ModCon boiler- Can I use the DHW Indirect loop to feed an air handler ?

Dave Carpentier

Looking at getting rid of our 20+ yr old oil boiler and going modcon gas (Nti NTVN series). We have in-floor heat for the main floor and an air handler for the second story. I'd like to run the boiler low temp (80-120?) for the slab (primary/secondary and a mixing valve), but am curious about running the air handler via the DHW indirect loop at a different setpoint temperature (140 ish). This is not a combi unit but does have an auto valve for the Indirect outlet.

EBEBRATT-Ed

I don't see why not

hot_rod

Isn’t the DHW indirect function a priority? If not, what maintains the radiant at lower temperature?

Some of the mod cons have 2 or 3 temperature outputs, but the prioritize them, one temperature load at a time.

Dave Carpentier

It would make sense that the DHW is priority. I do see in the IO a "CH/DHW Shifting Priority Timer" where it "Determines how long a CH or DHW demand can last before the priority is switched. First priority is with DHW." Can set that from 0-1440 mins where 0=indefinite DHW priority.
I think I would connect the "Aquastat" terminals on the boiler DHW to the airhandler either on the AH board or in some way to the tstat that controls the AH now. The AH doesnt really come on that much as heat rises from the main floor hydronic anyhow.
I guess there's a chance that an AH call followed by a regular CH call can result in hot water going towards the floor mixer.. but the mixer would just take care of that.

Dave Carpentier

Now that I think about it more.. I guess I wouldnt want the boiler to switch away from "DHW" in favour of the CH using the timer I just mentioned, since the AH fan would still be running (call from AH tstat) and then it would blow cool(er) air. Realistically, any calls for heat from upstairs would be solved way before anyone noticed a drop in temps in any zones on the floor CH (nice thick slab, so it has a looong period).
Another situ I will have is that the AH has its own pump, so the boiler's internal ECM pump and the AH pump will be teaming up. Not sure if thats a no-no.

hot_rod

Maybe just set the SWT at the highest temperature the AH needs, use outdoor reset, just blend down the radiant with a 3 way thermostatic. That is a common application, simple to pipe with a hydraulic separator or parallel P/S

I thought you were trying to do it without a mix valve, at first. Thermostatic mix valves need the hot supply to be 20-25 degrees hotter than the mix temperature, so if you want to ramp down the boiler SWT, either a motorized mix valve or the boiler needs to run 140.

Is the low temperature zoned, or one thermostat? I have used a manual mix valve that tracks along with the ODR. But you may not run the AH to low or it blows tepid, uncomfortable air temperature.  Even if it is covering the load. Like mini splits or air to air heat pumps😗

Dave Carpentier

Well, I was thinking to run the CH on a lower temp, even though Im using a mixer, to try and keep away from heat buildup in the boiler. It says to keep a min 2gpm flow, and I assume thats at max output (85k in this unit). My loops struggle to flow that much, so I think recirculating the primary might overheat the unit when its running high set temp ? The loops are fed via one circ (An Alpha-1 set to the highest constant pressure mode). There are 7 loops with their own zone valves and tstats, shortest is about 70ft of 1/2" wirsbo, the longest is about 400ft? (an attached garage). I use balancing caps on the supply side to try and keep things even. Everything works fairly well, but that Alpha rarely shows 3gpm at most. So Im hoping that low temps on the boiler and longer run times would be more stable ?
The AH was the wildcard where I need the higher temp.

mattmia2

How do the diagrams in the manual show the indirect piped? Is it primary secondary or directly to the boiler? You probably want to turn off dhw priority for this application if you will have both zones calling at once. Is the boiler circulator providing the flow in the high temp loop of the radiant mixing?

Dave Carpentier

There is a internal pump only diag option (for small single zones), but I would have to use a p/s with the "dhw" tap-off like this diag from the manual.
My AH would replace the Indirect, and my AH has its own circulator that should be mentally added to that diag.

mattmia2

Did you already buy the boiler? Can you get one without a diverter valve and internal pump and just do normal primary secondary or a hydraulic separator? A fire tube like an htp will require less pump on the boiler loop.

hot_rod

If everything is piped and working correctly you cannot “over~heat” that boiler. It has multiple controls and safeties to prevent that from happening.  The minimum flow is to assure the heat exchanger doesn’t develop hot spots around the fire tubes and prevent short cycling.
Still, the safeties will lock out the burner if that internal pump fails or does not meet the minimum flow.

The purpose of a Sep or closely spaced tees is to allow different flow rates between the boiler and distribution. No problem running a higher flow in the boiler side, that is common on multi zoned systems

Dave Carpentier

I didnt notice a theory of operation kinda thing in the IO, but are you saying that regardless of the ODR, the boiler would start to turn down (it's a 10:1) as the primary loop temp climbs due to lack of secondary loop flow ? At some point it would short cycle at low fire with a very low flow loop on the secondary ? This boiler would turn down to 8500btu at min, but Im not sure what power level it 'restarts' at. 8500 isnt a lot, but that super long garage loop struggles to touch 1gpm even when I test the Alpha-1 loop pump on max speed (but I normally operate it at constant pressure due to the differing loop demands). Something flows though, thats a 600 sqft attached garage and it stays stable at 15c even when its -35c outside.

Im not to the point of control logic yet, but I was guessing that I would just tie all the zone end switches together for a common "tstat" input on the boiler. Any or all zones would be a heat call for the boiler.

hot_rod

The manual mentions a mixing valve to protect low temperature zones. I don't see a piping diagram for a two temperature system, however.
It also mentions that 2 gpm minimum is to prevent short cycling.

What is the btu load of your smallest zone? Assume it matches the lowest turndown, on an 85K boiler that would be 8500 btu/hr.

So at a design day condition you could expect the boiler to run non stop if that load stays consistent. Once the load starts to drop off, yes the boiler will start to cycle on and off. How much it will cycle depends on what that load drops to. IF you want to lessen cycles on micro load zones you may consider a buffer tank.

As I see it that 3 way valve in the boiler is mainly to put the boiler in DHW mode, not for a two temperature heat design.

They also show the what the onboard ECM pump has available for distribution. So if the indirect coil matches the available pump output, that could be direct piped without P/S or addition pump. Same for the heating, they show a direct pipe option.

To learn more about control or piping it would be good to attend a NTI class. I have sat in on a few of Jasons 1/2 day presentation and he covers actual installations better than the manual can. Really no way to include the dozens of ways to pipe a boiler in a useable manual

Find the local rep and see what training they have available, or maybe a recorded training online.

Dave Carpentier

Yes I agree that I wont have two separate temperatures at the same time, when it switches to DHW/Indirect/AH mode, the slab (CH) heating will stop until the AH tstat is satisfied or the switchover timer forces it back to CH mode. There's also an option CH Delay Following DHW (0-30mins) that might be of some use.
Im feeling like I may have to remove the circ pump that's inside my AH, or at the least remove it electrically from the AH controller board and run it to the boiler "DHW Pump" line connection to avoid potential conflict/deadheads. It's a Grundfos UP15-42BUC5, 9gpm at 0ft to 0gpm at 15ft, so is a little below the boiler's pump (but enough to matter that they're both in series of the same water loop ?). Hard to physically remove as it's union on one side but sweat on the other (why? lol) and in fairly tight to the AH rad piping.

hot_rod

It is not uncommon to lock out heat supply during a DHW call from the indirect. The adjustable timed out function was added, and is important, so you don't leave the heat call off for too long. We see fin tube baseboard freeze from time to time when heat supply is shut down for too long. Inside the building!

I don't know a scenario where you would want one heat demand locked off, think about a design day and it switches to the high temperature load. How and when does it come back to the second load. If it is on a trimer, what happens if that load isn't satisfied when it switches to the other load?

You would need to look at the coil spec for the AH, the onboard boiler pump may not be able to cover that adequately, that is why they supply the "pump capacity available" info.
If the AH has a dedicated circulator, there may be a reason, like a high pressure drop, and the manufacturer wants to assure proper flow for adequate heat output.

A hydraulic separator cleans up all these concerns. Proper flow to all connected devices, regardless which one or combination of devices are flowing. You may be over-thinking the value of that DHW connection?

Dave Carpentier

"A hydraulic separator cleans up all these concerns. Proper flow to all connected devices, regardless which one or combination of devices are flowing. You may be over-thinking the value of that DHW connection?"
I may end up doing just that to keep the situation less complex control-wise. The boiler on its own primary loop with its own internal circ, then a p/s separation for the AH (with its own internal circ), and a p/s separation plus a mixer for the main slab heat (with its own circ).
Maybe something like a 90-130f range on the boiler could work for both systems.
But I will make sure we leave a union connection near the boiler for potential buffer tank tap-in point.
Thx

mattmia2

It would make more sense if you pick a model without the diverter valve inside. You are just going to be working around something yo don't need in your application.

Dave Carpentier

Not too many choices locally, unless I order in. It defaults into the CH only side and they say to remove the control wire and cap the outlet if not using it. But.. I guess it would leave me the option of using an indirect DHW at some point.

mattmia2

They are small enough that they are UPS shippable.

Dave Carpentier

True, they're quite a bit smaller than my 22yr old cast iron boiler. Im looking forward to the newfound space in my util room. Still, a 100 pound box sent to Canada would be $$.

mattmia2

Canada makes it more complicated even though most of the places that will ship boilers are closer to Ontario than most of the US. I have had enough issues with one supply house selling to me at a reasonable price while another will act like no one outside of the trade has ever bought something from a supply house before and why am I even asking that I will gladly order online and pay motor freight.

hot_rod

NTI and IBC are both made in Canada, I believe?

Dave Carpentier

NTi are made in New Brunswick apparently. Nice to know we still make something up here lol