Review ConMod / Radiant / Indirect Plans

MikeCo

We are replacing a dead steam / hyrdonic sidecar system with a new radiant system. Radiant loops are installed & pressure tested, and the next step is to finalize the boiler / DHW plan. We would like some feedback, as this is not something we have planned before, and it's been daunting to research all of the pieces and get this far.

Total thermal load across the radiant zones is approximately 32-35K BTU/Hr, so the Greenstar 151 should be able to modulate down well (close to sea level), and still meet the indirect tank peak loads (~115K BTU) w/ DHW priority. Total flow for radiant loops is ~6.5 GPM, approximate breakdown (not including head loss for manifold + boiler piping + balancing valves):
Mudroom Zone: 1.5 GPM @ 3.5 ft. head
1st Floor Zone: 2.3 GPM @ 2 ft. head
2nd Floor Zone: 1.6 GPM @ 2 ft. head
Basement Zone: 1.3 GPM @ 2.5 ft. head

Indirect Tank (Ideal): 14 GPM @ 3.5-4.5 ft. head

Based on these numbers, it looks like the 3 speed Grundfos pumps should fit our needs with the available curves. The heated space is ~2250 sq. ft. (including basement) with 2 bathrooms, 1 dishwasher, 1 washing machine (very unlikely we'll ever pull more than 3 GPM of DHW for regular sustained loads, and that's probably an overestimate).

As this is our first time planning a heat / DHW system, any and all feedback is appreciated, as we don't know what we're missing or over-engineering.

hot rod

You need a pump on the boiler side of the sep also, maybe one inside that boiler?

Do you need all those mixing valves? Run the boiler at the highest required zone, mix the others.

If all the temperature requirements are within 10 degrees or so of one another I would not bother with additional mixing, you might run everything directly from the sep without thermostatics?


I prefer the indirect on the boiler side, it sees the hottest SWT without blending thru the sep, and only need to run one pump. Not a big deal it's done either way.

MikeCo

@hot rod The boiler has an internal circulator pump, from the manual curves are:

From those curves, my interpretation was that the 14 GPM flow rate for peak indirect tank performance could cause cavitation or other issues if it wasn't on the secondary side of the separator (more precautionary). Alternatively could probably use closely spaced tee on the primary loop to create another secondary zone, seemed better to just put it on secondary side in this case since DHW priority should keep the blending temp close to the output temp? I'm also unsure where the output temperature sensor would go in this case (or if it's not used by the boiler in DHW priority mode)?

What I don't know enough to tell from the documentation is, at the low end of modulation range, how much (if any) cycling would be happening when not all zones call for heat at once, especially if using outdoor/indoor reset curves. The mixer before the manifolds I believe is necessary to ensure the safe maximum surface temperature for my engineered wood floor warranty. The mixing valve on the first "mudroom" loop may be unnecessary, since it's embedded in a slab overpour, which should mitigate temperature fluctuations if cycling occurs?

hot rod

Here is an example of the indirect connected directly to the boiler, only P2 runs and full output and temperature goes to the tank on DHW call. It is the quickest most efficient path from boiler to indirect. P2 sizes to whatever gpm you desire of the boiler can handle, 14 gpm might be a stretch for that boiler design.

Depending on the lowest turndown of that boiler, if the smallest zone calls alone, yes you will end up with some short cycling.

This pic shows how a buffer could be added if the cycling is too much to bear. The boiler may have control features to help limit cycling, ramp delay, anti cycling parameter, etc.

If you set up the ODR on the boiler you could set a not to exceed temperature to the zone that has a max. temperature limit, it would protect the floors. You could also add a thermostat with a floor sensor to watch and respond to actual floor temperature.

If the mud room slab needed a lower temperature, use a single mix valve for that. It would greatly simplify your piping and component cost.

I suspect you could run everything at one temperature, the mud room probably isn't SWT sensitive if you are not living on it

If you did a full on radiant design it should spell out not just gpm to each zone, but also SWT for comparing.

Zman

Does the aluminum heat exchanger in the Bosch have good track record in your area? That type of exchanger is pretty sensitive to water quality.

What are you basing the boiler sizing on? What are your DHW needs? Unless you have extreme DHW requirements, size the boiler to the heating load and upsize the indirect to give you more storage if needed.

Be mindful of the CV ratings of the mixing valves, the ones designed for DHW will have too much resistance.

There is no need to mix the higher temp zones, you can just modulate the boiler output to the design temp. Use outdoor reset if at all possible. You could use a non thermostatic valve for the low temp zone. It would then have a reset curve that is parallel to the hotter zone.

I agree with hotrod that the DHW would be better on the other side of the separator.

Zman

I don't think you need the mixer to protect the flooring. The boiler will run at whatever temp you set it for. Even if it puts out a slug of hot water when finishing a DHW call, it won't heat the floor much.