Lost in the design process...

flyboy182

Quick intro: New to forum, not a pro (that will become obvious!) and building a small separate rental unit onto our property. In serious need of some "heating help" as I don't want to mess up this aspect of the new build. Building permit was issued with a heating design as part of it (architect hired HVAC engineer). Builder and his plumber now suggesting a bunch of changes (which are permitted so long as inspector approves). I have concerns mostly about "short cycling" (loads are light/small unit), reliability/longevity of equipment, and having sufficient DHW as there is a long term rental in the unit (upstairs) and a secondary "AirBnB" type short term rental unit as well (downstairs). 2 full baths basically, but lots of use and certainly concurrent use of both baths.

Current Design: two (main) zones in-floor heat on the main floor (slab)...one zone is single car garage other zone is approx 400 sq ft living space (AirBnB suite is on main floor). Upstairs suite is approx 800 sq ft, with heating/cooling via dedicated hydronic air handler and AC unit. Additionally main floor living space has supplementary cooling control provided by small mini-split...wanted AirBnB unit to have separate AC control basically...typically summer rental. Heating load design for Air Handler: 21,000 BTU/h @ 140F&3GPM 525 CFM. continuous run HRV fitted to air handler. The radiant design shows a "Low Loss Header" off the main boiler loop. Downstream of the low loss header are three smaller loops. One for the main floor heating with its own pump and thermostatically controlled mixing valve. Other loop has no mixing valve but a pump and loop for the air handler. Last loop includes some kind of a heat exchanger, pump, and mixing valves etc with a separate "glycol" loop system for the garage floor. DHW is directly off the "combi" boiler for entire building, both upstairs and downstairs separate unit.

Equipment originally specified:
IBC DC15-96 "combi" boiler plus piping, valves and pumps (see above).

Main concern with the current design is the DHW which is coming off the Combi boiler (only outputs 2.1 GPM) and the potential for short cycling as the loads are so light for heat, plus cycling for DHW. There is room in the utility room for an indirect tank.

Looking at switching boiler to HC series IBC (smaller unit HC 13-50) and adding an indirect tank (say 30 gallon or maybe 40 gallon). Wondering if the air handler can simply take its feed (directly) off the DHW loop set at 130F or so and thus the tank would also buffer this load? Searching for smaller boilers like the HC13-50, only options I am finding are the Viessmann 200-W (B2HB 19-68) and also Navien NHB-55. Not sure if there are other options out there worth considering on the boiler side. Building is located in Ontario, Canada and I am learning that not all boilers out there are necessarily available in Canada but the above noted options are.

Any thoughts and suggestions on the design and/or equipment choices for this would be most appreciated!!
Thanks for reading my long note!

EBEBRATT-Ed

This is a lot to unpack. Sounds like what they are doing is ok. You have some hi temp loads and lo temp loads. It is hard to "see" without a piping diagram but I would think with all that going on with a low water content boiler that a buffer tank would be needed

Ironman

You could use a Heating only boiler + an indirect and a buffer tank if the boiler doesn’t have a low enough turn down.

The indirect could also be used for buffering if the space heating is connected to the hydronic side like this:

flyboy182

EbeBratt and Ironman thanks for the replies. I attach the requested diagram below. Note DHW on this diagram shown as on demand from the combi. On a separate notes sheet the designer outlines the heat load for the air handler I mentioned earlier but doesn't give the expected heat load for the floors (but I gave square footages). Ironman sorry for the basic question but in your diagram would the large hydronic load on the left side represent the air handler? Thanks.

Ironman

No, the zones on the left would be the radiant floor which could be controlled by the boiler's ODR curve instead of a mixing valve.
The AHU would be the loop tapped off of the hydronic connections beside the indirect. Just substitute the AHU for the micro zones above the indirect. No mixing valve would be needed there.

Ironman

Also, you're diagram is not correct: there's no need for the secondary loop to be closed and a pressure bypass valve (far right).

hot_rod

Another option is a manual 3 way mix valve. It floats along with the ODR on the boiler that runs the higher temperature demand, AH.

A motor could be added to the 3 way if you want a separate ODR function.

DHW from the combi.

flyboy182

Definately don't want the DHW from the combi because it won't keep up with expected demands. To get one that will, it is way oversized for the heat loads...hence to desire to introduce the tank. Ironman back to your schematic, is this indirect tank one that has two sets of outlets...I think I have seen them for solar applications. Is that was is used here, where one is the heat loop and the DHW is kept separate?

flyboy182

something like this I mean for the tank?

flyboy182

Ironman I stated that wrong, while this tank indeed has two sets of inlets/outlets, it also of course has two coils. If the bottom coil was hooked up to the boiler, and the upper coil hooked up to the main heating loop that fed the AHU directly, and then fed the in-floor loops tempered with mixing valves off of that main heating loop...would this design be workable? DHW directly off the tank (also likely with tempering valve).
My logic is that the tank would then buffer the whole system, reducing cycling. If the tank temperature had to be moved up in winter months to keep up with AHU demand, DHW would be tempered down accordingly The design heat load for the AHU was only 21,000 BTU/h at 140F, so tank temperature low point may be set to 130F for the DHW and the heat loads may seldom be the driving force calling on the boiler to fire. Let me know please what you think. Thanks!

Ironman

No, a double coil tank is not necessary.

The TT Smart tank in the diagram is a tank in tank design, but the concept will also work with a single coil tank: you would Tee off to the AHU from the HYDRONIC supply and return connections at the indirect. The hot water stored in the tank is the buffer and heat source that heats the hydronic loop to the AHU. As the temp of the water drops, the indirect's aquastat will turn the boiler back on to re-heat it. The outdoor reset curve of the boiler is bypassed during a domestic call from the aquastat, so the boiler sends 180* water to the tank. The aquastat should be set to at least 140* and an ASSE 1070 tempering valve be installed on the domestic outlet.

The part of the concept which you seem to be missing is that heat energy stored in the domestic side of the tank can be transferred back to the hydronic side and utilized by the AHU.

A heat exchanger is just a dumb piece of metal and it will transfer heat in either direction since heat moves toward cool.

We've used this set up multiple times and it works great.

A link to info on the Smart tank:
https://s3.amazonaws.com/s3.supplyhouse.com/manuals/1297270226861/47062_PROD_FILE.pdf

Ironman

A slab floor does not need a buffer since the high mass of the concrete IS the buffer.

Because of that high mass, the Supply Water Temp to the floor should be controlled by ODR which will vary the SWT based upon outdoor temp so that the mass of the slab won't overheat and continue to give off heat long after the the thermostat is satisfied.

The slab should only require 75-100* SWT, so it makes no sense to have the boiler make 180* water then send it to a buffer tank and then have a mixing valve reduce the SWT down to 100* or less.

The boiler will operate most efficiently with the coolest Return Water Temp to it. Letting it directly contol the SWT to the floor is the simplest and most economical approach.

flyboy182

Thanks Ironman...I might be actually starting to get this! Really appreciate you staying with the neophyte here. Best regards.

Ironman

Basically, just follow the diagram I posted and the pink and blue lines Tee'd off near the indirect would go to the AHU. You'll need the extra circulator and check valves that are shown, but not the mixing valve.

Ironman

You can also substitute a low loss header like what's shown in your drawing for the closely spaced Tess near the boiler in my drawing.

A Caleffi Sep4 would be an excellent choice.

flyboy182

Ironman; Since the air side of this design includes an HRV system that must continuously (tight house), I guess that means that I will need motor controlled thermostatic mixing valve on the AHU supply loop because the fan will be running all of the time to keep the HRV system operating (on circulation mode of the AHU). Not sure if a smart thermostat manages that mixing valve or the boiler control logic?

Ironman

No, just a switching relay to turn the pump off to the AHU. The relay would be controlled by the thermostat.
Taco makes a couple of them to specifically control AHUs. I can’t recall the model number at the moment.

motoguy128

A combi in a multi use, multi tenant setup is a bad idea. Especially with low mass radiation or fan coils. Plus you can then use a smaller boilers with better turn down ratio. But up size the DHW tank due to occupancy.

flyboy182

Thanks motoguy128...seems like I am on the right track with a change out to a tank based system and a somewhat simpler boiler. Some of the tank specs seem to specify minimum heat inputs...are those guidelines or hard rules. For example I was thinking of using about the smallest boiler available in my area (example IBC HC13-50) given the modest loads involved with this build, but tanks want more heat input like for example the 30 gallon TriangleTube brand that Ironman suggests wants 87,000 versus the 50,000 the above boiler can output. Is this a worry or does it simply mean the boiler will run a bit longer if there was a large draw on the tank for DHW?

Ironman

Size the boiler to the space heating load, not the domestic. Just increase the tank size for more storage if needed.

hot_rod

Kind of a guess on your best choice unless you define the DHW load. You either generate the DHW instantly with a combi or tankless, or store enough to cover large dump load draws. Then wait for the tank to recover to a useful temperature. Of course it is a BTU game regardless.

The more and faster you want DHW, to more horsepower you need. In some cases the DHW load is the larger and you need to size to it. Keep in mind with a lower output boiler the heat goes off while the tank recovers, so don't scrimp too much.
A modulating boiler is ideal for lopsided loads, a 199K with a 10-1 turndown would give you DHW horsepower and modulate to small heat loads.

Homeowners and tenants tend to learn what DHW is available and live within that, the Air BnB may be another guess as to what they want or expect. A tankless just for the AirBnB portion would be sweet, a 130K tankless would give you at least 2 gpm non stop.

With an indirect tank you can elevate tank temperature to 150- 160 or higher and add a thermostatic mix valve to extend the drawdown.
Actually with a rental you may want to include a listed mix valve set at 120° for liability reasons. Plumbing codes list 120- 122° as max DHW supply for residences. Elevated tank temperature also protects for legionella potential.

flyboy182

So I went back to the architect today to ask for the detailed heat loss calculations that were used for the design. He sent those along which is like 21 pages of detail. I don't understand a lot of it, but what is clear is that the heat load I indicated earlier for the AHU was overstated. The load for the whole unit for space heat (ie AHU plus the main floor in-floor) at the design winter condition is like 22,000 Btu/h. Very light load indeed. So this got me thinking...

Is it possible (and of course legal...can I get it to pass inspection etc?) to run a whole house off of a gas fired hot water heater (with a secondary dual coil indirect tank for the heat loads). This would give me DHW directly off of the main DHW tank "boiler". Then take a loop off of the DHW tank output/input to feed one coil in a dual coil indirect secondary tank. Off the other loop in the indirect tank you have a heat loop to feed heat to the AHU and the in-floor circuits. So you have a dual coil buffer tank for the space heat that is indirectly fed by a DHW tank.

Ideally if there is a product that can automatically provide outdoor temp based adjustment to the tank temperature of the indirect tank that is providing the space heat...that would be even better.

Am I talking complete nonsense?

flyboy182

I must be talking nonsense because no one is responding! Ha Ha.

Ironman

I believe that what I gave is the best and most economical design.

Zman

flyboy182 said:

I must be talking nonsense because no one is responding! Ha Ha.

I'm still scratching my head

flyboy182

OK Ironman thank you. I proposed your solution to my builder and he will be discussing it with his HVAC sub and let me know. Zman, sorry for the headscratcher! Best regards guys. Have a great day.