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Old 01-06-2010, 08:50 PM   #1
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Indoor Boiler Question


I picked up a 114,000 btu oil boiler tonight.

It did not come with an expansion tank, and I know it needs one.

I've been reading online about sizing and other requirements of a boiler system and decided to start a thread here about it.

The plan is to install a 19" x 20" heat exchanger in my return plenum and create a single loop to the boiler.

The boiler holds 13 gallons and has a circulator pump (i have to confirm the GPM on it). The boiler has a 1 1/2" in/out pipe size from the old setup it has.

Any boiler expert here willing to chime in with any wisdom?

Note: The plan is to use the boiler as the primary heat source when the temperature is below ~20 degrees, as my 5T heat pump cant keep up and the 20kw electric resistance heat kicks in. My thermostat has an outdoor probe and can be configured to select whichever heat source I choose.

This past month, my electric bill is around $400, and we've burned about 1 cord of wood too.

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Old 01-06-2010, 09:00 PM   #2
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The air handlers blower motor is not designed to take heat.
And you would have to shut off teh heat pump when the boiler is heating the house.
So you wouldn't be saving as much as you could if the heat pump was running with the boiler as aux heat.

How about putting a hydro coil in each supply trunk line. Then you can run both the heat pump and boiler at th same time.

Have a fair amount of systems out with a hydro coil as the aux heat. people like the savings of both running at the same time.

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Old 01-06-2010, 09:16 PM   #3
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I could purchase two heat exchangers and put them in the supply side, I'd have to redesign my plumbing plan as I was wanting to use only one circulation pump and would only have to plumb about 5' of line.

A couple of the heat exchangers I looked at, didnt have a huge drop of water temp at 1500 CFM, so I guess I could just loop them together....making the hot water go to the side that has the most vents first, and then catch the second side second.

Yes, I was planning on shutting the HP down whenever the boiler was running, as it would be pointless to use the hotter return air to heat up the A coil from the heat pump...and transfer "heat" back outside.

Hmmh, I am not seeing heat exchangers in the 10" x 24" size....

So if I did put it in the return side, the hot air would shorten the life of the blower motor...but by how much? I can get a replacement motor at wholesale cost...whenever it goes bad.
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Old 01-06-2010, 09:33 PM   #4
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Depends on the current motors temp rating. Some are only rated for 104F(40C) ambient temp.
Might want to keep 1 or 2 instock.

Doesn't have to be piped in series. Can be piped parallel and still only use one circ.
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Old 01-06-2010, 10:09 PM   #5
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Thank you, I'll check its rating and see what the cost would be for two heat exchangers.

I still need to do the pressure checks since I put the turning vanes in...maybe this weekend.

I got such a good deal on this boiler, I couldnt pass it up.
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Old 01-07-2010, 04:37 AM   #6
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Check into the cost of the cost of the tank, and oil in your area.
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Old 01-07-2010, 06:51 PM   #7
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Quote:
Originally Posted by beenthere View Post
Check into the cost of the cost of the tank, and oil in your area.
The pressure tank is around $30.
Oil is around $2.05/gallon.

I already have an underground fuel oil tank and supply lines from it to the basement that went to the old fuel oil furnace I took out.

I still have the chimney too.

If my math is correct, the boiler will be cheaper.

I have 20kw backup heat.
I pay .105 per kilowatt hour.
20kw of electric heat generates about 60k btu/hr.
1 gallon of diesel generates 140k btu/hr. If my boiler is 80% efficient, I'll get around 112k btu/hr.

So, for the same amount of heat, it will be about 50% cheaper with the boiler.
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Old 01-07-2010, 06:54 PM   #8
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Not bad.

Got it installed yet?

Did you get those static pressure checks done yet?
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Old 01-07-2010, 07:05 PM   #9
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Wow, here in Southern Maine the cash price is currently 2.549 and it goes up from there, way up as you head north.

Has your underground oil tank been tested to make sure it is in good shape? Around here undergrounds are supposed to be registered etc. with the state lots of leakage problems across the state over the years.
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Old 01-07-2010, 07:06 PM   #10
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Quote:
Originally Posted by beenthere View Post
Not bad.

Got it installed yet?

Did you get those static pressure checks done yet?
I am trying, working late doesnt help me get things done.

It is supposed to get down in the single digits tomorrow night....unfortunately, it wont be installed by then.

BTW, a buddy of mine works for AOSmith. He checked the motor of my air/handler, said it is rated at 131 degrees, which is at constant duty rating. Its LRC is rated much higher.

He suggested I run the boiler water temp around 150...which depending on the heat exchanger I buy, might result in an ambient air temperature around 130.
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Old 01-07-2010, 07:13 PM   #11
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then you won't be able to use the heat pump when the boiler is running. And will 150 water be enough to heat your house.

Depends on air flow, as far as what temp the air will be from the hydro coils.

Much cheaper heating if the heat pump and boiler are both running.
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Old 01-07-2010, 07:37 PM   #12
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Quote:
Originally Posted by meboatermike View Post
Wow, here in Southern Maine the cash price is currently 2.549 and it goes up from there, way up as you head north.

Has your underground oil tank been tested to make sure it is in good shape? Around here undergrounds are supposed to be registered etc. with the state lots of leakage problems across the state over the years.
Road DIesel is around $2.75/gallon here. Offroad is usually 60-70 less a gallon.

I havent checked it sine I moved in. I was going to tear it out this summer, but I may not not. I may still use the backhoe to dig near it and inspect and see if it has any rusting or leakage.
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Old 01-07-2010, 07:40 PM   #13
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Quote:
Originally Posted by beenthere View Post
then you won't be able to use the heat pump when the boiler is running. And will 150 water be enough to heat your house.

Depends on air flow, as far as what temp the air will be from the hydro coils.

Much cheaper heating if the heat pump and boiler are both running.
Yes, I understand.

The hottest I have ever seen my A coil get, is around 118. So, if I my boiler heat exchanger is running at 150+, it will be hotter than my A coil ever has been.

My plan was to only run the boiler at a specific outdoor temperature that is the tipping point of lack of efficiency of the HP. My HP and A/H pull around 20 amps when running....which is about 63 cents an hour.
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Old 01-07-2010, 07:54 PM   #14
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If your heat pump and air handler combined are drawing 20 amps.
Then at 240 volts. that 4800 watts. At a COP of 2.2(15 to 19 outdoor temp), that gives you 112,000 BTUs for $1.957

With oil at $2.05 a gallon, and 80% efficiency. Plus 1KW for the air handlers blower. You get 112,000 BTUs for $2.155

While in the example, the savings of the heat pump is small. Your strips are coming on before 20 outdoor temp. So the heat pumps COP will be higher. And will give you even more heat for even less money.

The hydro coils in the return will cost you more in fuel every year. then what little work, and cost the 2 coils in the supply will be.

Save a dollar now, spend 20 later.
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Old 01-07-2010, 10:38 PM   #15
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but can I find a heat exchanger to fit in a 10x24 trunk?

So far, not without having a custom one built.

Assuming oil and electric stayed at the current rates (we know they wont), based on those numbers, it'll take 2500 hrs of run time to make up the cost of one heat exchanger, which is I think about 6 years.

I like your idea, and if I can make it work, I'll go that route. I am going to stop at the local supply house tomorrow and see what heat exchangers they have available locally.

Online, there are a number of vendors, but none that sell any that will fit in a 10x24 trunk. My drop down box is 20x25, so I have plenty of room there. My crossover box under the unit, is 16x30...so ample room there too.

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