Wood Burner For Hydronic Heat

DeerWood

Hello All
I'm looking for some advice on using an add on wood furnace to aid in a hydronic heating system. I realize this may be a bad idea. My home had pex ran under the basement slab for hydronic heat but was never hooked up. Last year my neighbor (retired plumber) offered to help me hook it up. He said to save money and keep things simple I could use an electric water heater to heat the water. I like to save money so we went ahead with it without much research. It worked but my electric bills jumped up by over $300/month.
Is it possible and worth it to try and use my Daka wood furnace to aid in heating the water? Daka sells what they call a hot tube domestic water pre-heater that gets mounted inside the furnace. Would I just be adding more inefficiency to my system? From looking at the instructions it looks like I could just circulate the water through the water heater.
Thanks!

Zman

I think that would be a better idea than the electric water heater. You will want to give some thought to how to control the water temp. The last thing you want to do is send water that is too hot to a radiant slab.
The Taco I-series valve with outdoor reset is probably the easiest way to manage the temp.

Jamie Hall

Leaving out the problem that an electric hot water heater is quite possibly the worst and most expensive possible way to heat a home...(a properly sized mod/con LP boiler, built for heating a structure, not a shower, would have been only slightly more expensive to install -- and far cheaper to run).

Yes, it will work. It is not quite as simple as it sounds, however. The difficulty lies in controlling the wood heat; there's either too much of it -- or not enough. Your best be will be to circulate hot water through the wood furnace to a large buffer tank. This tank must be big enough to absorb the total output of the wood burner for the longest time it can run on a full load of wood without overheating, and be equipped with a temperature and pressure safety valve. This is a life safety issue: should the wood furnace get overenthusiastic while unattended, about the last thing you want is to heat your water to boiling.

Then you can take water from the buffer tank -- with another pump -- through a mixing valve as @Zman suggests, to supply the radiant floor. This circulation will be more or less continuous (it can be continuous, if you use outdoor reset as he suggested() and will give you nice even heat.

Larry Weingarten

Hi @DeerWood , If the present electric heater is big enough, you might be able to disconnect the lower element and power only the upper element for backup, using the wood burner as the main heating source for the tank. Perhaps a second tank can be added in series to get more storage ability. Still, safely controlling this is important as others have brought up.

Yours, Larry

GroundUp

There is a lot of misinformation out there about electric heating. A mod/con LP boiler assembly installed properly would have easily cost 3x what your electric water heater setup cost and wouldn't be likely to cost much, if any more in utility bills, often meaning a decade or better just to break even. There are 3412 BTU in one kWh of power. If your power costs $.10/kWh, $300 bought you 10,236,000 BTU. Those same 10,236,000 BTU would have required roughly 125 gallons of propane at 90% efficiency. If propane is $2.00/gal, that's $250. Obviously those prices are hypothetical depending on your location (that's very close to what they are here in my neck of the woods). Then there is maintenance of the mod/con system which is another cost that electric does not require.

Sorry- sidetracked. Anyway, do you have any idea what the insulation is like under and around the slab? What was the slab temp when you started and what water temp were you pumping into it? If the insulation is nonexistent like so many other basements, you're simply heating the earth below all winter and no heat source is going to do a good job efficiently. If it was simply super cold and you had to bring it up to 90 degree surface temp to feed through carpet or something, that again raises a whole lot of expense. More information about the system is required to figure out what's going on before you write off the electric heating as a whole.

As for the add-on DHW coil, sure it's an option. It will take some fancy piping and a bypass, ideally a buffer tank of some sort (perhaps your water heater tank), but it's possible to do safely. We should still get to the bottom of the heat load of the slab before we get too carried away though.

DeerWood

Thank you all for taking the time to comment! I know I left a lot out but I didn't want to get too long winded.
@GroundUp , My neighbor that helped me is also the person who laid out the pex and insulated it. I don't know what the insulation was other than he said it's fully insulated and all "top of the line". My manifold and pex is Rehau, which probably doesn't make a difference. I'll have to check for sure but on whether the heater is a 50 or 60 gallon. Also the radiant heat is covering about 1,000 Sqft of the slab.
I'm trying to remember a lot of this from last heating season but another thing I didn't mention is that the heater was having a hard time keeping up even when the slab and room were at the desired temp. I wasn't checking the slab temp but I know that the water going out would come back about 20 degrees cooler. This was what my neighbor had said it would be ahead of time. My first thought when I realized the heater couldn't keep up was that I needed to use a timer to stop the circulation giving the heater time to catch up. While this might help, I tried manually shutting off the circulation through the day and it helped the heater to keep up but I seemed to use the same amount of electricity. I do have a relay set up so that it's not circulating when heat is not needed. I also tried adjusting the water heater temp between 85 - 100 to see if I got better results. After 3 months of really high bills I gave up for the year. I wish I would have checked with this group at that time!.
I'm going to take a look at the set up tonight to see if I'm forgetting to leave anything else out.
Thanks again!

Jamie Hall

A key point in your comment, @DeerWood -- you note that the return temp was about 20 degrees lower than the feed temp. That tells me that you are putting about as much heat into the floor as it can absorb at that floor temperature. That it won't keep up to your heat requirements, then, isn't the water heater's fault, and perhaps worse raising the water heater temperature -- within limits -- won't help, as you can't run the floor too hot (80 is about as hot overall as a floor can be run, so inlet temps in the 110 degree range are about as high as you can go). Nor will shutting the flow off the "let the water heater catch up".

Nor, unhappily, will putting more heat -- boiler, wood furnace, what have you -- into the system help much: the floor is your limiting factor.

hot_rod

I would not count on that DHW heat exchanger giving you much output. Looks like something a handy person could build. Steel pipe would be fine if it is not domestic water.

Two things to be aware of, IF you pull the wood furnace temperature too low you may get creasote formation.
And too cold of a slab return will cause condensation on that HX coil inside.

Sooo, to be safe you would want to monitor the flue gas temperature, shed some load if it starts running low.
And a mixing valve on the radiant return to assure the temperature hitting the HX is not too cold.

What is the BTU capacity of the furnace? It may not have enough capacity to add much to the radiant??

GroundUp

In light of this new information, it's becoming apparent that your slab (and/or the entire basement) is not very well insulated. "Top of the line" only means expensive to some people, and despite decades of proof otherwise, still many "professionals" think bubble wrap is top of the line when it's little more than a vapor barrier. Turning off your pump to let the WH catch up does nothing beneficial whatsoever. The WH is still running wide open and making X amount of heat, using the same amount of power- so the only thing you're doing by shutting it off is losing ground with the ambient temperature in the basement. The odds are, that your WH only makes 4.5kw or 15,354 BTU at any given time when it's running. If it runs 24/7 for 30 days, you're at 11,054,880 BTU. A 1000 sq ft basement should have a heat load far less than that even in the coldest locations, assuming the upper level is already heated by another source. When the basement was unheated, what was the average temp down there in the winter would you estimate?

psb75

Try and "pull" a little more information from your neighbor. But first, what size is the pex in the slab--that should be obvious to you from the manifold. How many ports on the supply manifold? If the pipe is 1/2" pex and there are 4 ports, then each loop had better be 250' long. FIND OUT 'how much' piping is in the slab--total length? How he divided the loops? Ask if he insulated the EDGE of the slab? Does the snow melt very obviously around the bldg.? Find out WHAT MATERIAL he used for insulating the slab--beyond "top quality." Top quality "WHAT?"
You need to determine if the radiant system is even worth using. Things seem suspicious.

DeerWood

DeerWood

I actually did call my neighbor this morning.  It's 1/2" pex and laid every 12" with insultarp for insulation. I don't know if there was insulation on the sides.  I double checked with him that I was right about the 20 degree difference on the return temp and he said yes.  He says that's what it is supposed to be.  I'm a surveyor so who am I to question that especially when he was nice enough to help me .  He seems to think the heater should keep up and work good.  Or might possibly need a slightly bigger one.  My thoughts are it's something else like many of you are leaning towards.
As far as seeing snow melted around the building, I can't say that I noticed it.  It's an exposed basement and the pex is only ran on the finished side which is 3/4 of the total size.
I think the basement was around 60 degrees during the winter without the heat.  I run my wood furnace on the utility side which helps keep things warm.

Thanks!

psb75

I noticed that your manifold has three pex loops. If your neighbor spaced your pipe at 12" that would mean he used 1000' of pipe-- total. If divided equally, that's 333.333 feet per loop. Those loops are too long, if so. There should have been 4 loops.
A delta-t of 20 degrees is also not right. It should be 10 degrees for a radiant floor.
I believe the system is compromised. Certainly indicated by the picture. And as you are finding out.

GroundUp

My suspicions have been confirmed. Insultarp is garbage, and little more than a vapor barrier. They claim an R value of 5.9 but the actual R value is around 1, so it may as well not even be there. Assuming the tubing is at the bottom of the slab, there is more R value in the concrete than the Insultarp has so more than 50% of your heat is being sent into the earth below. Regardless of heat source, the radiant is always going to be an energy hog due to lack of insulation.

Zman

The installation is not great, but it should be workable. You will want to redo a few things, the circulator should be installed with the shaft positioned horizontally and there does not appear to be an air eliminator.

If this was mine, I would pipe it to the wood furnace and use the electricity as a backup. An outdoor reset mixing valve will manage your water temp and improve efficiency and comfort.

DeerWood

Sorry for the gap in time. I had some other projects going on but the cold temps will be coming soon and I'd really like to try to figure this out. A quick recap on this is everyone seems to agree that the slab is poorly insulated. My electric water heater can't keep up and is costing me a ton of money to use. Using my indoor wood burner seems to be and option but I obviously want to do this safe. I've tried to do some research to understand everyone's comments on how set this up but there seems to be very little info for indoor wood burners and I'm still confused.
@Zman, When you say to use a Taco I-series valve with outdoor reset, is this after the water is circulated through the floor and before going to the wood burner?
The instructions for the heat exchanger (using a pump) mentions using an aquastat. I'm assuming this is just so the I'm bypassing the wood burner when if I'm not burning wood.
Thanks again for everyone's comments! If anyone recalls another thread for a similar situation I would really like to see it. It looks like the heat exchanger has gone up $60 in the last month as well!

psb75

The pic of the item you have supplied--the horseshoe-shaped stainless coil IS the best way to extract heat from your wood stove. Mount it INSIDE the firebox, horizontally along the back wall, on the vertical axis. Try to keep the "bottom leg" out of the deep ash bed. Be sure to: INSTALL a TPRV on the hot supply outlet near the stove (behind usually) as in your diagram of OWB. Obviously there is much more to the install than this. A heat-dump is a very good idea. Esp. if its not a gravity thermo-siphon set-up and relies on electricity to circulate.

Jamie Hall

I have to go back and comment on one comment above. Watch out for your energy costs. One member posted electricity at $.10 per KWh. Where I live and work, it's $.25 per KWh. This does make a difference. Also, research your other fuels. Just because fuel A is cheaper than fuel B in one location, this may not be true in some other location.

DeerWood

@psb75, thanks for the suggestions! This is actually for an Indoor wood burner. The horseshoe coil is from Daka and designed to go with my Daka furnace similar to how you explained the install. I will have to look into a heat dump. My guess is that when just using the wood burner that it will struggle to keep up with heating my floor but It's definitely something I need to consider.
@Jamie Hall, I agree. All I know is when I started using the electric water heater my electric bill went up over $300/a month. I'm really just trying to take the chill out of the basement floor.

psb75

Check out this company:
Image 1 - 21T HILKOIL STAINLESS STEEL HOT WATER HEATING COIL -FOR WOOD OR COAL STOVES