Scored a new Vie$$man Vitocell 100B 79 gal Indirect

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http://www.viessmann-us.com/content/dam/internet-ca/pdfs/tanks/vitocell-b_100_tdm.pdf

It's a 79 gal Indirect DHW tank with 2 internal coil Heat Exchangers (HX)

Planning my next house heating system. (I like buffer tanks)

Zone 5 high altitude, 3 in-slab zones: 15Kbtu/h , 8K, & 8K

Indirect will be tank heated via bottom coil HX with a 30* delta aquastat control.

3 heat sources: Solar, Heat Pump, NG Burnham RV3 (already own), computer selected automatically, based on fuel cost & solar tank temp. etc.

Buffer Tank math:
t = (vbt*500*dt) / (qh - qload)
t = runtime in minutes
vbt = vol buffer gal
dt = buffer dt
qh = boiler output BTU/hr
qload = emitter output BTU/hr

Burnham says to derate by 18% for 9,000' altitude. 48,000 DOE x .82 = 39,360 btu/h
About 30 minute burn time, without system or DHW draw


Top coil HX will be used to pull heat out of tank for system heating: 3 zone valves + Grundfos Alpha pump.
much like this diagram:

http://www.houseneeds.com/learning-...kless-heat-combo-with-indirect-storage-tank-5

Comments ?
 
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Dana

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It's still useful to buffer a cast iron boiler if you micro-zone the hell out of a place. Before I scrapped the cast-iron beast in my house it was delivering 80-90 second burns when serving only the low-mass radiant zone in the family-room/addition (one of then only two zones, but the lossiest room in the house.) In the re-configuring the system it became 5 zones, prompted by major differences in heat loss characteristics between rooms. With 5-zones and a modest total load it would have driven the short-cycling numbers stratospheric.

The incumbent c.i. beast was so ridiculously oversized for the total heat load (about 4x) it got scrapped, re-configuring the system around a 48 gallon reverse-indirect HW tank/buffer slaving a cheap low-mass modulating burner to the tank's aquastat (set to 130F as the high limit, with ~7F of hysteresis.) When all zones are calling for heat the entering water temp at the burner is ~115F, and with the shower running when all zones are calling it settles in at about 105-108F EWT. But I digress...

I could as easily have installed a more appropriately sized 50-60K cast iron boiler there, but it would still have needed the buffering mass of the tank to avoid short-cycling like crazy on zone calls.

With the high thermal mass of slab radiation the RV3 could be configured to run without a buffer tank without short-cycling, but when using multiple heat sources like solar + fossil-burn and want to be able store the solar heat when it's available and the heat load is low, it's not a bad way to go.
 
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http://www.pmmag.com/articles/96765-alternate-methods-to-pipe-a-buffer-tank

"Short operating cycles continue to be one of the chronic complaints associated with modern hydronic heat sources. Even state-of-the-art mod/con boilers with 5:1, 8:1 or even 10:1 ratios between their maximum firing rate and minimum stable firing rate can’t always match the heating load imposed by a single small zone, such as a towel warmer radiator in the master bathroom, on a mild day. This is where the additional thermal mass provided by a buffer tank provides the “thermal elasticity” needed between heat supply and heat demand.

Over the last couple of decades, the North American hydronics industry has been learning the importance of buffer tanks as low thermal mass boilers, and water-to-water heat pumps have been increasingly combined with highly zoned distribution systems."
 
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My lot in Teller County Colorado, is a prime climate for solar. High altitude & clear skys.

From Craigs-List, I bought 300 ft2 of used hot water panels + a 1,000 gal heavy duty fiberglass tank (made for cooling Cray super computers)

If solar heat storage tank is above 140*F, it will be used directly to heat buffer tank.

If solar heat storage tank is in the range of 70-140*F, (via a TMV set to 90*F limit), it will provide source heat to my 32K btu Water-to-Water Heat Pump, to heat buffer tank.

If solar heat storage tank is below 70*F, it is more economical to use NG RV3 boiler. (at my todays electricity & NG rates)

I have Arduino code written & debugged to handle the automatic switching between heat sources.
 
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BadgerBoilerMN

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Just dressing up a pig. If you have low temperature emitters you have to protect the CI boiler from low return water temperatures and "batch feed" as we do in solid fuel boilers.

I am not against buffer tanks, just wonder when a high-mass boiler will benefit from a buffer and how much gas you can save by using the boiler as ballast and getting a ModCon with all the benefits and few foibles.
 

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Short cycling? Balance issues? Flow issues? Short zones, long zones? Taco ECM Verideon delta T series programmable zone circulators. Makes even the worst engineered system perform.
 
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Thought experiment:

You have a CI boiler: 50MBTU which is perfectly matched to house @ design temp. of 0*F
4 zones all 12.5MBTU each.

What happens when 1 zone calls for heat @ 50*F outdoor temp.
 
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Description for Taco PC700-2
Designed to improve performance in your hot water heating systems by regulating the supply water temperature of a single boiler based on the outdoor temperature. The control utilizes a heating curve to set the relationship between outdoor temperature and supply water temperature to provide optimum control and comfort. Thanks to its integral microprocessor, the PC700 is able to continuously adjust the boiler differential to avoid boiler short-cycling and large temperature swings, Standard functions include warm weather shut down, minimum boiler supply temperature setting, and a starting water temperature setting. A liquid crystal display clearly shows the boiler supply temperature and other monitored temperatures and settings for simple and effective control.

The PC700-2 is a microprocessor-based control designed to regulate the supply water temperature of a single boiler, based on the outdoor temperature. A wiring harness is provided to be easily connected to the Taco Expandable (-EXP) Controls.

The PC700-2 includes functions such as automatic reset ratio calculation, Warm Weather Shut Down (WWSD), Minimum Boiler setting, and an optional automatic boiler differential. The control has a digital, liquid crystal display (LCD) that normally displays the boiler supply temperature, but can display other temperatures and settings.


So it works sort of like how my Grandfather drove his 52 Cadilac to maintain 50mph:
Floor it for 2 seconds, foot completely off gas for 8 seconds, repeat, repeat.....stop for gas

As the burner is 100% on or 100% off

I have read that short cycling a boiler is bad.
 
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so in my:

Thought experiment:

You have a CI boiler: 50MBTU which is perfectly matched to house @ design temp. of 0*F
4 zones all 12.5MBTU each.

What happens when 1 zone calls for heat @ 50*F outdoor temp.


Assume that house don't need heat at or above 65* outdoors.

65* - 50* = 15*

outdoor range = 65* (65* - 0*)

That 1 zone needs:
(15* / 65*) x 12.5MBH = 2.88MBH

2.88MBH / 50MBH = .0577MBH or about 6% of boiler capicity

That's about 1 minute burn with 16.3 minutes off
or
30 second burn with 8.15 minutes off
or
15 second burn with 4 minutes off

I much prefer: 20-30 minute burn time into a buffer
 
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From Dana:
"It's not considered short cycling unless the boiler's burn time is very short. It's the burn time that counts, since every ignition cycle and flue purge it "throws away" a fixed number of BTUs. If it's burning only 30 seconds and purging for 60 it's an efficiency-disaster."
 

Dana

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Yup, 30 second burns and tens of burns/hour would be efficiency robbing boiler-abusing short cycling behavior!

Most of the economizers work by exercising the available thermal mass in the boiler + zone. When a microscopic zone like a towel rack calls for heat it draws from the boiler/system thermal mass without firing the burner until the boiler temp drops below either a "learned" or programmed minimum temperature. When the boiler fires it attempts to maximize the length of the burn to some minimum, even if the boiler temp blows over the programmed or learned maximum. Most of them work off recent burn history data to determine what the appropriate temperatures would be based on how quickly the return water temperatures ramped on recent cycles, the length of burn time & return water temp at which the thermostat was satisfied, etc, often cutting out the burner ahead of the end of the call for heat to purge heat from the boiler so that it is at a lower temp during standby, thereby cutting the standby losses. The control algorithms are proprietary, and there are significant differences, but the principles are usually discernable in the product literature.

With many multi-zoned systems short-cycling can be avoided with optimizing smart economizer controllers. That level of smarts comes pre-packaged with the ES2 and ESC series cast iron boilers, which Burnham calls the "IQ Control System", but it isn't much different from the Intellicon 3250HW+ or similar heat purging retrofit economizers.

The RV series doesn't come with those smart controls, but it does have internal primary/secondary piping an non-digital but responsive injection pump controller that makes it tolerant of system temps as low as 55F (!) without condensation on the heat exchanger plates. Without the heat purging smart controls it will be more prone to short-cycling in a low-mass radiation micro-zoned system than the ES2/ESC boilers. If the boiler is the primary thermal mass on the zone, the RV3 has maybe 55-60lbs water-equivalent thermal mass, so serving tiny zones without buffering or smarter controls to maximize the use of that tiny thermal mass it would short cycle pretty badly. But with sufficiently massive radiation on each zone it could still do OK without the buffer.

But this just isn't any buffer- it's plumbed for multiple thermal inputs, including solar thermal, which implemented could keep the tank from calling the boiler for heat at all much of the time.
 
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Burnham RV

The Revolution has a built-in bypass that connects the system return to the system supply. Branched into this bypass is the boiler return with it's variable speed injection pump and boiler supply with 10K sensor strapped on. If the boiler supply water temperature is below 150*F, then the boiler injection pump will run at reduced speed to allow the boiler to heat up due to reduced flow of cool return water. As the boiler transitions through 160*F, the pump will speed up until it eventually reaches 100% speed. You need to supply a system pump or pumps and these need to be on the system supply.

The Revolution utilizes a variable speed pump (007) within the internal bypass piping to control flow through the boiler and injection into system via a Tekmar based VSP controller. has been and continues to be very dependable, simple in design and operation, affordable and highly efficient
It is quite simple to determine if the injection pump is not working as desired on the Revolution boiler. Simply look at the boiler temperature gauge, the % On light on the VS3000 pump control and feel the system supply pipe. If the boiler is at or near high limit, the LED % light is on solid and the supply line is not hot, then the pump is not performing correctly.
 
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