Adding Outdoor Wood Boiler as backup to Existing LP Combi

PhillipJones

Hello! I'm a new post here. I've read from you pros for years, but it's time to stick a toe in, and get your thoughts. Attached is a photo of an LP firetube combi we installed in a farm shop a few years ago. Now the owner wants to add his outdoor wood furnace as an auxilliary source. I've sketched my game plan with some comments. I'd like to know if I'm on track, especially as it relates to having the zone circulator pull directly from the flat plate HX and the I-Series mixing valve. Your feedback is appreciated!

Phillip Jones

GroundUp

That's fine, but move the aquastat to the supply line on the unpressurized side of the HX and turn the temp up so that the gas always comes on when the wood boiler is below 120*

hot_rod

What you don't want to do is run the back up boiler and allow it to heat the OWF. That can get expensive

If you are running the OWF pump 24/ 7, this piping with a delta T controller is another option. The delta T (solar controller) has two sensors so it makes decesions based on two temperatures. It give you more, possibly better control.

GroundUp

Backfeeding the OWB with return water from the slab is barely enough to keep it above 40 degrees in most cases, and is a fantastic idea for freeze protection. Like, less than a dollar a day when I do it at my own home with an electric boiler.

You may want to fix your gas line though- that attempt at a dirt leg isn't doing anything.

leonz

What size central boiler forest eater does he have?

Are the circulators for the forest eater inside the access door of the forest eater or in the shop/home??

You can simplify you work by installing a simple pipe loop with a heat exchanger.

If you want a friend for life mention to him that he should fill the firebox half full of standard firebrick to create a huge heat sink to store heat from the fire and radiate it back into the firebox slowly over time by doing this he will have less smoke and at the same time he has created a huge volume of thermal mass to keep the firebox hot without adding water storage.

When I filled my hand fed boiler half full of standard firebrick to the flue breech I cut my wood and coal use by half.

hot_rod

leonz said:

What size central boiler forest eater does he have?

Are the circulators for the forest eater inside the access door of the forest eater or in the shop/home??

You can simplify you work by installing a simple pipe loop with a heat exchanger.

If you want a friend for life mention to him that he should fill the firebox half full of standard firebrick to create a huge heat sink to store heat from the fire and radiate it back into the firebox slowly over time by doing this he will have less smoke and at the same time he has created a huge volume of thermal mass to keep the firebox hot without adding water storage.

When I filled my hand fed boiler half full of standard firebrick to the flue breech I cut my wood and coal use by half.

If you fill the firebox 1/2 full of fire brick, you have 1/2 the space for the fuel, firewood.
Maybe that is why you use 1/2 as much wood

The water content is usually very high in those OWF so you should have plenty of thermal storage. The water is a better place to store heat compared to any brick or concrete mass.

The problem with OWF is your precious thermal storage, in the brick or water, is in an open container outside in the cold

So fuel to actual heat into the space may be 40%? Less? Depends on how efficiently you burn the wood. Wet, green or large diameter wood is not a great fuel source.

PhillipJones

Thanks for the advice, guys. Hot Rod, I'm interested in the Delta T controller you mentioned, and the idronics snips. I noticed that those show a load-side circulator, where my plan was to simply allow the zone circulator pull through the HX. Could I move my OWF circulator inside, and allow the Delta T controller to turn it off to prevent sending heat back out to the OWF? Ground Up, I also like the idea of moving the aqua stat. Thanks again!

hot_rod

In open system OWF the lower the pump the better you have a high head circ with only a few psi and high temperatures often above 190 you are always right at the cavitation point.

I think a two or three piece circulator is better. I think Taco has a specific OWF circ, 2400. Wish it was a stainless body. Since open systems tend to rust and corrode badly if the inhibitor is not maintained.

Unpressurized OWF are the worse application for circulators

I don't think you can shut off the circ to the OWF without risking freezing?

To connect multiple sources and take advantage of the delta t circ, you need the piping to match, has to be primary secondary 

That idronics issue shows a few piping options and the control wiring schematic that accompanies it

If you burn wood for heat, you may as well squeeze all the BTUs you can out of it. A bit of clever control logic can help with that.

GroundUp

That Taco 2400 is far and beyond the WORST possible circulator for a wood boiler, despite being "wood boiler specific". I have thrown at least 50 of them away in my career, not a single one more than 2 years old. To answer your question though, no you can not put the circ inside the building unless you want to replace it every year or less. It must be on the OWB for proper operation, and it must run 24/7. This is not negotiable.

If you think you need some sort of temperature control to prevent backfeeding the OWB, a simple 3 way zone valve to bypass the HX when the gas boiler is running will do the trick. But with that said, the odds of freezing the OWB go up very significantly under those circumstances. The $1 a day it costs to backfeed is money well spent, IMO.

hot_rod

MIs-apply any pump and they are destined to fail.
I don’t see NPSH spec on the 2400, but the curves indicate virtually every model would be running off curve on an OWF with 1” Pex and less than 12-15 gpm of flow.

Secondly at higher temperature you need adequate pressure on the circ inlet. Grundfos is kind enough to show that requirement in psi and actual distance the water level needs to be above the pump. At 190 f water you need 9’ , or 4 psi at the pump inlet. At 210 where OWF tend to go often, probably 20’ or more above the pump.

I suspect the high head 2400 NPSH numbers would be even higher?

So from day one these OWF circulator are running out of spec. Way out in some cases.
And I suppose without warranty if you go by the book.

Replacing 50 pumps without diagnosing the failure cause, hmmm, Crappy pump is usually the excuse to the homeowner.

leonz

I had plenty of domestic hot water and once the firebrick
became hot like the firebrick in a pizza oven and stayed hot
and the wood burned with little to no smoke and I know I used
much less Anthracite nut and stove coal so..........

The physics are the same the fire brick radiates heat much
more slowly than water in a closed system or an open system.

The geg thermal mass calculator is a great read.

GroundUp

hot_rod said:

MIs-apply any pump and they are destined to fail.
I don’t see NPSH spec on the 2400, but the curves indicate virtually every model would be running off curve on an OWF with 1” Pex and less than 12-15 gpm of flow.

Replacing 50 pumps without diagnosing the failure cause, hmmm, Crappy pump is usually the excuse to the homeowner.

Mighty bold of you to assume that the failure cause hasn't been diagnosed. Seized motor, 100% of the time, due to a crappy pump with unsealed components that can not handle the outdoor elements. All models of Taco circs have the same problem during their 6 month summer shutdown, but the 2400 is especially terrible. Most are indeed running outside their ideal curve, but Grundfos can actually take the abuse. 26-99 and 26-150 are virtually indestructible when installed at the boiler where they belong.

hot_rod

GroundUp said:

hot_rod said:

MIs-apply any pump and they are destined to fail.
I don’t see NPSH spec on the 2400, but the curves indicate virtually every model would be running off curve on an OWF with 1” Pex and less than 12-15 gpm of flow.

Replacing 50 pumps without diagnosing the failure cause, hmmm, Crappy pump is usually the excuse to the homeowner.

Mighty bold of you to assume that the failure cause hasn't been diagnosed. Seized motor, 100% of the time, due to a crappy pump with unsealed components that can not handle the outdoor elements. All models of Taco circs have the same problem during their 6 month summer shutdown, but the 2400 is especially terrible. Most are indeed running outside their ideal curve, but Grundfos can actually take the abuse. 26-99 and 26-150 are virtually indestructible when installed at the boiler where they belong.

Why can't the pumps be remote mounted, in the basement for example to get better NPSH?
Some OWF piping drawings show the pump inside as an option. That would put open frame motors in a better environment.

SteveSan

Any 00 series and 2400 series are not rated for outdoor use. It is stated in the IOM.

CAUTION: Taco 2400 Series circulators are
designed for indoor use only

PhillipJones

Listening to you guys bat this around is helping me a ton, so keep it up! Hot Rod, I'm interested in the relationship the hotter water has to the need for increased psi at the pump inlet. Has Siggy addressed this in any articles that you are aware of? I have his book, but I have missed this so far. Clearly atmospheric systems are less than desirable, and the quicker you can switch to pressurized, the more your pumps will appreciate you. Lately it seems that there is an old guy ready to quit cutting firewood for every young guy ready to quit buying propane. I need to get these hybrid systems down pat, for sure!

hot_rod

It has to do with a term called vapor pressure. in simple terms it is the pressure that needs to be present on a fluid surface to keep it from boiling.
It is very temperature dependent. So the higher the fluid temperature the higher the pressure required on it's surface to prevent it from boiling.
When you talk vapor pressure it is usually in absolute terms, PSIA. So at sea level the absolute pressure is 14.7psia. Gauges are built to read 0 (gauge pressure, psi) at sea level.

At sea level water boils at 212°F.
In Leadville, CO, water boils around 194°F. As you heat water at that altitude vapor pressure rises until equals the reduced atmospheric pressure at 10,000 feet elevation.

This can have consequences for open OWF systems also. This example attached below was ahead scratcher for me when Siggy first presented it in Idronics. Water boiling in the upper level fin tube, basically.

I imagine this would drive a troubleshooter crazy until they understood the relationship of all the conditions, temperature, vapor pressure and pump differential.

Lots of things working against you with open OWF devices. Heat loss from the tank. Ran an example today for a 36" diameter tank 75" long with 3" of foam R-12 all around, and both ends the loss from a tank at 50° located outside at 0° is 322 BTU/hr.

With an OWF you don't have much if any front or rear insulation, and a hole in the top belching BTU out. So knowing that you can get a good idea how much "fuel" would be required to keep the tank from freezing. Plus the in-ground piping loss. Plus the cost to run a 200W circ.

Then the mis-applied circulators with little to no NPSH. Idronics 16 explains NPSH best.

The wood to heat conversion of maybe 40- 45% on a good day. 8, 10, 12 cords of wood or more a year??

Almost enough to make you want to put that OWF and "free" firewood money towards an more efficient shell, with 120°, or lower, distribution temperature heat emitters and a 90% + appliance. Or a 3 COP heat pump

GroundUp

Why can't the pumps be remote mounted, in the basement for example to get better NPSH?
Some OWF piping drawings show the pump inside as an option. That would put open frame motors in a better environment.

Because unless the basement is 10 feet deep and 40 feet away, and the required flow rate is only 4 GPM, the NPSH is greater than it would be at the boiler where the circ belongs. An 007, 009, 15-58, NRF22/25 variety will do just fine in an average basement with a sub-100ft loop of 1" pex, but the reality is that 95% of these OWB systems need at least double the flow that the aforementioned circs will provide and most loops are 2-5x that long. Trying to move 8 GPM through a 400ft loop of 1" pex with a 2400 in the middle is going to suck that impeller into pieces within the week. The resistance to flow on the inlet side of the circ needs to be minimized, or this will happen every single time when trying to move any more than a few GPM through an atmospheric 180+ degree system. Just because somebody drew a diagram of something doesn't make it correct.

About 8 years ago I was called out to replace an 009 in the middle of a ~300ish foot loop of 1" from a Central 6048 OWB into a shallow basement (maybe 5ft of head from the top water level). That circ had been being replaced 3-5x annually by the OWB dealer, claiming normal wear and tear but he had since folded up shop so the HO found me instead. I repiped it to move the circ (upgraded to a 26-99 now, since the delta was upward of 100* with the 009) out to the OWB where it belongs and was able to attain a 23-24* delta under full load, and that same circ is still being used today plus the spot boiling was also eliminated. I just spoke to that HO this summer and he's still happier than a pig in mud that somebody was able to fix this mess. Circulators can not pull through an unpressurized system, they can only push. The correct place for the circ in any OWB system is ON the boiler itself, with as little piping as possible upstream- even if that means it's higher in elevation. Central Boilers for example, have the supply port about 2 feet below the maximum water level. Head is less than 1 PSI right? Never, not once in my 18 years and hundreds of these systems have I seen a shredded impeller or evidence of flashed steam in a circ that was properly mounted ON the boiler. I have, however, seen several dozen located inside the building which have suffered the vacuum fate. Beat me with diagrams and theories all you want, but they're not going to supersede facts and experience.

hot_rod

So the bottom line is the undersized piping and related pressure drop to supply the required gpm. Still not a pump problem but a mis-application. That seems to be the number 1 issue with OWF, the connecting pipe size. Long runs of way under-sized tubing to move the load.

As you confirmed with short run, small pump, adequate NPSH it will work fine to remote mount the circulator.

I guess it is debatable if the enclosure of an OWF qualifies as an indoor mounting location as @SteveSans mentioned?

GroundUp

The problem is that almost all underground piping specified for OWB is only 1" pex. Nobody owning an OWB, and I mean nobody, is going to spend $50/ft for the proper underground line to be able to move 8-10 GPM with an 007 sized circ. You get 1" or 1-1/4" pex, and that's about it. This means that the options include mounting the circ at the boiler where it belongs, and nothing else.

leonz

Thermal mass calculators can be found at geg calulators.

Water%20Thermal%20Mass%20Calculator%20-%20GEGCalculators.html

From Wikipedia:

Some basic information about firebrick.


"A fire brick, firebrick, fireclay brick or refractory brick is a block of ceramic material used in lining

furnaces, fireboxes and fireplaces.

A refractory firebrick is built primarily to withstand high temperature; but will also have a low thermal

conductivity for greater energy efficiency, usually dense firebricks are used in applications with Extreme

Mechanical, Chemical or Thermal Stresses, such as the inside of a wood fired kiln or a furnace, which is

subject to abrasion from wood, fluxing from ash or slag and high temperatures.

In other, less harsh situations such as an electric or natural gas fired kiln more porous bricks, commonly

known as "kiln bricks", are a better choice. They are weaker but much lighter and easier to form and

insulate far better than dense bricks.

In any case firebricks should not spall, and thier strength should hold up well during rapid temperature

changes"'.