Converting from outdoor wood boiler to indoor electric

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Old 01-24-09, 11:36 AM
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Converting from outdoor wood boiler to indoor electric

First off, I’m sorry if this is long, but I want to be as detailed as possible. Thank you in advance for all opinions and/or suggestions.

My wife and I purchased our house in August ’08, and it’s our first winter dealing with the outdoor wood boiler (or some call it a wood furnace). Besides a couple of electric baseboard heaters, we have no other back up heat. All in all, the wood boiler has been OK, it looks like we’ll go through about 8 to 10 cords of wood this winter (we’re in Manitoba, bitterly cold this year). Our biggest downside to the wood boiler is the maintenance and the fact we can’t go anywhere for a lengthy period of time without having somebody else monitoring the wood consumption. So in the spring/summer, I’m going to get rid of the wood boiler and put in an electric boiler.

The current wood boiler provides water to heat both my house (2200 sq ft) and detached garage (800 sq ft). The garage is not heated full-time, it is only heated when I decide to do work in there. In the house I have a Bryant air handler blowing air through a 3” 18x20 hydronic coil which is mounted in the plenum. In the garage I have a belt driven (1/4 hp) squirrel cage with a 2” 16x16 hydronic coil mounted to the blower. All the plumbing is 1” Kitec (which I also want to change out) and the circulating pump is a Grundfos 15-58FC. Currently the water temp is set to 150f at the boiler which is about 100 feet from the house and I believe there is about 100 gallons of water in the system.

I have purchased an Argo AT1243 Argo Electric Boilers (4 element, 40,944 BTUs) that I plan on using and I’m comfortable with both the wiring and plumbing of it. My only concern is heating my house, the garage is secondary which can be heated with other sources if need be unless it is practical to hook up the garage to the electric boiler loop also. The electric boiler will be mounted in my mechanical room and plumbed with 1 inch copper. I will also incorporate a 5 gal. expansion tank with an air bleed valve. The plan is when the thermostat in the house calls for heat, it fires up the electric boiler (which with the Argo also starts the circulating pump) and starts the fan in the air handler.

My question is, because of the short distance of pipe, there will only be about 4-8 gallons of water in the electric boiler system, will this be enough water? I know this will heat the water quite quickly and the Argo AT will automatically maintain the set temperature (I plan on setting it at 140f) while heat to the house is required. If I do incorporate the garage also, it will add another 5 to 6 gallons of water to the system. Is there any recommendations on other stuff I should incorporate in the system besides the usual valves and pressure release. I am open to all suggestions!!!

Thanks

Pharic
 
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Old 01-24-09, 11:49 AM
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The very first thing that strikes me as a possible downside to your plan is that you haven't mentioned having done a proper heat loss calculation! That is imperative given your weather conditions. I do not believe that a 40 MBH boiler is going to be big enough. I'm going to educated guess and say that your actual heat loss is going to be greater than 80 MBH.

I agree that you need backup... and that's fine... so install a properly sized system but KEEP the outdoor boiler! Plumb it in parallel with the new system... wood has GOT to be cheaper than electricity...

You are comfortable with the wiring etc... good. But, is your existing electrical service sufficient to support the new boiler? That is another imperative that must be considered.

Why change out the Kitec? Sounds like unneccesary work to me...

I don't think the volume of water in the system is relevant.
 
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Old 01-24-09, 12:28 PM
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Mr. Trooper,

Thank you for the reply, please let me clarify a couple of things. My HLC is 52,000 BTU's, although the Argo AT Boiler is only 40,944 BTU's, this is where my hydronic coil comes into play. By the following chart Air to Water Heat Exchangers my BTU's are increased almost 2 fold. I would totally agree with your assessment IF I was running infloor or radiant heaters. But in the house I have forced air through the hydronic coil. Basically if water is coming through the coil at 140f and at 8 to 10 gpm and my blower is pushing 1500 cfm I should be easily getting 60,000 BTU's. If I increase the incoming temp to 160f I believe I can get close to 80,000 BTU's.

For wiring, I have 200 amp service, I have more than enough room for the electric boiler (70 amp 2 pole).

For costs, this winter I have bought 10 cords of wood, at $150 a cord delivered. $1500 just for wood. I don't have access to cut my own. I think I will go through it all also if this weather keeps up the way it is. When I live in the city I never paid more than $1200-$1300 a year with a mid efficiency gas furnace. House sizes were comparable also.

The wood boiler is nice, but I bet it's no more than 50% efficient as I don't have radiant heating.

Thanks

Pharic
 
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Old 01-24-09, 12:36 PM
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uhhhh... no, I'm afraid you have a misunderstanding somewhere, somehow.

Your hydronic coils can in no way output more BTU than you are putting into them!

If your boiler can produce 40 MBH, then that's what you have available to heat the home.

Please explain how the water to air heat exchangers are able to put out more heat than you put into them.
 
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Old 01-24-09, 04:48 PM
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52000 heat loss calc for a 2200sqft in Manitoba ? Must be a fairly new house.
Im east of you in NW Ontario, 2000sqft 2 story w/attached garage (but i keep it at ~5C), and my calcs are about 79-85000 here. I use a 115000btuh boiler, and at -35c it runs alot. I figure I could probably step down a nozzle to make 90000, but Id be afraid of going lower.

Yes, that air handler could probably do (the spec's say) 80,000 for incomming water at 160 and 8 to 10gpm. BUT (the important BUT), thats only IF you can keep feeding 160f water to it at 8 to 10gpm ! Im thinking your electric is gonna start out hot then slowly get cooler and cooler and cooler... it'll be running full tilt (ie, hydro meter spinning like a top) and wont be able to keep you happy.
 
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Old 01-24-09, 05:14 PM
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Mr. Trooper,

I understand exactly what you are saying, do not get me wrong. Lets look at this as I currently have it:

- Water temp at the boiler is 140f, it travels 100ft before it gets to the circulating pump. Heat loss is probably apparent, I believe the lines are 3 ft under ground.
- There is approximately 100 gallons of water in the boiler as it takes 4+ hours of solid burning to get it to 140f from 90-100f.
- I do not know how many BTU's the wood boiler is putting out, but I do know most of it is wasted as my Bryant air handler only comes on as the house requires heat. Basically I'm heating water with wood that is not used all the time (if I had radiant baseboards I would disagree).

Now, if per se, I'm heating my house now quite well with the wood boiler with my current setup of supplying heated water to my hydronic coil at say 130f after the loss for the underground line (which I'm sure is way more than 10f). How well would 140f water heat my house?

It doesn't really matter how many BTU's the unit or wood boiler can provide, it's the constant temperature they can provide in water as the house requires heat. How fast the house heats up is dependent on the temp of the water going through the hydronic coil. The fan speed is constant, not variable.

Consistent water temp through hydronic coil + GPH of that water + CFM of the fan in the air handler = the heat in my house. Right now I can control the GPH and the CFM in my current setup, I cannot control the consistent water temp due to it being heated by wood.

I need water going through my hydronic coil at 130-140f when there is a demand for heat. A 40 MBH unit heating 10 gallons of water will easily get that water to temperature and keep it there as needed. As I said, my current setup is pretty much just keeping water at 140f (if the fire doesn't go out), and my house is being heated by that....

Look at my system as a car's rad. A cars motor is able to produce ALOT of heat, with a small amount of liquid through a rad, it's able to dissipate that heat with air flow through it's rad. My hydronic coil is the rad, the air blowing through it is my air handler, which heats my house.

Again my constants and controllable's are GPH and CFM, I need water at a constant temp to heat my house.... Will the Argo AT1243 be able to heat water to 140f and keep it there for a short amount of time? I know I will be able to control the temp on the Argo also.

Pharic
 
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Old 01-24-09, 06:37 PM
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I understand exactly what you are saying
It's obvious to me that you do not.

I'm sorry to say that you have a misunderstanding of what it takes to heat your home... that outdoor wood boiler is producing way more BTUs that you must think it does.

But you can do what you want with the wood boiler... personally, I would keep it... ESPECIALLY if you go through with your plans to install a 40 MBH boiler in your home. Mark my words... you'll be sorry.

That 40 MBH boiler WILL keep the water at 140°F on the outlet, but ONLY IF :

1. you pump no more than FOUR GPM through it

2. the water going IN to the boiler is no more than 20° cooler than the water coming out.

As your neighbor Dave stated:

Im thinking your electric is gonna start out hot then slowly get cooler and cooler and cooler... it'll be running full tilt (ie, hydro meter spinning like a top) and wont be able to keep you happy.
And what's worse, if you DO try to pump 8-10 GPM through that unit, the DELTA TEE is gonna spread so far apart that the boiler will come apart at it's seams from the stress.

It doesn't really matter how many BTU's the unit or wood boiler can provide, it's the constant temperature they can provide
It absolutely, definitely, most certainly DOES matter! Because if you start pulling out more BTU than they can provide, you will start to COOL the unit, and it will NOT provide the 'constant temperature' that you think it will.

I'm sorry to say that you will be miserably unhappy with your choices if you choose to ignore what I'm telling you.

Good Luck to you!
 
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Old 01-24-09, 07:11 PM
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I can only shake my head when people "think" this way.

Pharic, don't confuse heat with temperature. You can heat your home with 80 degree water IF you have an unending supply AND you can pump enough of it through a large enough fancoil assembly.

I don't think that the electric boiler you are planning on using will come apart at the seams but no way will it heat your house either except in milder weather. What you could do is install two, or possibly three of those boilers (with the necessary pumps and controls) and it would work okay.
 
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Old 01-24-09, 07:53 PM
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furd, where ya been? missed ya man!

I'm willing to bet that if he tried to run that boiler long enough to heat his home, he'd end up with cracks in it... of course, he's not gonna be able to run it that long, cuz it won't heat his home properly.

Here's the thing though... it would probably be FINE for perhaps 80-90% of the heating season. It's that 10-20% of the coldest part of the winter when he's gonna suffer badly. And THAT's when he's gonna wish he hadn't removed that wood boiler!
 
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Old 01-24-09, 09:13 PM
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I'm here every day, I just haven't seen anything I needed to comment on.

As for the electric boiler...I hadn't read the manual when I posted and just assumed (always bad) that it was a steel vessel with electric heaters. I went back and briefly scanned the installation manual and found this:
The Electric Hydronic Block is constructed with a cast iron boiler that conforms to the American Society of Mechanical Engineers (ASME) Boiler & Pressure Vessel Code. The interior design allows just enough water to be present for proper heating element operation - no excess water is stored which would cause undersirable [sic]thermal losses and longer recovery times.
So you may very well be correct that it might crack. I don't know why anyone would build an electric boiler out of cast iron, I just don't. An electric boiler could easily be built with steel pipe fittings and be better (in my opinion) than making the darn thing out of cast iron.

Anyway, I didn't look at the control scheme but I did see that they have a 24kW boiler with four 6kW elements. If they were staged according to temperature that would be a dandy boiler for him to use, assuming the cost of electricity was in his favor over other fuels.
 

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Old 01-24-09, 09:28 PM
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I just used an online calc, 24kw is about 82000 btu.

Now its all up to the flow rate.

Id think the other way to 'bridge' the gap where heat-source-btu is less that the actual current requirements is by storage mass right ?

Getting slightly off topic here.. but I wonder if theres a way to figure that.. put a system in even a little below design temp, and store the extra btu in some kind of rock/water mass etc ?
 
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Old 01-24-09, 09:59 PM
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Heck, with a calculated heat loss (assuming he did the calculation correctly) of 52,000 BTU/hr the 16 kW boiler should be fine. Heat loss calculations always have a bit of a fudge factor built into them. As long as he has the fluid flow to protect the boiler the rest is pretty much negligible. With a minimum of water in the system it will heat pretty quickly.

If the control boards and relays on the 12 and 16 kW boilers are the same (more than likely to reduce the number of specialized parts) he may be able to swap two of the 3 kW elements for 4 kW elements and have a boiler rated at 14 kW or 51,186 BTU/hr.

The idea of stored heat via a large tank is a good one except it would take a huge tank and the tank would need to be very well insulated. Yes, there is a way to calculate it but I gave up that crap when I retired.
 
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Old 01-25-09, 09:05 AM
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Thermal storage calc

Who may be the 'thermal mass' expert here. Here is my crude estimate:

It takes 8.34 btu to raise 1 gal of water 1 degree.

I am going to assume no losses, and lowering the temp gives off the same heat as if raising.

So if we go from 180 degree water to say, 70 degree, this is 180-70=110*8.34=917BTU per gal. A 45 gal storage tank could store about 41000 btu.

If the particular micro zone or zone heat loss was ~20k btu/hr, this tank would carry it for 2 hours. Useful for micro zoning and preventing short cycling, or matching uneven demand to source over a couple hours.

Again ignoring all losses (not realistic but a first order starting point)

By no means an expert here..... this could all be completely wrong.

At the same time Im not sure how thermal storage helps in a case where the total AVAILABLE energy is inadequate for the total CONSUMPTION rate.
 
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Old 01-25-09, 09:48 AM
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Calcs appear correct... but, while it may take appx 1K BTU to raise the temp from 70 to 180, you likely won't be able to extract ALL that heat back out... if for example, you are running fintube BB, there will be little to no useable output from water below about 100 or so... so when figuring the 'storage time' or 'run time' of that scenario, you might need to modify your bottom end a bit to match your emitters...

So, if we look at 180-100= 80 X 8.34 = 667 BTU/GALL
or, appx 30K BTU for the referenced 45 gallon storage tank.

At the same time Im not sure how thermal storage helps in a case where the total AVAILABLE energy is inadequate for the total CONSUMPTION rate.
All it will do is delay the inevitable.

If you take out faster than you put in, at some point yer gonna run out... for the accountants and economists among us, think "Cash Flow" ...

If yer taking out say 50 MBH, and can only put in 40... yer gonna need a 'bailout' sooner than later.
 
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Old 01-31-09, 08:10 PM
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no expert here (as nj can testify) but i heat with an OWB and if that fellow is using 10 bush cords of good hard wood (hickory)he is in fact (no not fact..but generally) using about 270,000,000 btu that is in the wood...now out of that much wood he is only getting about 200 million (the rest is used up to drive the moisture still in the wood) now lower that some more because his OWB is likley running around 60 percent efficient we get down to about 100 to 120 million btu

so, could the electric boiler he is looking at produce that many btu in the normal length of time of his winter season
of course i don't have a clue as too how many btu he would need on -30 day that is for you pros to figure out
 
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Old 01-31-09, 10:47 PM
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Originally Posted by NJ Trooper View Post
if for example, you are running fintube BB, there will be little to no useable output from water below about 100 or so...
Hrmmm, I would tend to disagree. If the BB is sized to heat at design day temps with 180* water, then at under 100 you would be down to 10 or 15% of the heat but you'd still have heat that would be ideal during milder days. If the BB is sized for 130* which would be ideal in a tight house with a modcon, 100* would still give you 30% of full output, whi8ch could easily get you down to the freezing mark outside for example depending on the system.
 
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Old 02-01-09, 06:25 AM
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I came into this late but The thought of running 8-10 gpm through a 40k boiler is not going to work. That kinda flow is an 8 - 10f delta T.
Delta T Formula
Delta T = Q/(490xF) Where Q= BTU's and F = Flow
Delta T = 40,000/(490x10gpm)
490 x 10 = 4900
40,000/4900 = 8f delta T
As a rule divide btu/10,000 to get flow so a 40k boiler, working on a standard 20 delta T is 4 gpm and a 40 delta T is 2 gpm. The fan coil will run cold.
He will not heat the home when it gets cold and lose comfort long before that.
Buy a second boiler and stage them.
Storage is a thought but you have to have enough water at a high enough temperature to satisfy the thermostat. The new problem then is enough time to recover before the next thermostat call. The bigger the tank the longer the recovery and the small the tank less usable hot water. Catch 22!!
 
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Old 02-01-09, 08:49 AM
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Originally Posted by Who View Post
Hrmmm, I would tend to disagree. If the BB is sized to heat at design day temps with 180* water, then at under 100 you would be down to 10 or 15% of the heat but you'd still have heat that would be ideal during milder days.
Sure, yer gonna get SOME heat out of it, you can't NOT as long as the space is cooler than the emitter. If had said "little" instead of "little to no", you would agree, correct? So use what numbers you are comfortable with! It was intended as an example that you might not get ALL the heat you put into a buffer tank as useable output.

We did kinda 'drift' the topic a bit, and I doubt the OP is gonna be back because he had his mind set that he was correct, and we were all wrong. He believes that he's gonna get more BTU out of his system than he's putting in... over unity energy! Don't we wish that were possible.

rbeck, thanks for those formulas! I want to point one thing out about them. We all hate 'rules of thumb', right? We often (usually) use 500 instead of 490 as a 'close enough rule of thumb' guesstimate because it makes the math very easy to do in your head. This is where the 10K BTU per GPM comes from... the 'real' number is 9800 BTU/GPM @ 20° DT ... Just in case anyone is confused by the numbers.

billie_boy said: "no expert here (as nj can testify)"
Nope... I won't testify! Can't make me do it!
 
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Old 02-02-09, 08:48 AM
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As usual you are right Trooper. I do use the 490 when I am ciphering (as Jethro Bodine would say) with one of those adding boxes with buttons.
 
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