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jmthunderbirdturbo HalfDork
12/27/17 4:23 a.m.

hello GRM, I have a question. 

I have a 70's era ranch house with a full basement. It's out in the country a bit. There is no gas service. We are on well and septic. The big power bill comes in December through march when we run the 95,000 kW electric heat. The stove, instant hot water heater, etc is all electric. The goal is to try and lower my winter power bill. I have done a metal roof with a double air gap and blown in insulation. We have all new double pane windows. We also have a VERY large wood burning stove in the front room, ground level. It works great, just get a good fire going, run a pedestal fan to circulate air, and the heat wont come on at all. However, we run into two problems. First, we have to have this fan running all the time to circulate air throughout the house, and while that works for the main rooms in the circle, the bedrooms get kinda chilly, and the basement gets downright cold. Two, whenever we leave for longer then 4 hours, or go to bed, the fire dies out, the temp drops, and the dang heat comes back on, and I can almost hear my wallet thinning...


What I'd like to do is this: find a simple yet effective way to heat water with the wood stove. Store that water in a 50-60 gallon tank to act as a heat battery. Then, run that water through a radiator in my central air ducting. This should solve both issues. The central air is ran to every room, basement included, meaning this would heat the whole house evenly (ish). And, the large insulated drum will keep the house warm longer when the fire goes out. 


I can figure out the make-up water, that's pretty easy. Hell, a toilet float mounted above the 50 gallon drum with a float ball would work. I can also figure out the circulation. CPVC pipe and any off the shelf baseboard water heater circulating pump would do fine.


Here's the part where I need help: First, max water temp in my mind would be about 150, keeping it 50 degrees under CVPC's rating, and reasonably safe should leaks occur. How should I control this? Dump water to my sump at 160*, let the cold well water make up drop the temp? Or have the firebox line 'drain dry' with the pump off, so when the pump shuts down at 160* the water just drains out of the 'firebox' lines, until the tank cools? This would require two pumps, one to run from the box to the tank to heat water, the other to run to the exchanger in the central air to heat the air. 


Second, do they make/sell flue stacks with heat exchangers in them? Or do I just get dimensions and buy a coiled copper heat exchanger and mount it in my flue pipe? Also, being a total GRMer (cheap bastard...) I'm trying to keep this under a winters worth of extra power bill, which is about $500. 





mad_machine MegaDork
12/27/17 8:29 a.m.

I am thinking for a similar price, you could put a pellet stove in and have warmth for as long as the hopper is full. Or you could set an alarm for every 4 hours. My grandfather's first home had a coal furnace that needed to be stoked every 4 to 5 hours, yes, even in the middle of the night

RossD MegaDork
12/27/17 9:16 a.m.

First, put your thermostat for the electric heat system (I'm assuming it centrally ducted system with a fan on a thermostat) on the 'ON' setting rather than 'AUTO'. That will help move some of the heat around the house with out the electric coil coming on.

Second, you are going down a rabbit hole that leads to a boiler system. Just consider if you ever want to sell your house and you have a crazy system that now you either need to bring up to code or rip out. Consider the safety devices that are required by code on a boiler system too.

Third, there are two types of combustion heating equipment. Ones that are condensing and ones that are non-condensing. What that means is that when you cool down the flue gases enough that stuff condenses out of the air stream. This is great for efficiency but very harsh to the heat exchanger or the rest of the burner system. I.E. you will destroy a non-condensing piece of equipment very quickly by pulling too much heat out of the flue gases. Minimum supply water temp of 170°F with 20°F drop is about the minimum for a non-condensing unit.

Since you don't talk about buying wood to burn, I'm guessing you have wood on your land or some that's free. I'm also guessing you don't have a choice of wood types available. Some burn longer than others to get your 4 hour burn time up.

There is no free heat but from the Sun or from the center of the Earth.

nocones UltraDork
12/27/17 9:26 a.m.

Look no further then your car and the toilet section for the answer.  I would run a 20 gallon tank by the fire with a 55 gallon cold resivoir in the basement.  I would probably run 2-3 insulated 55 gallon drums as your hot reservoirs.  Plumb a low PSI pump from your cold reservoir to a toilet tank shut off valve to the 20 gallon upstairs.  Gravity feed from that to a copper heating coil in the fire.  The cold water needs to enter low with the coil running up.  On the discharge of coil run a 160f automotive thermostat that drains to the hot reservoirs.  That way water will drain through the coil to the basement whenever the discharge is 160f.  The 2/ gallon tank will be constantly filled by the toilet valve.  Ideally you would have an emergency overflow line that would drain back to the cold reservoir if the toilet valve malfunctioned.  Run your heating coils to draw from the hot reservoirs.  Ideally they would return to the hot reservoirs until the temperature was low enough that you needed more heat.  Alternatively you could have a coil in the hot reservoirs and just circulate water in a closed system.  Tie it to the furnace so when the furnace is on the pump for that ciculates water.  You would need to drain the hot tanks to the cold reservoir whenever the temperature is to low.  


Good luck sounds interesting.  Make sure all the tanks are open/vented so in a malfunction you can't create pressure.  

frenchyd Dork
12/27/17 9:35 a.m.

In reply to jmthunderbirdturbo :

Look into wind generators.  They work 24/7 and excess electricity is sold to the grid at last years prices.   ( they have to buy it by law) It’s good for the power company because your excess is used locally so the power company doesn’t have the transmission losses they normally have.   You can supplement it with solar for when the sun is shining but the wind isn’t blowing.   

The bigger they are the quicker the payback. Plus there still is the whole tax credit thing. 

Im. Assuming you have done the energy audit.  You’d be amassed at the potential savings available.  Electric costs are going up by multiples of inflation and if you have older appliances lights  etc.  you can often pay for new stuff with the savings alone.  

RevRico UltraDork
12/27/17 9:41 a.m.
mad_machine said:

I am thinking for a similar price, you could put a pellet stove in and have warmth for as long as the hopper is full. Or you could set an alarm for every 4 hours. My grandfather's first home had a coal furnace that needed to be stoked every 4 to 5 hours, yes, even in the middle of the night

I'm going to second this.

In the early 90s, thanks to our combination wood burner and electric furnace, our electric bills were in the $400s every month during the winter. Then we got our pellet stove.

Heats the entire house on less than $600/year. As long as you have somewhere dry to store the pellets your golden.

No more trips to the woods pile through snow or middle of the night relights. One bag of pellets every 24 hours, and it's just that easy. 

Over 22 years, I think I have $450 in repairs/maintenance, which was mostly the control panel at the 14 year mark. 

mad_machine MegaDork
12/27/17 9:43 a.m.
frenchyd said:


 Electric costs are going up by multiples of inflation and if you have older appliances lights  etc.  you can often pay for new stuff with the savings alone.  

years ago my Mother went on Vacation and I pet sat for two weeks. while she was gone I replaced every lamp in the house with LEDs. I dropped her power bill by $25/month with that alone

WonkoTheSane Dork
12/27/17 9:54 a.m.

The other thing to be careful of with your Homebrew boiler setup is that if you take too much heat out of the flue, you can have creosote buildup issues.  Just something to keep an eye on.

wlkelley3 UltraDork
12/27/17 10:08 a.m.

Wait, $500 heats your whole house for the winter? Also see that you are in Florida. I'm in north Alabama and $500 isn't even 2 months of winter bill for my 2,000 sq-ft 80's era ranch without basement. Similar setup, edge of town, city water & electric, septic, electric central heat/air and water heater and no gas available. Have a fireplace with fan but don't use it. The hottest summer months A/C almost equals the winter bills. Looking at next year replacing the unit with a bigger more efficient unit which is supposed to lower heating/cooling costs because it works less hard. Plus have to recharge the A/C with the old style freon(hard to get & expensive $$) every spring. Don't have anything relevant about this anyway.

Ian F
Ian F MegaDork
12/27/17 10:16 a.m.

I agree with Ross - you are basically trying to build a boiler system. Some parts of this will be easier than others.  Some of your idea will be difficult, if not impossible from a physics POV.  The big problem is the size of the system and the inherent efficiencies of a system at various sizes.

So... let's think about each part of the system.  My background is electrical design, but I spend a lot of time doing the electrical engineering for these types of mechanical systems in industrial facilities.

1. Heating the water:  Here, you are likely looking at wrapping copper tubing around the stove flue as close to the stove as possible, wrapping it wit high-temp insulation (rockwool) and then enclosing that with another wrap of metal. The run copper tubing from there to your storage tank. Insulate the tubing. I would not be comfortable with saying the water temp will never stay below the temp rating of any PVC. Maybe try PEX if you feel lucky.

2. Moving water:  You are going to need a boiler circulating pump as those are designed for the temperatures you will be dealing with here (could be damn near close to boiling). You will need to keep that water moving so that it doesn't boil at the flue. You will need to keep air out of the system. Air in the system is bad.  Because of that, you'll need an expansion tank somewhere to self-adjust for the differences in water temperature.

3. Storage. Here is where things get tricky.  I'm trying to think of how you can use a standard hot water tank, but nothing has come to me yet.   Also, to really store enough heat to do what you want, the tank will need to be big. Really big.  And really well insulated.

4. Using the heat:  To be honest, here is the most difficult part of your plan.  What you are planning to do is basically use a water heating coil in your air duct.  Blowing air across the coil will cool the water very quickly. The problem there is the BTU output of the coil vs. how much BTU storage you can have without constant replenishment of a running boiler.  To put it bluntly, I can see a practical storage tank providing heat for about 30 min - maybe an hour - before the BTU capacity of the storage tank has been depleted.  Because of this, I'm not sure the return on your $500 investment will be worth it.

Hot water works better for radiant type heating (cast iron, baseboard or in-floor) than for a forced air heating coil.  If you google them, there are wood fired boilers, but they are typically used for radiant type systems as mentioned. 

Electric forced air is common because it's easy and cheap to install and they need very little maintenance.  However, it can be expensive to run.  Systems less expensive to operate tend to be more expensive to engineer, install and maintain.

Pick your poison.

iceracer UltimaDork
12/27/17 10:51 a.m.

I grew up with wood and mostly coal.  Even my gand father often used coal even though the wood shed was full.  

I have often thought about how cheap wood heat really is.  Not counting the wood there is the expense of preparing the wood from the tree to the stove,  a large storage area,  often feeding the stove new fuel, removing the ashes and someplace to put them. 

My son in law installed a pellet stove in their house and raved about the heat.   They then moved to another house and I noted that they did not take the pellet stove.  When I asked, he said it was not that much cheaper and too much extra work. they now heat with propane.

All said,  the wood stove in our camp would warm up the place quickly even in winter.   But someone had to be the care taker.

OK, I sound negative.  I just like to look at the rest of the story.

93gsxturbo Dork
12/27/17 11:04 a.m.

What about a supplementary wood heater/firebox plumbed into your existing electric forced air?  They are pretty common up in the north.  Its basically a separate wood stove that blows hot air through the fire box.  No water to mess with.  Load it up before you go to bed and it will run at least till midnight, get up and reload or at that point let electric take over, or just pulse the electric in the morning to get the house up to comfortable.


I am probably a bad example, we keep our house at 53 during the day and 60 when we are home.  Sweatpants are cheap.  Spend around $400/year on heating oil in Wisconsin. 

oldeskewltoy UltraDork
12/27/17 11:11 a.m.

I've run "weird" heat for all my home ownership.   When we lived in the Poconos, we used a coal furnace, with an oil burner for those times of year a draft was difficult to sustain.   In Oregon, we had electric, but then bought a pellet stove, and a small circulating fan.  Our heating bill fell by about half.

WonkoTheSane Dork
12/27/17 11:15 a.m.

Another thought: what about upgrading the wood stove?  I bought a few year old Vermont castings defiant non-cat last season.  My old stove was like yours (worked hard to keep the house above 65° and cooled down every 4 to 6 hours).  The new stove only needs to be loaded twice a day (I do mornings and evenings) and it's hard to keep the house less than 70° and I only have to empty the ash pan once every other week or so.

I burn 2.5ish cord per season which equals about $650ish per year, an that's seasoned, split, delivered. 

That said, all wood stoves are a pita for sure. I miss oil fed baseboard from the last house, but this new stove went a long way towards making it better.  And it works without power and theoretically, I can feed it for free from my property, but ain't nobody got time for that!

The0retical SuperDork
12/27/17 12:17 p.m.

Bunch of good stuff in here.

What you're looking for is a Wood Furnace. They operate much like a wood stove but can be plumed into your HVAC as either an add-on heat source or primary where something like the oil burner would be. That will retain the position of your condenser for your AC as well.

Some of the better ones alo have a boiler provision which will allow you to recover some of the extra heat from the furnace to use for heated floors or the heatsink like you're suggesting. They're kind of expensive though.

Englander from Home Depot (The 28-3500 had a water loop)

Caddy (You'll want the hot water loop attachment)

If you just want the loop for an existing stove:

Hilkoil is referenced a lot in regards to that type of setup.

I'm looking at doing basically the same thing or at least plumbing it into my HVAC.

jmthunderbirdturbo HalfDork
12/27/17 1:53 p.m.

Couple of responses. 

I am in Ohio. Florida is on my profile, and in my heart, but I'm stuck in Godforsaken Columbus..

Wood comes from my FILs farm, and is free. He pulls it off the fields when trees fall, we saw it up and split it. He sells what I don't burn to pay for the fuel in the splitter/saw. Its a mix of ash, cherry, and oak.

My air handler is in the basement. I have no desire to haul wood or pellets down stairs. Replacing the current stove is not an option because of reasons. 

I'm not sure everyone is giving credit to how much heat would remain in the house if I were to actually heat the basement and all of the bedrooms. I wouldn't need very much liquid heat storage as was suggested, considering how much longer it would take for the heat to dissipate, since my basement wouldn't be 55 degrees anymore.

This may turn out to be too much of a mess, but I'm still thinking it over.



ProDarwin PowerDork
12/27/17 2:25 p.m.

Where is your air return compared to the stove?  Seems to me the lowest hanging fruit at this point is to set your minimum fan-on time so you are using your central HVAC system to circulate the hot air all around the house vs. the wood stove.  If the air return is anywhere near the stove, that should certainly help other rooms (and the HVAC system) stay up to temp somewhat.

Good luck with this, I am intrigued by it.  I fell like all homes should have a liquid heat transfer system built into them as it makes adapting heat sources to the rest of the house much easier, as well as harvesting waste heat from any number of sources.


jmthunderbirdturbo HalfDork
12/27/17 5:28 p.m.

Each room has its own return, which is nice in the summer, cause you don't get hot spots, and can somewhat control the temps in each room to have colder bedrooms. It sucks in the winter though, as its almost impossible to not have much warmer bedrooms (when heat is on), and much colder common areas/basement.


I've thought about using convection to do this as well, but I can't think of a way to make it 'sexy'. The punch line is a fairly large duct/hood very close to my beautiful hearth and cast iron stove, running mass amounts of air downstairs to my central unit. I could plumb this air via 16 or 24 inch duct into my main return, wrap my incoming filter in a trash bag (stop return air flow from upstairs), and let the flowing air do the job. But it's ugly, will take up a lot of space, and make a lot of noise. 


ill keep thinking.



iceracer UltimaDork
12/27/17 5:41 p.m.

My gas for heat and hot water runs roughly $700 year and I don't have to do anything.

BrokenYugo MegaDork
12/27/17 8:09 p.m.

Given your constraints there is no good solution.

I know from personal experience that putting the stove in the basement and leaving all the doors open works ok, but you still have to get up at 3AM and kick the fire, or let the central heat set at 50 kick in and carry you over to morning. I doubt 55 gallons of hot water will change that.  

Pulling the override on the furnace fan doesn't work that well, even with the big filter service hatch opened and a few feet from the stove. 

I think the only practical long interval between refueling central heat from wood system is one of those big ugly outdoor water heaters. 

Ian F
Ian F MegaDork
12/27/17 9:32 p.m.

I'm not sure everyone is giving credit to how much heat would remain in the house if I were to actually heat the basement and all of the bedrooms. I wouldn't need very much liquid heat storage as was suggested, considering how much longer it would take for the heat to dissipate, since my basement wouldn't be 55 degrees anymore.


Oh I know what you're trying to do with using water for heat storage. You basically need a battery. But instead of a battery storing electricity, you need one that stores thermal heat energy. It's not a new idea. However, when they are used as a thermal source for a heat pump, reservoirs considerably larger than any common water tank is used. The tanks are huge. And buried deep in the ground below the frost line so the water temperature stays fairly constant. While generally kept at lower temperatures, it works because it's a large thermal mass to pull energy from.  A water tank in your basement will only store so much energy and a heating coil will deplete it quickly. Especially a coil that will be somewhat equivalent to a 95 KW electric heater - which is over 300,000 BTU's. That's a lot of heat.  Think about it.  If you want a water tank to store enough energy to keep you somewhat warm overnight, that tank will need to store over a million BTU's worth of energy. That's a big "battery".

ProDarwin PowerDork
12/27/17 9:36 p.m.

Ian, do you mean 9.5KW?  95kw is a bit on the insane side, and I don't think its possible on any residential electric service.


Ian F
Ian F MegaDork
12/27/17 9:55 p.m.

In reply to ProDarwin :

That's what the OP said in the first post... (he actually typed 95,000 KW, but that is even more insane)

But now that you mention it, that does sound really high.  He probably meant he has a 95,000 BTU furnace and misread the nameplate.  That would still be a big feed, but plausible. The oil fired heater for my little 600 sqr ft house is 85,000 BTU's.

frenchyd Dork
12/27/17 9:56 p.m.

In reply to wlkelley3 : look very carefully at your A/C system.  Central air usually is an expensive and inefficient way to cool a house. Unless your air conditioning comes out near the ceiling you are forcing it in the room wrong!  Heat naturally rises.!! Cool  naturally settles.  Blowing A/C out floor registers means you need a lot more of it to stay cool. 

If  it comes out at the ceiling it will settle down over you where you need it most ( your head & upper body not your feet and legs.)

central Air is big expensive and doesn’t get replaced when new designs come out using less power because of that.  

The affordable solution?  Take modern efficient window air conditioning units and put them through the wall at the ceiling.  You can usually get by with several smaller units for a fraction of what a new central unit costs  with no loss of window view or light.  

I bought units that have a remote control and leave it near the light switch.  That way if I switch off the lights because the room isn’t in use I can turn off or on the A/C  as needed.  My worst A/C  Bill is about $80 a month for a 5500 sq ft house.  

I like the house overlycool in the summer and overwarm in the winter.  

Rethinking central air will make that possible.  A few $200 window units is a minor expensive compared to what modernizing a central air conditioning.  

Besides you can leave unused spaces without A/C 

In addition please rethink your fireplace plans.  To retain heat use thermal mass.  Brick stone concrete all have big thermal mass numbers.  If you surround a fire with those a large amount of heat can be retained and slowly yield it to the house.  Follow the basic laws of thermal dynamics  and you can pump heat anyplace you want at no energy expense or cost. 

jmthunderbirdturbo HalfDork
12/28/17 1:51 a.m.

yeah, thats incorrect. its 5 coils, all of them 7.5Kw, or 7500 watts.  total power requirement is 37,500 Watts, or 37.5kW. the BTU *IS* 95,000. 


i have individual returns in each room. 



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