DHW heat source plus HX to closed RFH

peter_19

Hi everyone, I am leaning toward a Polaris DHW as my heat source, and I want some feedback on the alternatives I am considering.

I have a 2800 sf space with a max of about 60000 BTUh heat loss (about 21 BTUh/sf). I'm using in-floor PEX where there are new floors going in, and either ceiling or staple up in the retrofit areas. I also want a lot of DHW capacity (far more than the "tankless" coil inserts of the typical "combi" boilers). I have some serious hot water pigs in my family. I mean I want to run the dishwasher, a shower and fill my whirlpool tub at the same time. And then have enough left for a 20 minute long shower with the 3-head 8GPM shower wall. ;)

As I see it I can serve both of these needs with a single heat source and a heat exchanger. I can either connect the heat source directly to the RFH, and indirectly to the DHW, or do the reverse... connect the heat source directly to the DHW, and indirectly to the RFH.

In the first scenario, the best option seems to me to be to go with a mod/con boiler and an indirect DHW tank.

In the second scenario, I see a couple of alternatives. You could go with a condensing tank water heater (e.g. Polaris) and then use a pump and HX to supply heat to the closed RFH system. A cheaper (but less efficient) alternative would be to replace the polaris with a tankless DHW (e.g takagi) connected with a pump to a well insulated buffer tank.

My concern with the mod/con boiler and indirect DHW is that due to the weather here in the SF Bay Area and the fact that I want a fair number of zones, it is likely that I will have short-cycle problems unless I go with a pretty small boiler. But I don't think a small boiler can provide me with the DHW service that I want, at least not without a huge (e.g. 119 gal) tank, which would be both expensive and inefficient (the tank would be in an unconditioned area), and would have a very long recovery time. Its also a pretty expensive combination.

Reversing the heat exchange and going with a DHW heat source seems like it would work better. I am leaning towards something like the Polaris. When it comes to the DHW its a pretty killer appliance, although quite expensive for a water heater (but still probably cheaper than the boiler+indirect). The efficiency is quite high which is good for the earth. The only negative I can see is the expense when compared to the tankless option.

And so we come to the tankless. 200KBTUH input provides a lot of hot water, and combined with a tank you eliminate any low flow issues, or simultaneous use issues. But its not exactly what the appliance was designed to do. Its not too far off, but I would worry a lot about how much less efficient it would be, compared to the Polaris. If its just the difference between condensing and non-condensing, then I think the expense of the Polaris is difficult to justify.

So I have these questions:

Would a mod/con boiler + indirect be a good solution? How hard will it be to address short-cycling? How big a boiler/tank do I need to get the kind of DHW supply I want? Which boiler? (I like the Vitodens 100, but most people around here seem to use Munchkin)

If not using a boiler + indirect, which DHW heat source would you choose? How much efficiency difference do you think there would be between a Polaris and a Takagi-TK3 plus tank combo? I'd suggest a Takagi TH1 but its about the same cost as the Polaris, so I can't see any advantage. There is a new tankless from Navien that claims some ridiculously high efficiency at very attractive low cost, but I'm not sure I want to be a beta tester. :)

Finally, for my retrofit areas, would you recommend staple-up or ceiling? No one around here seems to do ceilings, so its likely to be staple-up. If it is, do you think transfer plates are worthwhile? I think it would be cheaper to run two different water temps (hotter water in the staple-up, cooler in the in-floor sandwich). Oh yeah, one last question... Gypcrete or WarmBoard-style for the sandwich? I'm leaning toward Gypcrete but I worry about response time.

Thanks in advance!

hot_rod

Here is another option

I just finished a winter with this set up. It is a HTP Phoenix modulating water heater 130,000 input. It also has a solar coil in the bottom.

I added a Taco RMB radiant mixing block with a HX. That supplies 3 zones of radiant floor. It is a bit complicated looking as it is a test bench of sorts, notice the additional Grundfos zone pump.

Even with small radiant loads it doesn't cycle often.. It has the 80 gallon volume and the modulating feature to provide nice long run times. 130K supplies plenty of DHW, almost non stop at a 3 gpm flow rate.

The Phoenix is available with or without the solar coil in the bottom. I expect to get 100% soar DHW most of the summer months with 40 square feet of panel.

With SRCC listed panels this should qualify for the $2000.00 Federal rebate plus any state rebates for solar DHW and heat.

Morgan Muir up near you sell this package of Taco and HTP.

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peter_19

Thanks for the reply hotrod. That sounds a lot like what I am thinking of doing, except with the Phoenix instead fo the Polaris. How long have you had that setup? I like the idea of getting the solar coil, although I am not planning on solar at the moment, but I would like the option to add it in the future.

What kind of radiant floor have you got (gyp? warmboard? staple up?)

hot_rod

I have a few Polaris

with plate HXers out there also.

I like the Phoenix a bit more with the modulation feature. It seems to run a bit quieter also. I just have one heating season on this. But I have Polaris systems out there, with external HXers since 1999. Two out of six have leaked and were replaced under warranty. The new version polaris has some nice improvements. Be sure to elevate it on a couple concrete blocks to make servicing easier.

I have 3 zones of radiant in slab.

I am a big fan of Warmboard. it may be the ideal dry system. The Roth foam panels work well also for retro fitting, or maybe even new work. About the same gauge aluminum as WB but a bit of foam r-value.

Gyp is a good installation, a little tougher for nail down hardwood applications, however.

I like the instant response of WB, Roth, or any aluminum covered dry systems. Not as pleased with the bare tube above floor systems.

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peter_19

I was thinking Gyp might be a better idea with nail down hardwood... if the installers miss the wood firring strips they hit the gypcrete instead of poking a hole in the PEX.

hot_rod

wood furring strips

and gyp pours are a bit of a hassle. They need to be wide enough to nail into without splitting. A flat 2X4 works best. Good luck finding straight ones that are stable. they cut them from such young trees these days.

All the water in gyp makes them swell up a bit, then the shrink back down when the heat is on. The must be glued and screwed to keep them tight and noise free.

You need to cross through them somewhere, somehow to make continous loops. Protect the tube in those grooves in the furring strips.

Gyp seems to always shrink down a bit when it dries. So the hardwood really isn't in contact (conductive transfer) with the gyp.

Calculate how much of the square footage will be in furring strips. sometimes it's hard to get enough tube in the room.

The seems to be more hardwood waste as you cut to fall on the nailing strips.

There is a place for gyp radiant, don't get me wrong. It is a nice medium mas system and offers thermal mass, sound and fire proofing, etc. By far a floating engineered flooring product is a much better match for going over gyp.

Check out some of the wood flooring from www.launstein.com They have done a lot of testing of wood over radiant and have some unique attachment methods.

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