of water it's temperature is rising 14,000BTU/hr /17lbs= 824F p/hour, or (/60=) 13.7 F per minute.
So, with a high/low differential of 5F the burn cycles would be about 5/13.7= 0.365 minutes, or about 22 seconds long.
In that 22 seconds the amount of heat buffered in the thermal mass of the system is about 86 BTU. With the fin-tube emitting at 2800BTU/hr, or 46.7 BTU/minute, it cools off the 5F in less than 2 minutes before re-firing, so a complete firing-cooling-refiring cycle takes less than three minutes, and you would get more than 20 burn cycles per hour.
With a differential of even 10F you're looking at burns less than 45 seconds long, and more than 10 cycles per hour. This is short cycling.
You can't cheat the thermal physics with magical plumbing.
But,say, you have massive hydraulic separator with 18 gallons of water, plus the and 2 gallons of distribution plumbing fin-tube, boiler, etc you're looking at 20 gallons or about 167 lbs to work with, and a 5F swing would take about 220 seconds, more than 3 minutes, and a 10 F swing would take more than 7 minutes of burn time. At a 5F differential the buffered BTUs is 5F x 167lbs= 835 BTU, and with the fin tube emitting 2800 BTU/hr the cooling time takes 835/2800= ~0.3 hours or 18 minutes, so even with a 5F swing you're at only a couple of burns per hours, and at 10F swing it will be barely more than one burn per hour.
But an 18 gallon low loss header like a Boiler Buddy BB-18 is a pricey piece of hardware (about a grand, with shipping at internet pricing), which is quite a bit more expensive than another 40' of fin-tube to bring it up to 54'. At an AWT of 120F and 200 BTU/hr per foot, a 54' stick of fin tube emits 11,000 BTU/hr and there is now "only" ~5,000 BTU/hr of excess, not 14,000, and you've picked up another ~7lbs of water, tripling the burn times, shortening the off times, but still bringing it to under 10 burns per hour with a 10F swing.
But with average heat loads a slow as implied by the total amount of baseboard band-aiding an oversized minimum-burn rate with a massive low loss header is still the "wrong" solution. Massive low loss headers really only make sense for micro-zoned houses, and it's still counterproductive to size the boiler to where it cycles rather than modulates with the whole-house load.
(The tall tank next to the boiler with the vent on top)
vs.
(The fatter pipe with the connections to the boiler, which is Navien's pre-made low loss header.)
The napkin math:
It takes 1 BTU to raise 1 lb of water 1 degree F.
The typical high/low differential on a condensing boiler is between 5F and 10F.
At 110F average water temp typical fin-tube puts out about 200 BTU/hr per foot. A 14 foot zone delivers thes 2800 BTU/hr.
The output of the NCB-240 at that temp is about 95% of the 18,000 BTU/hr minimum input, or about 17,000 BTU/hr.
So the excess heat going in is about 14,000 BTU/hr.
With a low-mass low-loss header, 50 feet of distribution plumbing the 14' of fin tube and the internal volume of the boiler adds up to about 2 gallons of water, x 8.34 lbs/gallon is about 17 lbs.
With 14,000 BTU/hr going into 17lbs of water it's temperature is rising 14,000BTU/hr /17lbs= 824F p/hour, or (/60=) 13.7 F per minute.
So, with a high/low differential of 5F the burn cycles would be about 5/13.7= 0.365 minutes, or about 22 seconds long.
In that 22 seconds the amount of heat buffered in the thermal mass of the system is about 86 BTU. With the fin-tube emitting at 2800BTU/hr, or 46.7 BTU/minute, it cools off the 5F in less than 2 minutes before re-firing, so a complete firing-cooling-refiring cycle takes less than three minutes, and you would get more than 20 burn cycles per hour.
With a differential of even 10F you're looking at burns less than 45 seconds long, and more than 10 cycles per hour. This is short cycling.
You can't cheat the thermal physics with magical plumbing.
But,say, you have massive hydraulic separator with 18 gallons of water, plus the and 2 gallons of distribution plumbing fin-tube, boiler, etc you're looking at 20 gallons or about 167 lbs to work with, and a 5F swing would take about 220 seconds, more than 3 minutes, and a 10 F swing would take more than 7 minutes of burn time. At a 5F differential the buffered BTUs is 5F x 167lbs= 835 BTU, and with the fin tube emitting 2800 BTU/hr the cooling time takes 835/2800= ~0.3 hours or 18 minutes, so even with a 5F swing you're at only a couple of burns per hours, and at 10F swing it will be barely more than one burn per hour.
But an 18 gallon low loss header like a Boiler Buddy BB-18 is a pricey piece of hardware (about a grand, with shipping at internet pricing), which is quite a bit more expensive than another 40' of fin-tube to bring it up to 54'. At an AWT of 120F and 200 BTU/hr per foot, a 54' stick of fin tube emits 11,000 BTU/hr and there is now "only" ~5,000 BTU/hr of excess, not 14,000, and you've picked up another ~7lbs of water, tripling the burn times, shortening the off times, but still bringing it to under 10 burns per hour with a 10F swing.
But with average heat loads a slow as implied by the total amount of baseboard band-aiding an oversized minimum-burn rate with a massive low loss header is still the "wrong" solution. Massive low loss headers really only make sense for micro-zoned houses, and it's still counterproductive to size the boiler to where it cycles rather than modulates with the whole-house load.
Last edited:
DR-DEATH
Member
gotcha. I found a local plumber who seems to know his stuff when it comes to mod cons and he seems to like HTP and says his main supply house carries them and the local support is great in NH. I'll update when I have a quote but I'm expecting $1700-1900 for the boiler and another $2,000 in labor and materials. My gas company still has a $1500 rebate which should put this around $2300-2600 out of pocket. At least I hope that's a pretty accurate idea of cost.BTUs in has to equal BTUs out to keep it from cycling. Without significant thermal mass to work with the cycles will be short, thus, the only way a low loss header can address short cycling is if it has a lot of water mass, say a 20+ gallon hydraulic separator, not Navien's pre-built closely spaced tees:
(The tall tank next to the boiler with the vent on top)
vs.
(The fatter pipe with the connections to the boiler, which is Navien's pre-made low loss header.)
The napkin math:
It takes 1 BTU to raise 1 lb of water 1 degree F.
The typical high/low differential on a condensing boiler is between 5F and 10F.
At 110F average water temp typical fin-tube puts out about 200 BTU/hr per foot. A 14 foot zone delivers thes 2800 BTU/hr.
The output of the NCB-240 at that temp is about 95% of the 18,000 BTU/hr minimum input, or about 17,000 BTU/hr.
So the excess heat going in is about 14,000 BTU/hr.
With a low-mass low-loss header, 50 feet of distribution plumbing the 14' of fin tube and the internal volume of the boiler adds up to about 2 gallons of water, x 8.34 lbs/gallon is about 17 lbs.
With 14,000 BTU/hr going into 17lbs of water it's temperature is rising 14,000BTU/hr /17lbs= 824F p/hour, or (/60=) 13.7 F per minute.
So, with a high/low differential of 5F the burn cycles would be about 5/13.7= 0.365 minutes, or about 22 seconds long.
In that 22 seconds the amount of heat buffered in the thermal mass of the system is about 86 BTU. With the fin-tube emitting at 2800BTU/hr, or 46.7 BTU/minute, it cools off the 5F in less than 2 minutes before re-firing, so a complete firing-cooling-refiring cycle takes less than three minutes, and you would get more than 20 burn cycles per hour.
With a differential of even 10F you're looking at burns less than 45 seconds long, and more than 10 cycles per hour. This is short cycling.
You can't cheat the thermal physics with magical plumbing.
But,say, you have massive hydraulic separator with 18 gallons of water, plus the and 2 gallons of distribution plumbing fin-tube, boiler, etc you're looking at 20 gallons or about 167 lbs to work with, and a 5F swing would take about 220 seconds, more than 3 minutes, and a 10 F swing would take more than 7 minutes of burn time. At a 5F differential the buffered BTUs is 5F x 167lbs= 835 BTU, and with the fin tube emitting 2800 BTU/hr the cooling time takes 835/2800= ~0.3 hours or 18 minutes, so even with a 5F swing you're at only a couple of burns per hours, and at 10F swing it will be barely more than one burn per hour.
But an 18 gallon low loss header like a Boiler Buddy BB-18 is a pricey piece of hardware (about a grand, with shipping at internet pricing), which is quite a bit more expensive than another 40' of fin-tube to bring it up to 54'. At an AWT of 120F and 200 BTU/hr per foot, a 54' stick of fin tube emits 11,000 BTU/hr and there is now "only" ~5,000 BTU/hr of excess, not 14,000, and you've picked up another ~7lbs of water, tripling the burn times, shortening the off times, but still bringing it to under 10 burns per hour with a 10F swing.
But with average heat loads a slow as implied by the total amount of baseboard band-aiding an oversized minimum-burn rate with a massive low loss header is still the "wrong" solution. Massive low loss headers really only make sense for micro-zoned houses, and it's still counterproductive to size the boiler to where it cycles rather than modulates with the whole-house load.
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I'd be surprised if you can get out of it for under $5K (before rebates are applied), but maybe. If it's replacing cast iron there will be the additional costs of the plastic venting & combustion air supply, and the exhaust condensate management stuff, etc.
The UFT-80W (and most other fire-tube boilers) usually doesn't need hydraulic separation/low-loss headers to function well, so as a drop-in replacement it should be pretty easy to plumb-in, could even be simpler than what the prior boiler needed.
The UFT-80W (and most other fire-tube boilers) usually doesn't need hydraulic separation/low-loss headers to function well, so as a drop-in replacement it should be pretty easy to plumb-in, could even be simpler than what the prior boiler needed.
DR-DEATH
Member
I'm probably being a little too optimistic. I hope not. It's a smaller company ran by 1 master plumber and he will need a helper for the day. New venting will only need to be around 5 feet of plastic. I'm forgetting the permit which will probably be a few hundred so 4500-5k before permit is probably closer but even then $3500 for new boiler installed ain't bad plus I'm hoping to save 50-90 bucks a month on gas in the winter. My largest gas bill in winter so far was $245 so I hope to cut those types down to $150-175 maybe. Plus I already have a condensate pump right under where the new boiler will hang - that's for my rinnai ruc98i. Can they feed two sources of condensate into one pump or should I have two pumps?
DR-DEATH
Member
Dana or anyone who knows about this stuff,
I have received a couple quotes for replacing my system with the uft-80 and some are wildly different. One quote replaces all older zone valves with pumps and piped in a primary and secondary manner. This quote is around 8k. Another quote I received was 5k and that is keeping my taco zone valves but using a new controller for it. I'm not sure if this quote is factoring in p/s set up. I know the brochure for the uft states no p/s configuration required but the higher quote stated they will only install p/s as its way less wear and tear on the boiler. Is this important for this boiler? and would piping in that manner really increase the cost that much? My set up is completely accessible with full drop ceiling and all within 10 feet of everything to be piped.
I have received a couple quotes for replacing my system with the uft-80 and some are wildly different. One quote replaces all older zone valves with pumps and piped in a primary and secondary manner. This quote is around 8k. Another quote I received was 5k and that is keeping my taco zone valves but using a new controller for it. I'm not sure if this quote is factoring in p/s set up. I know the brochure for the uft states no p/s configuration required but the higher quote stated they will only install p/s as its way less wear and tear on the boiler. Is this important for this boiler? and would piping in that manner really increase the cost that much? My set up is completely accessible with full drop ceiling and all within 10 feet of everything to be piped.
In 19 out of 20 systems there would be no need to pipe the UFT-080W primary/secondary. That is true of most fire-tube boilers, due to the low pumping head and tolerance of high temperature differences on the heat exchanger.
Replacing zone valves with pumps costs over $100 per zone in just hardware, and you's also be into it for the primary pump too, and the extra plumbing, etc., plus the additional labor involved. Maybe that adds up to $3K, maybe it doesn't, but it is not likely to be necessary.
If you stick with zone valves you should probably swap out the circulator with an ECM drive smart pump (a ~$150-200 cost adder on the hardware front) to be able to tweak the flow under various conditions, but there is no reason you can't go that route. Setting it up a smart pump for a constant 20F temperature difference would usually be super-kind to the boiler, but you may want to set it up for constant backpressure instead, depending on the particulars of the system. If it were a different type of heat exchanger the primary/secondary might make sense in some cases, but in most residential systems it's possible to specify a flow & pump that would work fine without primary/secondary anyway.
Spending another $3K to be nicer to a $1700 boiler that is also a design tolerant of a wide range of flows & temperature differences doesn't make much sense to me. If the contractor quoting the $5K seems competent and stands behind the work (get references), go with it. Find out what they intend to use for a pump, and if its a standard 1-speed, discuss the ECM drive smart pump concept. With a dialed-in right-sized mod-con the pump will be running pretty much 24/7, and if you go with a low efficiency sub-optimal 1-speed it will end up using more than 5x the electricity of a smart pump.
Replacing zone valves with pumps costs over $100 per zone in just hardware, and you's also be into it for the primary pump too, and the extra plumbing, etc., plus the additional labor involved. Maybe that adds up to $3K, maybe it doesn't, but it is not likely to be necessary.
If you stick with zone valves you should probably swap out the circulator with an ECM drive smart pump (a ~$150-200 cost adder on the hardware front) to be able to tweak the flow under various conditions, but there is no reason you can't go that route. Setting it up a smart pump for a constant 20F temperature difference would usually be super-kind to the boiler, but you may want to set it up for constant backpressure instead, depending on the particulars of the system. If it were a different type of heat exchanger the primary/secondary might make sense in some cases, but in most residential systems it's possible to specify a flow & pump that would work fine without primary/secondary anyway.
Spending another $3K to be nicer to a $1700 boiler that is also a design tolerant of a wide range of flows & temperature differences doesn't make much sense to me. If the contractor quoting the $5K seems competent and stands behind the work (get references), go with it. Find out what they intend to use for a pump, and if its a standard 1-speed, discuss the ECM drive smart pump concept. With a dialed-in right-sized mod-con the pump will be running pretty much 24/7, and if you go with a low efficiency sub-optimal 1-speed it will end up using more than 5x the electricity of a smart pump.
DR-DEATH
Member
I will check into the ECM pump option. One plumber also stated with our fin tube we won't see 96 percent efficiency and will only probably run at 86-88 percent max. That is only on coldest times because it has to heat the water to 180 correct? How cold are we talking? Should I be able to condense when it's 25-35 degrees outside? Or will I only condense when it's 45-60 degrees?
The water most likely NEVER has to be 180F, not even on the coldest days.
Without completely re-reading this thread, if you run a fuel-use heat load calculation, at your 99% outside design temp, and divide the heat load by the number of feet of baseboard, you'll come up with a number. Say the load comes out to 34,000 BTU/hr @0F or whatever your local design temp is, and say you have 96' of fin-tube baseboard. The ratio is 34,000/96'= 354 BTU/hr per foot of baseboard.
You can then use this spec for a typical baseboard product to estimate the required water temp at that outdoor temperature. To get the 354 BTU per foot-hour out of it takes something like 145F average water temp (say 150F out, 140F return), not 180F.
With return water at 140F or a bit higher it will be running about 87% combustion efficiency, but 99% of the hours in a year it won't need temps that warm. With return water at 125F (130F average water temp, say 135F out) most mod-cons will be running 90-91% efficiency, and it rises pretty quickly to the mid-90s as return water temps drop below that.
So, even if it needs 140F or higher water 1% of the time, you'll still be averaging over 90% most of the season.
When it's 35F outside the heat load is about half what it is at 0F, and the BTU/ft needed to stay warm, so what was 354BTU/ft @0F becomes 177F. If you look at the baseboard spec you'll see that takes an AWT of about 115F, which would be returning water cooler than 110F if you set it up right, which would put you in the 92-93% efficiency range.
The output characteristics of fin-tube isn't very linear at average water temps below 11oF or so, but that will be in the mid-90s range for most mod-con boilers. There are many existence proofs of systems with baseboard that average in the 95% efficiency range- it's just a matter of the ratio of load to baseboard length. If you need 140F on design day you probably won't beat 95% for and average, but y0u'll definitely beat 90% if you tweak in the reset curve.
So, to estimate the outdoor temp at which when it stops condensing it's necessary to know:
1> What is your heat load?
2> What is your 99% outside design temperature?
3> How many feet of baseboard do you have?
Without completely re-reading this thread, if you run a fuel-use heat load calculation, at your 99% outside design temp, and divide the heat load by the number of feet of baseboard, you'll come up with a number. Say the load comes out to 34,000 BTU/hr @0F or whatever your local design temp is, and say you have 96' of fin-tube baseboard. The ratio is 34,000/96'= 354 BTU/hr per foot of baseboard.
You can then use this spec for a typical baseboard product to estimate the required water temp at that outdoor temperature. To get the 354 BTU per foot-hour out of it takes something like 145F average water temp (say 150F out, 140F return), not 180F.
With return water at 140F or a bit higher it will be running about 87% combustion efficiency, but 99% of the hours in a year it won't need temps that warm. With return water at 125F (130F average water temp, say 135F out) most mod-cons will be running 90-91% efficiency, and it rises pretty quickly to the mid-90s as return water temps drop below that.
So, even if it needs 140F or higher water 1% of the time, you'll still be averaging over 90% most of the season.
When it's 35F outside the heat load is about half what it is at 0F, and the BTU/ft needed to stay warm, so what was 354BTU/ft @0F becomes 177F. If you look at the baseboard spec you'll see that takes an AWT of about 115F, which would be returning water cooler than 110F if you set it up right, which would put you in the 92-93% efficiency range.
The output characteristics of fin-tube isn't very linear at average water temps below 11oF or so, but that will be in the mid-90s range for most mod-con boilers. There are many existence proofs of systems with baseboard that average in the 95% efficiency range- it's just a matter of the ratio of load to baseboard length. If you need 140F on design day you probably won't beat 95% for and average, but y0u'll definitely beat 90% if you tweak in the reset curve.
So, to estimate the outdoor temp at which when it stops condensing it's necessary to know:
1> What is your heat load?
2> What is your 99% outside design temperature?
3> How many feet of baseboard do you have?
DR-DEATH
Member
I can't pull up all my bills but a bill from 12/10/15-01/12/16 shows I used 90 therms. I don't have access to my boiler currently but I do know one of the BTU's is 130k I don't know if that's input or output but I estimate my current boiler to be 75-80 efficient. The heating bulb show's -3 fo (design day temp?)
Heating days:
Description:,"Fahrenheit-based heating degree days for a base temperature of 65F"
Source:,"www.degreedays.net (using temperature data from www.wunderground.com)"
Accuracy:,"Estimates were made to account for missing data: the ""% Estimated"" column shows how much each figure was affected (0% is best, 100% is worst)"
Station:,"MANCHESTER AIRPORT, NH, US (71.44W,42.93N)"
Station ID:,"KMHT"
Month starting,HDD,% Estimated
2015-10-01,444,0.1
2015-11-01,598,0
2015-12-01,765,0
2016-01-01,1107,0.1
2016-02-01,973,0
2016-03-01,737,0
2016-04-01,552,0
2016-05-01,258,0
2016-06-01,66,0
2016-07-01,19,0
2016-08-01,10,0
2016-09-01,100,0
1705 for heating days at 65f.
I calculated a difference of 65f to design day -3 is 68 and followed the instructions for: "http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new" and it showed me i'm at around 11-12k heat loss. I have no idea if that's even close because I am horrible with math - but if it is correct or close then dividing that by 100f of base board puts me at 110f water temp which is definitely condensing. Does my math sound correct? I guess even if my heat loss was 34k-40 k that puts me at 155 degree temp or so.
Heating days:
Description:,"Fahrenheit-based heating degree days for a base temperature of 65F"
Source:,"www.degreedays.net (using temperature data from www.wunderground.com)"
Accuracy:,"Estimates were made to account for missing data: the ""% Estimated"" column shows how much each figure was affected (0% is best, 100% is worst)"
Station:,"MANCHESTER AIRPORT, NH, US (71.44W,42.93N)"
Station ID:,"KMHT"
Month starting,HDD,% Estimated
2015-10-01,444,0.1
2015-11-01,598,0
2015-12-01,765,0
2016-01-01,1107,0.1
2016-02-01,973,0
2016-03-01,737,0
2016-04-01,552,0
2016-05-01,258,0
2016-06-01,66,0
2016-07-01,19,0
2016-08-01,10,0
2016-09-01,100,0
1705 for heating days at 65f.
I calculated a difference of 65f to design day -3 is 68 and followed the instructions for: "http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new" and it showed me i'm at around 11-12k heat loss. I have no idea if that's even close because I am horrible with math - but if it is correct or close then dividing that by 100f of base board puts me at 110f water temp which is definitely condensing. Does my math sound correct? I guess even if my heat loss was 34k-40 k that puts me at 155 degree temp or so.
December 2015 is a lousy month to use, since it had several days toward the end of the month that had a cooling load instead of heating. (I recall skiing in 70F weather the afternoon of 24 December!) With super-mild weather the error from solar gains and other gas appliances is greater, compared to months with a consistent and deeper heat load.
That said...
I downloaded a daily HDD for the past year rom station KMTH, and truncated it to the period from 12/10/2015 and 1/12/2016. You don't know what time of day the meter was read, but you have to throw out either the first day's or last day's HDD to be valid, since they didn't read the meter at 12:01 AM on December 10th, then 11:59 PM on January 12th.
Adding it up from 12/10 through 1/11 it comes up with 930.2 HDD, and adding up from 12/11 through 1/12 it's 910.2 HDD, so spitting the difference it's likely that the fuel use covered about 920 HDD.
90 therms is 9,000,000 BTU. Burned at 80% efficiency it put at most (0.8 x 9,000,000=)7,200,000 BTU of heat into the heating system.
That's 7,200,000/920= 7826 BTU per HDD, or (/24=) 326 BTU per degree-hour.
A design temp of -3F is 68F cooler than the presumptive 65F heating/cooling balance point base, so the implied load is:
68F x 326 BTU per degree-hour= 22,168 BTU/hour.
As a sanity check, for 1420' of conditioned space that works out to 22,168/1420= 15.6 BTU/hr per square foot of conditioned space. That's a credible number, but a bit on the high side for a tight 2x6 framed house. But if the existing boiler was 130,000 BTU/hr (in or out), the thing is really more than 4x oversized for the design load, and probably delivering 70-75% efficiency (best case.) So the real load is probably closer to 22,168 x 70/80= 19,397 BTU/hr with a load/area ratio of about 13.7 BTU per square foot, which is also a credible number.
If you have 100 feet of baseboard, that's 222 BTU/hr per foot needed when it's -3F outside, which can be delivered at an average water temp somewhere between 120F- 125F. That's definitely in the condensing zone, even at design condition, which means you can probably set it up the reset curve to run with 110F-115F output & 94-95% efficiency most of the season, only dropping to the low-90s during the coldest weather.
The plumber who says you're limited to 87-88% either can't or didn't do even the most rudimentary math on it!
That said...
I downloaded a daily HDD for the past year rom station KMTH, and truncated it to the period from 12/10/2015 and 1/12/2016. You don't know what time of day the meter was read, but you have to throw out either the first day's or last day's HDD to be valid, since they didn't read the meter at 12:01 AM on December 10th, then 11:59 PM on January 12th.
Adding it up from 12/10 through 1/11 it comes up with 930.2 HDD, and adding up from 12/11 through 1/12 it's 910.2 HDD, so spitting the difference it's likely that the fuel use covered about 920 HDD.
90 therms is 9,000,000 BTU. Burned at 80% efficiency it put at most (0.8 x 9,000,000=)7,200,000 BTU of heat into the heating system.
That's 7,200,000/920= 7826 BTU per HDD, or (/24=) 326 BTU per degree-hour.
A design temp of -3F is 68F cooler than the presumptive 65F heating/cooling balance point base, so the implied load is:
68F x 326 BTU per degree-hour= 22,168 BTU/hour.
As a sanity check, for 1420' of conditioned space that works out to 22,168/1420= 15.6 BTU/hr per square foot of conditioned space. That's a credible number, but a bit on the high side for a tight 2x6 framed house. But if the existing boiler was 130,000 BTU/hr (in or out), the thing is really more than 4x oversized for the design load, and probably delivering 70-75% efficiency (best case.) So the real load is probably closer to 22,168 x 70/80= 19,397 BTU/hr with a load/area ratio of about 13.7 BTU per square foot, which is also a credible number.
If you have 100 feet of baseboard, that's 222 BTU/hr per foot needed when it's -3F outside, which can be delivered at an average water temp somewhere between 120F- 125F. That's definitely in the condensing zone, even at design condition, which means you can probably set it up the reset curve to run with 110F-115F output & 94-95% efficiency most of the season, only dropping to the low-90s during the coldest weather.
The plumber who says you're limited to 87-88% either can't or didn't do even the most rudimentary math on it!
Nice thing is with the mod-cons, well at least with the HTP UFT-80W anyway... is it literally takes less than 60 seconds to dial in a new outdoor reset curve to experiment with. There are only four variables to input:
Maximum Outdoor Temperature
Minimum Outdoor Temperature
Maximum Supply Temperature
Minimum Supply Temperature
I'm currently using a "shoulder season" reset curve to prevent short cycling on those 55f days when the house just needs a 1deg morning bump inside, I have plotted another couple of curves for the actual winter season to tryout.
Maximum Outdoor Temperature
Minimum Outdoor Temperature
Maximum Supply Temperature
Minimum Supply Temperature
I'm currently using a "shoulder season" reset curve to prevent short cycling on those 55f days when the house just needs a 1deg morning bump inside, I have plotted another couple of curves for the actual winter season to tryout.
DR-DEATH
Member
Awesome. That makes me feel better because the price for install have been much higher than expected and I was second guessing if it was worth it. I should have a final quote but it's looking to be at 5300 or so with a ECM pump. Keeping existing taco zone valves to lower cost. (Hopefully that doesn't bite me in the ass, but other quotes go replace have been 3-5k higher. )
A ~$5K quote for a relatively simple swap-out seems fair, and about what it costs to drop in a 2-3 plate cast-iron beastie that only hits' mid-80s efficiency. The ECM pump will pay for itself in fairly short years on the electricity savings alone, and allows you to optimize the system a bit better.
Pumping the system at ~2 gpm (~1000 lbs /hour) and a delta-T of 20F will deliver 20,000 BTU/hr, which is your approximate heat load. So setting the low end of the curve initially to something like 125F @ 0F and the pump at 2gpm constant flow will probably work just fine. When both zone valves are open both zones will be getting ~1 gpm, which is sufficiently turbulent for good heat transfer, and when only one is open and taking the full flow it's still a reasonable flow. That's neither a stress on the boiler or the plumbing. The delta-T will be smaller when the water temps are lower, since the fin-tube is emitting less, but that's fine.
At 326 BTU per degree-hour the ~7600 BTU/hr min-fire output balances at 7600/326 = 23F cooler than the 65F presumptive balance point, or ~42F. For 100' of fin-tube to balance it would only need to emit 76 BTU/ft to balance which would be fairly low water temp something like an average water temp 100F. At 2 gpm that would be a delta-T of 7.6F, so setting the minimum supply temp to something around 105F would be about right, then play around a bit with the maximum outdoor temperature. If you set the maximum outdoor temp to 40F it would probably keep up when it's 40F or lower, but if it won't fire at all at warmer temps than that you'll be pretty chilly during the shoulder seasons. Odds are pretty good that your true heating/cooling balance point is about 60F, so you might start there, or even lower. There's plenty of room to play around with it, but the goal is very long, near constant burns, at a reasonably low flow. Taking the flow under 1 gpm may have paradoxical results, since it may not be turbulent enough for good heat transfer due to laminar flows insulating the walls of the fin-tube pipe from the warmer flow in the center.
Pumping the system at ~2 gpm (~1000 lbs /hour) and a delta-T of 20F will deliver 20,000 BTU/hr, which is your approximate heat load. So setting the low end of the curve initially to something like 125F @ 0F and the pump at 2gpm constant flow will probably work just fine. When both zone valves are open both zones will be getting ~1 gpm, which is sufficiently turbulent for good heat transfer, and when only one is open and taking the full flow it's still a reasonable flow. That's neither a stress on the boiler or the plumbing. The delta-T will be smaller when the water temps are lower, since the fin-tube is emitting less, but that's fine.
At 326 BTU per degree-hour the ~7600 BTU/hr min-fire output balances at 7600/326 = 23F cooler than the 65F presumptive balance point, or ~42F. For 100' of fin-tube to balance it would only need to emit 76 BTU/ft to balance which would be fairly low water temp something like an average water temp 100F. At 2 gpm that would be a delta-T of 7.6F, so setting the minimum supply temp to something around 105F would be about right, then play around a bit with the maximum outdoor temperature. If you set the maximum outdoor temp to 40F it would probably keep up when it's 40F or lower, but if it won't fire at all at warmer temps than that you'll be pretty chilly during the shoulder seasons. Odds are pretty good that your true heating/cooling balance point is about 60F, so you might start there, or even lower. There's plenty of room to play around with it, but the goal is very long, near constant burns, at a reasonably low flow. Taking the flow under 1 gpm may have paradoxical results, since it may not be turbulent enough for good heat transfer due to laminar flows insulating the walls of the fin-tube pipe from the warmer flow in the center.
DR-DEATH
Member
Gotcha. Still waiting on one last quote before we make our decision but we just got word today there's a $3,000 rebate we qualify for so this upgrade may be super cheap! Once I get this installed (time frame mid November for plumber) I'll probably post back if I can't figure out how to tweak the settings for our use. I'm super excited to get this thing installed!
DR-DEATH
Member
Just updating now that I have my final quote in. System will be installed in the next couple weeks as I'll post pictures of install but total price should be at 3k after rebates. Not too shabby!
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DR-DEATH
Member
Installers are here for the boiler. They brought a taco vt 2218 ECM. From my research I believe this pump is great to manage delta ts but some people had issues getting it to work correctly with the HTP uft series. I was specifically looking at the taco vr 1816 as it can maintain constant pressure. Is this a big deal? Should I have them get the 1816 or will the 2218 work just as well but has more features?
When I was researching CH pumps for my mod-con (HPT UFT-80W) it seemed DP pumps were preferred over DT pumps on direct piped (not P/S piped) systems.
The best explanation I read was.... "having a DT pump on a mod-con is like having two people using the steering wheel of a car at the same time".
Since the boiler controller and the DT pump controller don't communicate with each other- they start reacting off each other which can produce counter productive results.
The DP pump only controls flow rate to reduce the risk of over or under pumping.
FWIW- I went with with the DP Grundfos Alpha on my direct piped UFT boiler w/zone valves.
The best explanation I read was.... "having a DT pump on a mod-con is like having two people using the steering wheel of a car at the same time".
Since the boiler controller and the DT pump controller don't communicate with each other- they start reacting off each other which can produce counter productive results.
The DP pump only controls flow rate to reduce the risk of over or under pumping.
FWIW- I went with with the DP Grundfos Alpha on my direct piped UFT boiler w/zone valves.
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DR-DEATH
Member
That's what I was reading as well. I just needed further clarification. They are going to install the DP pump. My plumber is familiar with HTP products as we are near MA but didn't seem to be fond of the UFT series. He mentioned the supply house that sells this boiler said they have 8 out in the area that are having issues. I'm trusting everyone's input though.. if this thing has issues my wife will murder me lol
