Domestic HWH for OWB backup

gg2

I have a Central Boiler 2400 OWB which I will install in new construction, 1200 sq ft main floor radiant in joists and 1200 sq ft basement radiant slab. Total house heat loss 60K worst case. Considering indoor boiler back up to OWB with indirect fired HWH. I would like to use a DHW heater for my back up "boiler" in series (no heat exchanger) with my OWB. Considering using a David White 75 gal, 75K btu TTW2. Questions:



Will the TTW2 handle the 185 degrees water from the OWB? If not, are there DHW heaters that can?



Can a DHW unit operate in tandem with the atmospheric pressure system of the OWB?



Will propylene glycol antifreeze or rust inhibitors used with the OWB bother the DHW unit?

Zman

How??

How did you arrive at 60K . Is it a tent?

How do you plan on keeping the system from air locking? Is the boiler going to be installed on a hill behind the house?

How are you going to mix the infloor water to the appropriate temp?

Seriously,

The house should be pressurized.

You should use a heat exchanger.

The floors should be controlled via outdoor reset.

You should not use hot water heaters for heat(use a boiler)

I understand that the OWB folks are telling you otherwise. I have redone several systems that were originally piped as you are suggesting.

Carl

gg2

Reply to OWB backup

Thank you Carl:



Heat loss calculated using two different methods. Log home. 23K btu main floor, 37 K btu basement. Basement probably not that bad. Fill in formula lacked some criteria and I would never keep my basement at 70 degrees.



I was hoping to direct plumb the OWB to a boiler...then run the indirect DHW off of that. I have not addressed air lock issues with Central Boiler. OWB is 112 feet away at about the same elevation as house. i would control output temp with mixing valves to control what hits the floor.



What are advantages of pressurized system?



What about the OWB to a heat exchanger as you suggest....to an 80 gallon storage tank....with a circulator to a wall mount on-demand unit? I could also use the 80 gal storage tank to heat the indirect fired HWH?



Is there a line of conventional boilers and/or wall mount units that you prefer?

Ironman

Central Boiler

I install wood boilers on a regular basis, as well as all other forms of hydronics. Central Boiler, and every other manufacturer of outdoor wood boilers that I've installed, does not understand proper hydronic design. Their main concern is to sell you their stove.



Some OWB installs are fairly simple and don't require much knowledge of hydronics. Others, like yours, require extensive knowledge that few in this trade have.



You will save yourself alot of $$ and frustrations for years to come if you get a radiant pro to design your system and heed the advice that Zman gave.



The best minds in the business frequent this site and contribute. Ask questions, take time to learn and you won't regret it later.



Here's a link to a Caleffi webinar by one of the best on wood boilers and hydronics.

Choose the webinar on wood boilers and hydronics by John Seigenthaler.





http://www.caleffi.info/webinars/#





Let me add Some things:

1. Your radiant system must be pressurized. No option here. You also need to use good manifolds such as Rehau or Uponor with flow setters, purge valves, etc. Don't use the chinese: they're inferior and notorious for leaking.

2. You cannot properly control slab water temp with a thermostatic mixing valve: it will over heat. Variable speed injection mixing is the proper method.

3. A tankless water heater is just that: a water heater. It's not designed, controlled or certified for space heating. There are numerous threads on here addressing this.



Hope this helps.

gg2

Boiler for OWB backup

Thank you Bob. Obviously, I have some more homework to do and that is fine. If I bring the water from the OWB to a heat exchanger on a HW storage tank, I can keep the inside water separate and pressurize the system....circulating water through my radiant zones wirth an indirect HWH for domestic. When the OWB is off, I can use a indoor boiler to pick up the slack. Question: Noritz makes a wall hung boiler NH 199-DV-LP (non condensing 199k BTU/hr) designed for hydronic systems combined with an indirect HWH for domestic. It is the only one Noritz makes designed for this purpose. In your opinion, should i still stay away from it?

gg2

Boiler for OWB backup

Thank you Bob. Obviously, I have some more homework to do and that is fine. If I bring the water from the OWB to a heat exchanger on a HW storage tank, I can keep the inside water separate and pressurize the system....circulating water through my radiant zones wirth an indirect HWH for domestic. When the OWB is off, I can use a indoor boiler to pick up the slack. Question: Noritz makes a wall hung boiler NH 199-DV-LP (non condensing 199k BTU/hr) designed for hydronic systems combined with an indirect HWH for domestic. It is the only one Noritz makes designed for this purpose. In your opinion, should i still stay away from it?

Zman

How often?

How often will you use the backup? If the answer is seldom, I would use a conventional atmospheric boiler. Bob's suggestion of an injection mixing pump would be easy for this arrangement.

If you plan on using the backup more often(or just want a sweet setup), I would suggest a mod/con boiler. The piping and controls become a bit more complicated. I recently worked through a design with tekmar using a (2) mod/con boiler controller and a 0-10 vdc circulator on OWB exchanger making this circulator  boiler 1 (fixed lead).The Mod/con is boiler 2 (fixed lag)



I think your heat loss assumption is high. Unless you have huge DHW needs, you wont need a boiler anywhere near 199K.

How were you planning on doing the tubing? Most radiant installs will not emit more than  about 30K per foot. There is no point in having more boiler than your emitters can radiate. You will be guaranteed short  cycling. Siggy has some great stuff on OWB's.

Carl

gg2

OWB Backup

The use of the OWB backup will vary. Will likely use back up exclusively in summer months for DHW and in winter when away on travel.



The Caleffi site on OWB was very informative. They address the issue of interfacing the non-pressurized OWB system with a pressurized indoor system simply by using a heat exchanger. However, most of their suggested system designs seem to place the OWB and inside back up boiler hooked in parallel.



I am looking at a 1200 sq ft main floor radiant system of (6) 250 foot circuits, suspended one inch under the joists, 1/2 inch hpex, 8 inches OC, calculated to produce max 24 BTU/hr/sqft at 130 degrees. My worst case heat loss (likely over estimated as you indicated) would require 21 BTUs. Floor will be mix of ceramic tile and composite hardwood. I don't like carpet.



I like the wall hung mod/con as back up if possible. However, i notice as I research them, many are not really designed for this. Maximum return water temperatures of 110 degrees is one issue. I am not thrilled with the inefficiency of mixing down my return water. The Noritz tech said I should circulate the water continuously through the system and not cut it out when not in use.



Based on everything I have read so far, I am considering bringing the 185 degree water in from the OWB to a heat exchanger to a 50 to 80 gallon buffer tank. The low pressurized buffer tank would supply the main floor radiant heat system, the basement slab radiant heat system, and an indirect fired HWH for domestic. Mixing valves would control the ouput......although it looks like i need some additional protection for the slab. I would interface a wall hung mod/con which would only come on when the OWB kicks off. According to Callefi, the heat retained in the buffer tank allows the OWB to continue heating the system even though the fire is getting low. Also, using the buffer tank in conjunction with the mod/con will reduce the recycling.



This idea will probably change tomorrow as I receive more input but it seems to make sense at the present. My respect for the folks that do this for a living has increased significantly as I learn how complicated it is. Truly fascinating though....gg2

NRT_Rob

first of all

get away from the idea that you want to pipe the boiler in series. that's not really what you want to do. either the wood boiler is doing the job or it isn't. pick one and run it. the window for "boost" heating is very small and if you're there often enough for it to be an efficiency question you'll cycle the boiler to death in all likelihood.



50 to 80 gallon buffer tank isn't much for these boilers. but if you use a tank in tank indirect, wood boiler to the inner tank, you've got your buffer on the atmospheric side and heat on the pressurized side. Might be cheaper/easier than external heat exchange if you're going to add a buffer anyway, but honestly, if you're going to add mass, add some serious mass. 50 to 80 gallons is negligible for wood boilers. Large tank with coils in it is really what happens when you want to add mass storage to a wood boiler. the Central boilers usually have a couple hundred gallons inside them and even that is really not ideal for long burn/draw cycles.



Personally, I would generally advocate for pressurized wood boilers. but those are usually indoor units and if you don't have a chimney, you might not have a lot of choice. If you must go unpressurized, then you might as well keep as much water mass as you can unpressurized.

Ironman

Mod/Con not Designed for this??

I don't know where you got that info, but it's dead wrong. Also, the thing of max return temp of 110* is way out in left field.



The lower the water temp, the more efficient the mod/con operates, but they're designed to operate up to 180* or more just like a conventional boiler.



Like Rob pointed out, you don't pipe it in series. You should create a primary loop with your heat sources injecting into it and you loads extracting from it.



The buffer tank is a good idea, but the size and necessity of it will vary depending on how quick the OWB recovers, how much water it holds and weather your radiant loads are high mass, low mass or a combination of both. If you choose a tank in tank design, the inner tank must be pressurized or the outer tank will crush it.



I have a "Heatmor" Phase 1 that holds about 300 gal. and it does fine with that alone. Most of the ones I install hold 115 gal. and that is sufficient due to the Heatmor's quick recovery from its forced draft system. The Central Boiler is natural draft and does not recover anywhere near as quickly.



If you haven't purchased the OWB, I'd recommend that you go with the Heatmor; it's a better stove IMHO.



As far I know, Noritz doesn't offer a mod/con. Again, don't confuse an on demand water heater with a boiler. There are a lot of Internet peddlers who claim to know how to design hydronic systems who will try to sell you one in place of a boiler so they can turn a quick $$. Buyer Beware! You'll be the loser after it's done. Could that be where you're getting the info of 110* return temp and a mod/con not designed for this? An on demand water heater is not a mod/con.



It's good for you to educate yourself as much as possible about hydronics, but as I stated previously, you need to get a design pro to lay it all out and go with what he spec's.



I do consulting and design as well as Rob and some others here. The cost would only be a small portion of the overall job cost, but it's the most important element. Don't omit that part.



You can contact me through the "Contact This User" tab under my handle.

SWEI

long, hot burn cycles

Are what you are after, both for combustion efficiency and to minimize the amount of time you spend feeding the beast.  You want to be able to burn a full load in one pass (no choking off the air) and capture every BTU that puts out.



For an unpressurized system, I'd probably choose a Garn -- take a look at the amount of water in one and you'll start to see what we're talking about.



For a pressurized boiler (which I prefer, especially once you start adding heat sources to the design) I'd probably choose an AHS, or one of the EU imports like a Tarm.

NRT_Rob

the problem I have with unpressurized

is the need to treat the water. in the garn, that's 2000 gallons of treated water you either need to pour out and pour back in every few years or replace.. $$.

Plumdog_2

Just curious, Ironman

why do you say "you cannot properly control temperature with a thermostatic mixing valve"? I have done it a lot, and with accurate results; carefully following the recommended piping and pumping layout. Now, a manual, non thermostatic mixer is another animal, and will not control temps very well. 

Ironman

On a Slab...

Radiant system you have high mass that will continue to give off heat long after the call is satisfied. If the water is not regulated to match load conditions, such as with outdoor reset, the slab will overheat the zone due to the flywheel effect. Any fixed temp mixing device in this application will cause issues. If injection mixing is used and the reset curve is dialed in correctly, the thermostat really only functions as a high limit and is almost un-necessary.



If it's staple up, which is low mass, then a thermostatic mixing valve is fine.



I know some like to use a thermostat with a slab sensor and control it with a thermostatic mixing valve and that might be okay in some applications with a small slab, but it's an inferior method that gives less comfort control (IMHO).