Indirect tank off propane modcom boiler OR Independent Electric Water Heater

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annelisemcl

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Hello all! Please help me make the efficient choice.

Here are the stats:
- Alpine mod con Propane boiler 150,000 95% AFU installed 2011
- Suburban Philadelphia temp zone
- ranch home slab on grade with an internal mechanics room concrete slab containing the boiler, current 80 gal WH, & electrical panel
- run the boiler maybe 5-6 months total during the year as we supplement heat with a soapstone woodstove & I make everyone put on socks & sweaters :)
- Propane cost per gal = $2.49 (underground storage tank)
- electricity cost per kwh = 0.20
- Daily average total water use is 85 gal (family of 4). I do not know how much of this is hot water.

Estimated Costs:
$4118 - 50 gal indirect tank Alpine Alliance SL - cost + install + 2 year install warranty plus US boiler limited lifetime tank warranty

verses
$1600 (internet estimate tank only no supply price + install quote yet), 80 gal AO Smith Conservationist electric .95 energy factor
or
$960 (internet guestimate tank only no supply price + install quote yet), 80gal Bradford White High Efficiency Energy Saver .91 energy factor

Is it accurate to break the kwh vs lp to btu cost in the below manner?

With 1 gal of LP = 91,333 btu & 1kwh of electricity = 3412 btu, would that mean it would take me 26.77 kwh of electricity to equal 1 gal of LP?

At my current prices am I correct in thinking that 1 gal of LP at $2.49 is a lower cost than the equivalent btu's from kwh cost of $5.35?

If this is correct, does this mean that a 50 gal indirect tank on my Burnham Alpine ALP150W-2L02 boiler running those 6 months in the summer still would be less expensive than the operating cost of an 80 gal electric water heater with a .91 or .95 EF for 12 months?

I feel as if I am missing something or not delving deep enough into the algorithm to have a true comparison.

I am getting a bit panicked that I need to make a decision (very slow leak from my 16 yr old BW) and I want to make the most efficient least wasteful choice I can.

Thanks,
Ann Elise
 

Jadnashua

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A mod-con boiler is typically designed to be able to cold start. This means that the boiler would only turn on outside of the heating season when needed to reheat the indirect, and the rest of the time, would just cool off to ambient. You'll have much faster recovery rates with an indirect verses an electric, and might want a bigger electric tank than an indirect. Many of the indirect tanks are almost a one-time purchase, although if they do fail, you only get a new tank, not the labor to replace it (for most, anyway). You'd typically connect the indirect as a priority zone so it would have priority over heating the house, but since your boiler is almost certainly oversized, it may not make much difference. By having it a priority zone, though, it's easier for the logic to only run up hot when the WH needs it, and run at a lower temperature for space heating - depends on the controls and how it is setup. If you had a boiler that was not designed to cold start, and it needed to maintain some minimum temperature all year long, it would be a different story.
 

Dana

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How long do you expect propane to stay at $2.50, and how long do you think electricity will stay $0.20?

My gut (and analysis of drilling starts in shale country) tells me that heating oil and propane won't dwell at this year's prices unless the Iranians get all luvvy and give up their nuclear projects and peace breaks out in Syria & Iraq (at which point I'll have to start sharpening my ice-skates...)

$2.50/gallon propane burned at 90% efficiency (probably won't do better than that serving an indirect) delivers 0.90 x 92,000= ~83000 BTU/gallon. That's 1,000,000/83,000= 12 gallons/MMBTU, at a marginal cost of $2.50 x 12= $30/MMBTU.

At 20 cents/kwh and an EF of 0.91 the electric tank delivers 0.91 x 3412= 31o0 BTU/kwh. That's 1,000,000/3100= 323 kwh /MMBTU which costs 323 x $0.2= ~$65/MMBTU more than twice the marginal price.

A pretty-good stainless heat pump water heater with an EF of 2.2 like the ATI-66 costs a couple grand, and the tank should be good for 20 years (you may have to replace the backup heating elements every 10-15 if you run it in that mode much). That would bring the cost of water heating down to about $65 x (0.91/2.2)= $27/MMBTU, which is cheaper than heating water with a propane mod-con.

The recovery rates for these things are pathetic, but unless you're filling big tubs or taking endless showers on a regular basis you don't need anything like the hot water performance of a 150KBTU/hr mod-con.

In my neighborhood third party ownership solar companies are falling all over themselves trying to get people to sign $0 down 20 year power purchase agreements (PPAs) for 15-16 cent/kwh electricity in exchange for permission to put PV panels on your roof. The deals are getting sweeter every year. Cheap PV will be driving both the wholesale and retail prices of electricity down in the coming decade- it's at a tipping point now, since even UNSUBSIDIZED the lifecycle cost of PV output is well under 20 cents/kwh (and with the federal tax and other subsidies it's getting close to 10 cents/kwh, which is why the solar companies are happy to photon-farm your roof and still undercut the utility price to you.) The installed cost of PV has a learning curve of about 20-22% cost reduction every time the installed base doubles in capacity, and the doubling rate worldwide is roughly every two years, and accelerating. That means every 5 years the levelized cost of that power will be cut in half.

Bottom line, now that even small-scale PV has crossed the retail-price threshold on it's way down, I don't think electricity can keep climbing in price, and may see a significant deflationary trend in less than 10 years.

If you have the right roof lines and shading factors you can probably get electricity for well under 20 cents, since PA is one of the states that expressly allows third party ownership of rooftop PV. A cheap electric tank and a PPA from a solar company would be low cash outlay and potentially high return overall, but if you can finance the PV at 3-4% and buy the solar yourself and take the tax credits you can do even better.

I think there is a HIGH likelihood that propane will be bouncing on $4/gallon or higher within 10 years.
 

spete112

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There is one other option that would give you the best of both worlds. Keep your electric water heater and make your own heat exchanger using 1.5 copper pipe with 3/4 inch in the middle for your domestic water . I just did this a month ago and it works great. That way if propane gets too expensive you simply turn the power back on to your electric heater. Most of the parts and pumps that you would have to buy you would need for the side arm heater as well as for your indirect heater. You can google this or if you would like more info just ask and I can get you some more info. You can also do the same thing with flat plate heat exchangers which work lots better then a tub exchangers except two things the price and the plate exchangers need cleaning from time to time so they don't plug up.
 
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spete112,
some questions:
Have U priced 1.5" copper pipe lately vs an Ebay 120,000 btu flat plate HX?
How long is your DIY HX?
How many continous GPM at what temperature production?
How do U control the the hot DHW flow through the primary side? Pump continuous or sensor based pump control?

3gpm x 80*F temp rise (120 - 40) x 500 (a constant for btu) = 120,000 btus
 
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spete112

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spete112,
some questions:
Have U priced 1.5" copper pipe lately vs an Ebay 120,000 btu flat plate HX?
How long is your DIY HX?
How many continous GPM at what temperature production?
How do U control the the hot DHW flow through the primary side? Pump continuous or sensor based pump control?

3gpm x 80*F temp rise (120 - 40) x 500 (a constant for btu) = 120,000 btus

I called a manufacture of a flat plate exchanger and was quoted a exchanger that costed over $600.00 I don't remember the btu of that. Plus you have to factor in the y fittings so you can clean out the exchanger and they were also very expensive. I have a total of 20 feet of exchanger pipe. The first set of exchanger tubs were made for me by my uncle. He actually used 2 inch dwv copper with two half inch copper running through the middle for the domestic side. This was all brazed together so there was no fitings used making this. I tried it and it worked however my return was still high compared to the domestic return so I decided to make one more set of exchanger pipes my self. For that I bought a 10 foot piece of 1 1/2 coper and ran the 3/4 through it. I bought four 3/4x1.5x3/4 tees for this. I just files of the stop so I could slip the 3/4 through the tee. I also wraped #10 wire around the 3/4 pipe. After this was installed I can get my boiler return to 3 degrees warmer them my Dhw return. As for as gph I am using a Grundfos domestic recirculating pump that is pumping no more then 5 gpm and my boiler is pumping at 15 gph. I am using glycol in my system to that it reducing the exchange rate a little. You may not need to use that so hopefully your heat exchanger will be a little more efecient. As far as controlling it I bought a 24 volt control that uses a capolary sensor that I slip between the insulation and the tank. When it calls for heat it simply open up a set of contacts so it really acts like any other thermostat. That intern starts my boiler and both my recirculating pumps for the boiler and dhw. Hopefully this helps let me know if I was not clear I will see if I can't post some pictures of the one that I made.
 

spete112

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I called a manufacture of a flat plate exchanger and was quoted a exchanger that costed over $600.00 I don't remember the btu of that. Plus you have to factor in the y fittings so you can clean out the exchanger and they were also very expensive. I have a total of 20 feet of exchanger pipe. The first set of exchanger tubs were made for me by my uncle. He actually used 2 inch dwv copper with two half inch copper running through the middle for the domestic side. This was all brazed together so there was no fitings used making this. I tried it and it worked however my return was still high compared to the domestic return so I decided to make one more set of exchanger pipes my self. For that I bought a 10 foot piece of 1 1/2 coper and ran the 3/4 through it. I bought four 3/4x1.5x3/4 tees for this. I just files of the stop so I could slip the 3/4 through the tee. I also wraped #10 wire around the 3/4 pipe. After this was installed I can get my boiler return to 3 degrees warmer them my Dhw return. As for as gph I am using a Grundfos domestic recirculating pump that is pumping no more then 5 gpm and my boiler is pumping at 15 gph. I am using glycol in my system to that it reducing the exchange rate a little. You may not need to use that so hopefully your heat exchanger will be a little more efecient. As far as controlling it I bought a 24 volt control that uses a capolary sensor that I slip between the insulation and the tank. When it calls for heat it simply open up a set of contacts so it really acts like any other thermostat. That intern starts my boiler and both my recirculating pumps for the boiler and dhw. Hopefully this helps let me know if I was not clear I will see if I can't post some pictures of the one that I made.
 

spete112

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Good job. thanks.

Your DIY HX heats a DHW tank from your boiler.

For some reason I was thinking about an on-demand deal like they do quite a bit in Europe,
where boiler or heat pump heats a tank directly & when any DHW spigot is opened, a pump moves tank hot water through HX which heats only the water being used.

On cleaning a flat plate HX:
Only the domestic side will build up, boiler side stays clean.
Put brass tees close to HX & then a ball valve on each domestic side leg.
Close ball valves & remove plugs in side of tees. Flush

Your #10 wire is to create turbulance & increase heat exchange?
 
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