I just received some interesting news today

 
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Post by lsayre » Wed. Jun. 26, 2019 8:45 am

McGiever wrote:
Wed. Jun. 26, 2019 8:41 am
Could be that low mass design contributes heavily to the functional condensing process...
But these were non-condensing models.


 
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Post by McGiever » Wed. Jun. 26, 2019 8:52 am

lsayre wrote:
Wed. Jun. 26, 2019 8:45 am
But these were non-condensing models.
Oh, I see.

Boiler construction materials, of course, do add some mass...cast iron vs steel...
Last edited by McGiever on Wed. Jun. 26, 2019 8:58 am, edited 2 times in total.

 
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Post by lsayre » Wed. Jun. 26, 2019 8:55 am

I guess that when you can pulse a NG or Propane fire on and off instantly there is no need for water storage within a boiler any longer.

 
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Post by McGiever » Wed. Jun. 26, 2019 9:02 am

lsayre wrote:
Wed. Jun. 26, 2019 8:55 am
I guess that when you can pulse a NG or Propane fire on and off instantly there is no need for water storage within a boiler any longer.
Mass at the emmitter side could be one other way to add mass, but few would want to change existing.

 
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Post by CapeCoaler » Wed. Jun. 26, 2019 9:06 am

Or you could just use a plain-old natural draft DWH as your 'Boiler'...
For way less money...
And have heat when the power goes out...
You would need to rely on gravity feed...

 
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Post by McGiever » Wed. Jun. 26, 2019 9:12 am

CapeCoaler wrote:
Wed. Jun. 26, 2019 9:06 am
Or you could just use a plain-old natural draft DWH as your 'Boiler'...
For way less money...
And have heat when the power goes out...
You would need to rely on gravity feed...
Solves the mass problem, for sure. And Larry has solar/batteries/inverter for limited pump use. ;)
Now...what about that resell value???

 
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Post by lsayre » Wed. Jun. 26, 2019 9:37 am

McGiever wrote:
Wed. Jun. 26, 2019 9:02 am
Mass at the emmitter side could be one other way to add mass, but few would want to change existing.
I'm not about to change my zones lengths (as in combining zones) to add more water mass via that route (although the literature does suggest it as a valid possibility). I'm already resolved to adding a 20 gallon buffer tank to accomplish this task.

I know a few people who own relatively modern hydronically heated (via baseboards) ranch style homes which are heated by a single zone. They literally must be built this way to avoid short cycling a "low mass" boiler for the case of not having a buffer tank. To help even out the heat distribution they generally split the single zone on each side of the house and run a one size larger common return down the middle.


 
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Post by CapeCoaler » Wed. Jun. 26, 2019 10:40 am

In the P&S you agree to install or give a certain value by closing...
You deal with what others want when you need to sell...
Create a contingency fund for said install...
But install what you want for now and save the money...

 
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Post by lsayre » Wed. Jun. 26, 2019 10:42 am

The zone emitters must not be undersized with respect to the lowest turn down ratio of the boiler or else the boiler will short fire, be inefficient, soot up, and wear itself out in short order. For example:

Boiler rated for 60,000 BTUH output
Boiler turn down ratio = 4:1 (wherein it will low fire to generate 15,000 BTUH)
Zones designed to exhibit a Delta-T of 20 degrees.
Outdoor reset set to an "EWT" (entering water temperature) of 105 degrees when it is 50 degrees or warmer outside (ergo supply and return are 125 degrees and 105 degrees respectively).
Shortest zone in house has 40 feet of HWB's, with HWB's rated for 190 BTUH/Ft emittance at a nominal water temp of 115 degrees.
It's 55 degrees outside.
T-Stat calls for heat on shortest zone.

40' of HWB's x 190 BTUH/Ft radiation = 7,600 BTUH total zone emittance.
Boiler emits 15,000 BTUH
Result = boiler rapidly overheats and shuts down, then wash, rinse, and repeat...

Solution: Combine 2 x 40 ft zones into one so radiation for outdoor reset at 105 degree EWT is 15,200 BTUH.

I have 4 zones, one with 38 ft of HWB's, one with 43 ft, one with 41 ft, and lastly the garage with 26 ft.

This "short cycling" seems to be the main reason why modern "low mass" boilers fail so often. Condensing or not. My zones have roughly 3.7 gallons of water in them, and when coupled to a boiler with 1.1 gallon, the total is 4.8 gallons for the case of 1 zone calling for heat, and about 16 gallons for the case of all of them calling for heat.

 
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Post by lsayre » Wed. Jun. 26, 2019 12:18 pm

How short is a short cycle? The absolute minimum recommended time between boiler fire and boiler cut-off for a low mass boiler has been arbitrarily set at 10 minutes. So how long does a boiler fire when it is emitting 15,000 BTUH, and a zone is designed for 20 degrees of Delta-T, and there are 4.8 gallons of water total for that zone (including the boiler). The best case scenario would be for the T-Stat to call when the boiler is just about to fire on its own anyway, meaning the temperature is at 105 degrees in the loop (for conditions as in my post above).

To raise 4.8 gallons by 20 degrees and shut off the boiler requires:

4.8 x 20 x 8.34 = 801 BTU's

If a boiler is outputting 15,000 BTU's per hour it is outputting 15,000/60 = 250 BTU's per minute.

And if the HWB's on the zone are emitting 7,600 BTU's every hour, they are emitting 127 BTU's per minute.

250-127 = 123

800/123 = 6.5

For this case the boiler will fire for 6.5 minutes.

But for the case where the T-Stat calls and the loop is already at 125 degrees, it fires for zero minutes. So the average will be between these extremes, or 3.25 minutes.

 
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Post by lsayre » Wed. Jun. 26, 2019 1:26 pm

CapeCoaler wrote:
Wed. Jun. 26, 2019 10:40 am
In the P&S you agree to install or give a certain value by closing...
You deal with what others want when you need to sell...
Create a contingency fund for said install...
But install what you want for now and save the money...
Good points!!!

 
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Post by lsayre » Wed. Jun. 26, 2019 1:37 pm

To raise 24.8 gallons by 20 degrees and shut off the boiler (as would be the case if a 20 gallon buffer tank is added) requires:

24.8 x 20 x 8.34 = 4137 BTU's, but we are looking for the midrange, which is half, or 2068 BTU's

If a boiler is outputting 15,000 BTU's per hour it is outputting 15,000/60 = 250 BTU's per minute.

And if the HWB's on the zone are emitting 7,600 BTU's every hour, they are emitting 127 BTU's per minute.

250-127 = 123

2,068/123 = 16.8 minutes of average firing time.

Well better than the arbitrary minimum of 10 minutes. A 10 gallon buffer tank would provide for just about precisely 10 minute average firing times. To me 20 gallons seems to be a safer bet though.

 
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Post by McGiever » Wed. Jun. 26, 2019 3:36 pm

Now what happens to the whole mix when your hypothetical four persons need separate but consecutive showers and it's 10*F below zero outside? :)

 
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Post by lsayre » Wed. Jun. 26, 2019 3:44 pm

McGiever wrote:
Wed. Jun. 26, 2019 3:36 pm
Now what happens to the whole mix when your hypothetical four persons need separate but consecutive showers? :)
With a combi boiler capable of 3 gallons per minute hot water with a 70 degree rise, all of them get to take hot showers for as long as they like. And heating the house is not at all impacted by this since a combi-boiler is two boilers in one box.

Example: The Navien NCB-150E Combi-Boiler. Home heating input BTUH = 60,000. DHW input BTUH = 120,000.

120,000 x 0.87 = 104,400 BTUH gross output at 87% efficiency for DHW

104,400/(8.34 x 60 x 70) = 2.98 never ending GPM of 120 degree DHW for the case of ground water at 50 degrees.

Divide by 1.15 to net out any BTU's typical of most homes to be lost to un-insulated pipes traversing between the boiler and the shower head, and the on demand continuous DHW GPM drops to 2.6, which is more honest. This is the application of the 1.15X "pick up factor". Pick-up is quite an inappropriate term and in modern parlance this is where gross and net hot water BTUH differences come from. Net BTUH output presumes that a ballpark 15% of Gross BTUH boiler output will be lost to piping hiding behind walls and running across basement ceilings, as it gets the hot water from emitter to emitter. For steam the presumed transmission loss is 30%. Consider it to be quite like line losses for electrical wire.
Last edited by lsayre on Wed. Jun. 26, 2019 3:59 pm, edited 1 time in total.

 
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Post by McGiever » Wed. Jun. 26, 2019 3:58 pm

Where can I get a coal fired combi-boiler? :lol:


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