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Indoor Boiler Question

14K views 134 replies 7 participants last post by  beenthere 
#1 · (Edited)
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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#2 ·
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.
 
#3 · (Edited)
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.
 
#4 ·
Depends on the current motors temp rating. Some are only rated for 104°F(40°C) 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.
 
#5 ·
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.
 
#7 · (Edited)
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.
 
#10 ·
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.
 
#9 ·
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 :eek: lots of leakage problems across the state over the years.
 
#12 ·
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.
 
#11 ·
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.
 
#13 ·
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.
 
#14 ·
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.
 
#15 · (Edited)
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.
 
#18 · (Edited)
Because you indicated it should be after the HP A coil in the air handler....

The A coil is on the bottom of the unit and the fan above it, and above the fan, the 20kw electric heat.

As you can see in the above photo, my supply trunk which goes left and right, is 10" x 24"

THis is an upflow setup. Air comes in from the bottom and exits out the top.
 
#19 ·
You get the coil made to fit your duct work.
The coil can be bigger then the duct, was my point.
because the ends of the coil would restrict air flow if inside the duct.
 
#22 ·
Any idea how close the heat exchanger coil could be to my 20kw e-heat?

http://www.houseneeds.com/shop/Heat...exchangers/airhandlers/heatingcoilerstech.asp

The E-heat will basically be "disconnected" because the boiler is taking its place. I might even take it out and put it back in the box to help with my static pressure....but if I didnt, I'd wire it to W3 on my t-stat and configure the t-stat to never use it unless I manually energize it.
 
#23 ·
20 KW of strip is 68,260 BTUs.

As you see, hydro coils are very restrictive to air flow.

Take the 18X18C3.5 coil. At 1500 CFM, it has a .24”wc PD.
If you were to use that coil with only 1000CFM of air flow going through it. Its PD would only be .105"wc.

Hydro coils need to be selected for both the BTU you need, and the PD your system can overcome.

And often. A large BTU coil is preferable to use at a lower PD. Then a smaller higher rated PD one rated for the lower BTU you need.
 
#24 · (Edited)
You lost me there.

If I was willing to live with putting a larger hydro coil in front of the A Coil in the 20x25 drop down box, that would be better to my airflow and WC then a smaller 12x12 on the supply side, right?

Also, the air output on the AH is about a 12x12 opening....because of the way it was made with the heat coil slot...you can see one off the panel that limit the width to 12" instead of the full width of the AH (23")

I could put a 12x20 coil in the blue area of the first photo pretty easily.
 

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#25 ·
I was refering to using 2 coils on the supply side. One for each supply trunk.

For a single coil. You would need the 22X25X3.5, rated at 2700 CFM and a PD of .32"wc.
Because when the air flow is only 2000CFM through it. Its PD would only be .18"wc.

If your air flow is less then 2000CFM through it. Its PD will be even lower.
But. As the air flow drops, the air temp coing from it will increase. And your back to harming your blower motor.

Next. What about refrigerant migration. With the coil heating up the air that is being blown over the coil. You will cause liquid refrigerant migration. And could cause oil problems with your heat pump.

Putting a hydro coil before the indoor coil is a real bad idea. Could cost you more in repairs then you save on your heating bill.

Put the larger coil on top of your air handler. And slant the transition as much as you can.

Or use 2 coils as I suggested earlier. And you'll get better air flow.

Before the coil can be put in. You still need to get your Static pressure down lower. Or you'll end up not being able to use the heat pump for heating or cooling no matter what the outdoor temp is.
 
#26 · (Edited)
Would frigerant migration be less of a concern if my HP has a crankcase heater?

The manufacture PDF says :
Crankcase heater (where indicated)


Why dont I hear concern with oil migration on a standard A/C system that is after say a fuel oil or LP furnace? Does it have anything to do with the reversing valve?
 
#28 ·
Because an A/C doesn't run in the winter.
It gets restarted in the spring on a warm day. A heat pump will be required to run again on a cold day.
Crack case heater will help keep oil and refrigerant from being mixed in the crank case. But not in the outdoor coil.

Whats the PD of the 12X24 coil you found.

2 large coils one in/at each supply trunk will have less then 1 big coil with all the air going through it.

The 18X18's i posted about. Would only add.105" PD.

Will the 12X24 you found add less?
 
#33 ·
Whats the PD of the 12X24 coil you found.

Will the 12X24 you found add less?
They didnt list the info online, I'll call them tomorrow and find out what the specs on it are.

He doesn't have a lot of room between his air handler and the trunk line.
Already has an air flow problem.

The hydro coil will compound his air flow problem.
Correct, I don't have the room and I already upgraded all my trunks from 8x24 to 10x24. If I was to make the trunk any bigger, I'd have to redo my return dropdown box and the box the AH sits on....I am not about to spend the money to do that again.
 
#30 ·
He doesn't have a lot of room between his air handler and the trunk line.
Already has an air flow problem.

The hydro coil will compound his air flow problem.
 
#31 ·
I am not much of a duct guy so I may be wrong but I thought it was best to pull through a coil. I thought when you pushed a coil most of the air goes through the center portion of the coil and when you pulled through you pull air through the entire surface. Is that not why air handlers started pulling through the coil as opposed to pushing the air through as they used to many years ago.
 
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