Help with heating DHW as well as air

[TheNeverStill]

Hello all! I’m relative new to wood boilers (came with the 1200sqft home I purchased in April). It’s a 120k BTU hardy that is connected to my HVAC vent stack (pex to a copper radiator in the vent).

Our water heater is on the way out (incredibly 30 years old), and I’m thinking I’d like to go “tankless” and utilize the wood boiler to heat my DHW on demand (ie no hot water tank in the house).

I have a general idea on how this could be setup, but I could really use some advice on how this should be setup (as well as product suggestions on what exactly to buy).

Right now I have:

- 2 indoor thermostats (one for A/C & heat via heat pump and electric/emergency heat, another for wood boiler heat)
- a box by the hvac stack that I believe (but not certain) has to do with controlling the boiler pump (when instructed by the indoor boiler thermostat, ie not the one on the boiler that controls the boiler fan) and potentially managing the hvac fan between the two indoor thermostats
- a pump
- a supply pipe and return pipe from boiler to hvac radiator (connected to the above pump)


One way I think I could add DHW tankless heating would be to cut into the supply pipe (from the boiler), insert a plate heat exchanger that is 300K+ BTU, and then for my DHW take the cold->hot supply line, add a flow switch, run it through the plate heat exchanger, then run it through a 120F mixer.

I do a lot of DIY and remodeling, but not a lot of hvac and complex plumbing. I’m not sure exactly which flow switch to buy (guessing 0.25/0.5gpm, but dunno where to get it), how/where to connect the flow switch to have it turn on the pump without messing up the existing pump control, etc. I also feel like this setup will be a little inefficient and someone more knowledgeable could instruct me how to set it up via zones.

Thank you in advance!!

 

[Bad LP]

What water temps is your boiler set to run at?

What water temp do you wish to have in the DHW storage tank?

I'm adding a heating zone to a room as we will be moving there full time shortly and I didn't mind doing without the space for long weekends but no way am i going without it now.
I've been unhappy with the DHW off my wood boiler so its going back onto the LP boiler when the new zone gets tied in.



One of two CI radiators roughed in sitting on leathered granite bases. Wood cover to go over PEX to hide pipes and some wires. Room was roughed to be a 3 season room at design and build then I changed my mind for the better.

 

[TheNeverStill]

Boiler set to 165-180F (testing to see which works better for my setup), 120000 BTU, 100 gallons.

No DHW storage tank - this is a tankless design.

---

Why are you unhappy with the DHW off your wood boiler?

 

[E Yoder]

What size lined (PEX) do you have coming in from the Hardy boiler? 1" will do much better than ¾" with a flat plate heat exchanger.
You'll want to circulate continuously if you're using a flat plate, if the underground pipe is cold you won't get hot water for several minutes when a flow switch activates. My opinion anyway.
How will you bypass flow from the hot air coil when you're not in heating season?
I would pump continuously to the flat plate to provide on demand hot water instantaneously. Then use close tees and an additional pump to create a secondary loop through the coil in the duct that only circulates with a call for heat.

 

[TheNeverStill]

Right now it is 3/4” to the hvac radiator, but I don’t mind running more/better lines.

Honest questions to follow, newbie boiler owner eager to learn:

• What makes 1” better than 3/4”? My assumption is that I should match the ports on the exchanger, the larger exchangers going up to 2” ports.
• Why so slow to heat up the plate exchanger // transfer to DHW lines? My hvac radiator is hot within a couple seconds. Are plate exchangers much slower to warm up?
• Bypass heat coil: an unsophisticated solution would be to add a bypass controlled by some ball valves. I’ve seem some more sophisticated valves managed via controllers based on what is powering the pump.
• Interesting suggestion on the second loop provided the first is running constantly. I’ll have to give that a good thinkin’. I’m a little wary because I was hoping so save electricity with this solution and a 24/7 pump for sporadic hot water use (and constant plate heat) seems pretty inefficient.

 

[hedge wood]

I think trying to run tank less with a wood boiler will be a problem if you don't run the pump for the feed on the flat plate 24/7. The lag time for the lines from the boiler to warm up will be a problem. I heat my domestic year round with my Garn I run a 5x12-20 flat plate to a 40 gallon LP hot water heater. I run my Garn water 120-180 in the summer and have Ranco controller that turns the pumps on and off to maintain water temp in the hot water heater. I have a bypass on my furnace coil to bypass the coil in the summer so the coil doesn't heat up. My system works well and my LP side never kicks in unless I let the Garn water temp get down to 100 degrees before I fire the Garn back up.

 

[TheNeverStill]

Thank you for sharing your setup @hedge wood — how long would you say it takes for your plate exchanger to heat up after the pump cuts on? Also how long would you say your pump runs once your controller turns it on (and what is the temperature rise over the 40 gallons while it runs)? (E.g. it runs 10 min to heat the 40gal tank from 110 to 120).

A lag time of a couple seconds is something I can tolerate, but if it is as @E Yoder was suggesting and it takes several minutes, I might have to reconsider.

 

[TheNeverStill]

Looks like enough googling with varying keywords resulted in something similar to the setup I am describing: https://www.hpacmag.com/features/only-as-needed/#

I think a key point here is that our wood boilers have 100+ gallons of 180 degree water, so they are the storage tank. So even if the boiler is 120k BTU and the plate exchanger is 300k BTU, you’ll be able to heat at least 90+ gallons of DHW to 120 degrees at the plates’ 300k BTU rate before you slow down to the boiler’s 120k BTU rate.

 

[TheNeverStill]

I don’t suppose anyone knows where I can find a reed/magnet based (ie not paddle based) flow valve that will turn on my pump when it detects 0.25-0.5GPM flow?

Attached is my relay/control panel and pump connection.

 

[Bad LP]

Why are you unhappy with the DHW off your wood boiler?

I’ve found in real winter conditions my 50 gals of hot water isn’t really hot enough because the zones are trying to keep up with demand. It seems that the DHW zone ends up being used in the transfer of heat to those zones. You can’t make 180 water with an as low as 125 hot water loop. Tank never satisfying after a draw down from say a shower so the pump never shuts off trying to make more.
That’s not something a bigger boiler is going to fix.

The boiler manifold does have stubs to add an exchanger to my central AC system but I don’t think I’ll ever use it for hydro air. We don’t change the temperature programs on the t’stats now cause it’s long weekend use now. Once living there we would only change it a few degrees anyway because the RFH does not respond quickly in concrete.

 

[TheNeverStill]

That makes sense! Thank you for sharing!

 

[Bad LP]

That makes sense! Thank you for sharing!

Forgot to add my LP boiler flips back on at about 120 degrees. Nothing makes hot water for free but my boiler and run to the tank holds maybe 2.5-3 gallons. I can easily turn the hot water priority switch back on.
I'm currently using the boiler in a dumb mode only. Outdoor reset is being done by the TACO control.

 

[hedge wood]

Thank you for sharing your setup @hedge wood — how long would you say it takes for your plate exchanger to heat up after the pump cuts on? Also how long would you say your pump runs once your controller turns it on (and what is the temperature rise over the 40 gallons while it runs)? (E.g. it runs 10 min to heat the 40gal tank from 110 to 120).

A lag time of a couple seconds is something I can tolerate, but if it is as @E Yoder was suggesting and it takes several minutes, I might have to reconsider.

I can't say I have ever measured any of these values. I have 450 feet of line from my Garn barn to the house so it takes a bit for the line to warm up. Time to heat the water depends on if the Garn is at 180 or 120 water temp. I had a tank type hot water heater that wasn't very old when I put the Garn in 1999. That's why I made the set up I have. I don't have enough BTU on my propane lines to be able to run my furnaces and a tank less water heater at the same time so I will always have a tank type. I think if you use a big enough flat plate exchanger you should be able to make this work. Don't have any flow switches for you. We used a lot of flow switches when I did factory maintenance work. You will need a water wheel type of flow switch.

 

[salecker]

When i built my heating system i wanted to add DHW to it.
The guy who helped design what i wanted talked me out of it.
There is no such thing as "free hot water"
You will use more wood if you heat your DHW with your heating system,you have to add more hardware to the system costing $$$ for your time and parts,both building the DHW system,getting more wood a year plus tending a boiler all year
If you have wimenz living with you they will use lots of unlimited hot water.
I had 2 of them in my house,plus my own septic in poor ground and i haul my own water
I would never see a ROI for heating DHW with the boiler.
Electric hot water heater is what i have.I cannot see tending a boiler or wood burning heating system when the ground isn't frozen.

 

[TheNeverStill]

When i built my heating system i wanted to add DHW to it.
The guy who helped design what i wanted talked me out of it.
There is no such thing as "free hot water"
You will use more wood if you heat your DHW with your heating system,you have to add more hardware to the system costing $$$ for your time and parts,both building the DHW system,getting more wood a year plus tending a boiler all year
If you have wimenz living with you they will use lots of unlimited hot water.
I had 2 of them in my house,plus my own septic in poor ground and i haul my own water
I would never see a ROI for heating DHW with the boiler.
Electric hot water heater is what i have.I cannot see tending a boiler or wood burning heating system when the ground isn't frozen.

Haha

In our current setup our DHW runs through a coil in the boiler before entering our ancient electric HW heater. The boiler gets it from groundwater temp to about 80-90F. (No pump, just HW usage pulls it through the boiler.)

I ran this setup through summer to test out the wood usage - only have to feed the boiler 1/2 way full of wood every few days. We’re fortunate to live on acreage with a lot of trees.

The boiler itself is on its way out (previous owner burned anything and everything in it, plastic, you name it). Looking at the new, EPA models I don’t see a lot with inner DHW coils, so I’m doing this transition now instead of when it dies.

Definitely not free hot water, but good exercise and already feeding it in cooler months. I’m not concerned about extra pump wear and a plate exchanger is cheaper and less risky to me (and easier to replace) than an electric water heater every decade or so.

Also, we’re a family of 7, so lots of HW usage (showers + laundry). Fortunately we’re on well and septic so not paying for water/waste.

 

[E Yoder]

The lag time from when a float switch would start the pump on the stove and get the flat plate hot would depend on several factors. Length and diameter of piping. Pump model. How long it's been since the last cycle.
Running a small circ 24-7 costs peanuts and prevents stratification and boiling in the water jacket on the outdoor stove.

 

[TheNeverStill]

The lag time from when a float switch would start the pump on the stove and get the flat plate hot would depend on several factors. Length and diameter of piping. Pump model. How long it's been since the last cycle.
Running a small circ 24-7 costs peanuts and prevents stratification and boiling in the water jacket on the outdoor stove.

Theoretically:

• 50’ of 3/4” PEX = a little under 2gal of water in pipe
• 1.4amp // 10GPM pump = a little under 6s to get boiler water to exchanger
• exchanger is near instant transfer so I’ll ignore that (along with flow switch) since above is really 5.49s

So theoretically a max additional 6s wait compared to having an indoor DHW tank at desired temp.

1.4amp // 120V // 30days // $0.1/kW (our rates) = $12/mo in electricity to remove those 6s plus however much extra wood.

I feel like 6s would only ever bother me when washing hands if I wanted warmer than indoor air temp water (our indoor well pressure tank is sufficiently large that water will generally be at indoor air temps). Tub/shower - 6s pass while still getting towels/etc ready.

 

[maple1]

It takes a lot longer than that for my flat plate to put out max hot water. Takes a while for everything to get heat soaked. I use mine in conjunction with an ordinary 60 gallon electric hot water heater. It goes electric only in the non heating seasons. Just not worth it to me to put up the extra wood every year along with the hassles of making fires all year.

 

[Bad LP]

It takes a lot longer than that for my flat plate to put out max hot water. Takes a while for everything to get heat soaked. I use mine in conjunction with an ordinary 60 gallon electric hot water heater. It goes electric only in the non heating seasons. Just not worth it to me to put up the extra wood every year along with the hassles of making fires all year.

I’d rather eat glass than burn for hot water.

 

[E Yoder]

I typically am moving about 4-6 gpm on the main incoming line with 1" , you'll get max of about 3 or 4 gpm on ¾ pex. 10 gpm would take a huge pump.
I use a lot of Grundfos 15-58 and have checked it with a clamp meter and depending on flow it runs about .5 amp with 1" PEX. With smaller piping the amp draw would drop a bit, the impeller bites less water.
What you're not factoring in is tho the initial water will get there within 30 seconds it won't be hot if your lines have had time to cool off. That could take several minutes if you're only moving 3 gpm which is realistic on 3/4” pex at any distance. The lines soak up the heat for a bit.
I think what you're doing will work if it's a short distance from the outdoor boiler, the longer it gets the wait time gets longer.

 

[maple1]

I’d rather eat glass than burn for hot water.

I did do it for the first couple of summers, I had quite a bit of junk wood to get rid of. But yah, no thanks. And I have a pretty efficient set up for doing it with.

 

[TheNeverStill]

Thank you all for your responses!

I had a spare moment today to try out my theoretical numbers in practice.

Actual 3/4” ID (7/8” OD) PEX from boiler to HVAC coils is 90’.

Pump specs noted above are for my hardy pump (https://altheatsupply.com/taco-009-bf5-j-circulating-pump.html).

After several (time spaced) trials, it takes 25-30s for boiler water to reach the HVAC coils and make both in/out pipes too hot to touch.

Given that the plate heat exchanger would be inserted 15’ earlier in the pipe run, this means it should take, in practice, about 20-25s delay before “unlimited” hot water from the exchanger. Given a home run set up, it takes only about 2 to 3 seconds for hot water to reach any of our faucets and tubs from the centralized (and very close to all bathrooms) tank/exchanger location.

I’ll have to do some pondering on if that delay will work for us. Some of my extended family have gas tankless water heaters and it takes a full minute for hot water to reach their faucets.

 

[Bad LP]

You couldn't pay me to own a tankless on demand water heater.

 

[E Yoder]

Thank you all for your responses!

I had a spare moment today to try out my theoretical numbers in practice.

Actual 3/4” ID (7/8” OD) PEX from boiler to HVAC coils is 90’.

Pump specs noted above are for my hardy pump (https://altheatsupply.com/taco-009-bf5-j-circulating-pump.html).

After several (time spaced) trials, it takes 25-30s for boiler water to reach the HVAC coils and make both in/out pipes too hot to touch.

Given that the plate heat exchanger would be inserted 15’ earlier in the pipe run, this means it should take, in practice, about 20-25s delay before “unlimited” hot water from the exchanger. Given a home run set up, it takes only about 2 to 3 seconds for hot water to reach any of our faucets and tubs from the centralized (and very close to all bathrooms) tank/exchanger location.

I’ll have to do some pondering on if that delay will work for us. Some of my extended family have gas tankless water heaters and it takes a full minute for hot water to reach their faucets.

Sounds like you have some better info now to decide if it fits your needs. Best wishes, hope it works good for you whatever the final version is.

 

[TheNeverStill]

If we didn’t have 7 people in the house (and before long 5 of them will be teenagers), I wouldn’t be so concerned about tankless. But as is we easily exhaust our 80gal tank (at 130 degrees) and inevitably end up with a cold shower. Never mind trying to balance that with the dishwasher and laundry washing machine.

I have all the info I need for best fit, but I still lack a link to where I can buy a lead free flow switch for <= 0.5GPM (ideally 1” fittings, but can add adapters for anything over 3/4”).

I can find a couple online, but none are verified lead free and I question their quality. Any help here would be greatly appreciated. I’m likely just not googling the right way. All I can find are paddle switches for >4 GPM (too high flow) and reed switches for 0.25-0.5GPM but <= 1/2” fittings (too small fittings).