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Tankless On Demand For Heat Source

 
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Old 06-21-2015, 02:20 PM   #1
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Tankless On Demand For Heat Source


Here goes what i am doing. In my other home is this old hot air furnace that at one time heated the home. Home is 3600sq ft with old windows and is constructed of poured concrete. The last time the furnace ran on oil it was consuming 200-250 gallons a week so it was shut down and woodstoves were used for many yrs. 15yrs ago a Woodmaster outdoor wood boiler was installed and a 250k BTU heat exchanger installed in the plenum of the old hot air furnace. This things has worked awesome for the last 15yrs now. Now, my mother lives in this home with a lifetime lease as well as her obese disabled husband. She just had a heart attack and a quadruple by-pass so this boiler is out of the question for use. Yup, o could feed it but i will not be traveling 10 miles one way 2 times a day to feed this thing. So, being my home and my responsibility i am looking to replace the boiler. My thoughts are a tankless on demand propane unit for a pool/large spa rated for 250k BTU with the same 1" ports to feed the heat exchanger in the hot air furnace. Then have a 1k gallon propane tank set in place to feed the unit.

I havent done any kind of heat loss figures or anything like that but i can say the wood boiler has worked awesome for yrs as well as the heat exchanger set up in the plenum of the hot air furnace. I plan to drain the wood boiler but still keep it plumbed in if need be. I am also purchasing a large storage tank of water to sit idle by so if i need the wood boiler in an emergency then i can fill the wood boiler without pulling off the well and have it ready to fire in a matter of 30 minutes or so.

I have seen smaller homes heated off tankless on demand propane units and they work great. Also i am on a bit of a tight budget so replacing the whole system is out of the question. I am trying to work with what i already have. As of now the research i have done this will set me back around $7k doing most of the work myself. What are the thoughts from others on here and has anyone experimented with anything similar?
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Old 06-21-2015, 02:49 PM   #2
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Re: Tankless On Demand For Heat Source


Heat pump minisplit units. I would need to see a load calculation to give a figure on what it would cost, but the propane is going to hurt in the long run!

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Old 06-22-2015, 12:56 AM   #3
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Re: Tankless On Demand For Heat Source


Not sure if this is the way i want to go. I am not worried of the cooling, only the heating. I prefer to stick with propane as the stand by generator is only rated for 8k watts so it will run any required ignition and circulators but it will not operate anything with a big amp load. My thoughts were with the on demand unit being fairly efficient is it would only run when the fan operates and the returning water would already be close to operating temp so it wouldnt be heating the water very much. Maybe a difference of 20-30degrees versus the normal difference of 70-90degress depending on the max temp i set it for and the water temp of the inlet side. I know most of these units go as high as 160 or 170 deg and i should be able to get away with 140deg being a forced hot air unit. Right now it operates at 180-190deg so dropping down the temp would only mean longer fan operation to satisfy the thermostat.
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Old 06-22-2015, 03:29 AM   #4
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Re: Tankless On Demand For Heat Source


As a personal preference, propane might be the right solution for you. I just know it is an extremely expensive fuel to burn. If you are talking about a considerable upgrade, and doing it on a budget, why not expend then money in a product that will generate quicker return, not to mention increased comfort and control in both heating and cooling seasons. Get it sized properly and operating in peak performance, as well as you being able to do almost the entire install minus the mounting and charging of the units as an electrical contractor.

I have seen first hand the benefits of switching fuel sources away from propane. Do yourself a favor for the long run and compare the numbers.
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Old 06-22-2015, 09:01 PM   #5
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Re: Tankless On Demand For Heat Source


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Originally Posted by woodchuck2 View Post
.... My thoughts were with the on demand unit being fairly efficient is it would only run when the fan operates and the returning water would already be close to operating temp so it wouldnt be heating the water very much......
Edited: You will get the highest system thermal efficiency by running the water as hot as possible.
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Old 06-22-2015, 09:11 PM   #6
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Re: Tankless On Demand For Heat Source


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Originally Posted by woodchuck2 View Post
My thoughts are a tankless on demand propane unit for a pool/large spa rated for 250k BTU
Pull up an online manual for your candidate unit and see if they recommend against this type usage, for Whoknowswhat? reason.

You'll need a manual in any case.
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Old 06-22-2015, 09:28 PM   #7
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Re: Tankless On Demand For Heat Source


Install manual for larger Noritz unit, heating to 180, and suitable for space heating (in the manual). http://support.noritz.com/download.p...allationManual
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Old 06-22-2015, 10:20 PM   #8
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Re: Tankless On Demand For Heat Source


Don't like hot water coil units for one reason: When the coils leak, it takes too long for a replacement to come in, and spendy at that. Might be able to find one on-line, but they don't fit exactly into furnace, so jury rigging is inevitibale.

I would either go with an electric furnace coupled with a heat pump, or a condensing furnace coupled with a heat pump. Propane is spendy, so, one would want to use the electrical or propane as little as possible.

Good thing about electric furnace is that the parts are readily replaceable, and l am assuming that this place is out there..
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Old 06-22-2015, 11:44 PM   #9
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Re: Tankless On Demand For Heat Source


http://www.firstco.com/Products/Comm...l-Closet/CW-HW

If you want to go with fan coil unit, here is one you could couple with your tankless unit. It has a coil for a hp, so that you could have the best of all worlds (cost effective heating for mild cold, cooling, and hydronic).... Not crazy about the water coil thing, but, otherwise easy to work on, in case of emergency.
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Old 06-23-2015, 05:29 PM   #10
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Re: Tankless On Demand For Heat Source


Do the water heaters you're looking at modulate or condense? Because the last thing I'd want to be using for space heating is a unit that has an over 200KBTU input at low 80% efficiency.
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Old 06-23-2015, 06:50 PM   #11
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Re: Tankless On Demand For Heat Source


Last year the oil furnace was used as an oil furnace. It was using 200 to 250 gallons a week? What has been done to the house to lower the heating needs, so that you don't end up using 257 gallons of propane a week.

While Navian and others make tankless to do what you want. It may not prove to be as cheap operating as you want it to be.
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Old 06-23-2015, 07:58 PM   #12
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Re: Tankless On Demand For Heat Source


Tankless water heaters are not a good option as a heat source for a number of reasons;
1: If it is a condensing unit, you wont get anywhere near the efficiency specified because they are designed for cold incoming mains temperature, the low difference in incoming/outgoing water temperature actually decreases their efficiency.

2: The majority of energy consumed for heating is in the shoulder seasons, i.e. when the demand is around half-3/4 the maximum output. Tankless water heaters aren't designed to modulate (with the exception of electric) like boilers, so it would have to cycle a lot to keep the temp, shortening its lifespan and further reducing its efficiency.

3: They aren't tested to any space heating standards, so you really have no idea what you're buying, and depending on how strict your building inspector is, they might not let you install one. Most residential tankless water heaters are tested to CSA P.3 or in the US 10 CFR, Part 430, Subpart B, Appendix E, which is all premised on domestic hot water draws and total energy consumed in the test. This results in and Energy Factor (EF), to which there is absolutely no correlation to the space heating efficiency rating (AFUE).

Some, like the one CarpenterSFO linked to get around this by being slightly outside of the scope of those "residential" standards by bumping their input rating greater than 75,000 BTU into the more commercial rated DHW equipment tested to ANSI Z21.10.3-2004/CSA 4.3-04 ...which is a farce of a test for thermal efficiency only under idealized DHW conditions....but still doesn't tell you anything about how it would operate space heating conditions.


.... I'm ranting, but sufficed to say, you can find ones that say they do it, and they will probably provide heat, but that doesn't mean they're right for the job.

The ductless mini-split or a modulating condensing boiler would be a better option.
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Old 06-24-2015, 04:40 AM   #13
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Re: Tankless On Demand For Heat Source


Navian has a 15 to 1 turn down ratio. So they do modulate. Some are made with dual heat exchangers to they qualify as comfort heating appliances.

While they won't condense supplying 180°F water, they will still be very efficient, specially since when they do shut off, there won't be 20 to 40 gallons of water sitting in them losing heat to a flue.
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Old 06-24-2015, 05:53 PM   #14
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Re: Tankless On Demand For Heat Source


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2: The majority of energy consumed for heating is in the shoulder seasons,
Also the HDDs? And at my latitude? I'll have to check some previous bills.

https://en.wikipedia.org/wiki/Turndown_ratio
In electronics, this would be called dynamic range.

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Old 06-24-2015, 08:40 PM   #15
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Re: Tankless On Demand For Heat Source


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Navian has a 15 to 1 turn down ratio. So they do modulate. Some are made with dual heat exchangers to they qualify as comfort heating appliances.

While they won't condense supplying 180°F water, they will still be very efficient, specially since when they do shut off, there won't be 20 to 40 gallons of water sitting in them losing heat to a flue.
You're talking about a combi boiler, not just an on demand, right?
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Old 06-25-2015, 12:34 AM   #16
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Re: Tankless On Demand For Heat Source


Looks like i have some homework to do. The house hasnt really had any upgrades since my old man bought it in 1970 and now that i own it the attic was insulated, wood boiler with heat exchanger mounted in the duct of the old hot air furnace, new electric service and stand by generator. Still has the old single pane windows and drafty doors. The old man passed away 20yrs ago and convincing my mother to let me do anything with the home is like talking to a wall. I opted for the boiler back 10+ yrs ago because my mother insisted on wood heat and at the time i was getting wood for free and often still do. I will be doing some more research on this, i will definitely be posting what i decide on and the outcome.
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Old 06-25-2015, 05:36 PM   #17
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Re: Tankless On Demand For Heat Source


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You're talking about a combi boiler, not just an on demand, right?
Mostly combis. But also just on demands.

Have a couple of them connected to a hydro coil, plus they provide the DHW also. No problems heating or supply DHW. I do use a mixing/tempering valve on the DHW. Navian set to commercial use. But its used in a home. Works fine.
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Old 06-25-2015, 08:21 PM   #18
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Given the climate in your area, what payback period/investment horizon are you looking at?
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Old 06-25-2015, 10:17 PM   #19
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Re: Tankless On Demand For Heat Source


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Originally Posted by woodchuck2 View Post
Looks like i have some homework to do. The house hasnt really had any upgrades since my old man bought it in 1970 and now that i own it the attic was insulated, wood boiler with heat exchanger mounted in the duct of the old hot air furnace, new electric service and stand by generator. Still has the old single pane windows and drafty doors. The old man passed away 20yrs ago and convincing my mother to let me do anything with the home is like talking to a wall. I opted for the boiler back 10+ yrs ago because my mother insisted on wood heat and at the time i was getting wood for free and often still do. I will be doing some more research on this, i will definitely be posting what i decide on and the outcome.
Is this a two story building? If so, is it possible to set it up for Mom and Mom's husband to live primarily on the first floor? Then, heat primarily the first floor? Somehow, I would think, you gotta figure out how to reduce the heat load.
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Old 06-26-2015, 10:20 AM   #20
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Due to the stack effect the second floor should be warmer unless it's more leaky than the first.

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