radiant system topology question

Grappa

Hello all,  I have what might be a simple question.  I've been pondering a low temp radiant system for an upcoming house we're planning.  I'm dreaming of system that uses solar water for the first stage and a modcon for the second stage. I'd like to use the solar to fill in for the cooler water calls.  The solar would probably be added later in life, but I'd like to prep for it in the design. The solar I envision is a drain back / big open tank design.



My though is to pipe the boiler and the solar as secondaries, and use a 2 stage controller to run the show.  I think I want to use a submerged coil HX in the solar tank to inject into the primary.  My great idea was to use a Taco 00-VV voltage following pump to pretend to be a modulating "boiler", still with me? 



In my mind when a stage 1 call asked for more heat, the 00-VV would speed up and we'd inject more heat into the loop, perfect!.  BUT, I think there might be a flaw in my logic,  and that leads to my real question.   When I speed up the pump, the solar secondary flow will increase, wouldn't that solar water temperature go DOWN because of the increased flow through the solar HX OR does the greater amount of cooler water injected into the primary do what I want it to do??  Fire away!



The less you know about something, the easier it seems to do it...



Thanks,

Paul

Ironman

Solar Control

The proper solar control would slow down or turn off the solar pump and secondary pump if the panels weren't contributing sufficiently. Good question.

Grappa

Missed the mark

Thanks Ironman, but my question was about getting the heat out of the tank, not into the tank.



For the sake of discussion let say I already have this big tank of warm water ready to go.  I get a zone call, the control asks the solar first stage for more heat, the solar injection pump will increase speed.   What happens next is the heart of my question.



PK

Charlie from wmass

Caleffi hydronics

has some good simple books that give a basic understanding of how to set up solar as you would like. In fact I am reading it right now. It is their July 2009 Idronics catalog. Page 38 figure 5-16. That also has a wood boiler adding heat to the system. It is the # 6 edition at this link http://www.caleffi.us/caleffi/en_US/Site/Technical_library/Idraulica_magazine/index.sdo

Grappa

Close,

Thanks Charlie, I've read that already, that's where the idea got started.



The boiler/stove/solar idea is the closest, but they used the storage tank as the transfer path from the boiler to the heat loads.  My uneducated guess is using 2 water connected heat exchangers isn't the best way to move heat, but I could be wrong.



PK

Ironman

Don't Think I Missed

I may not understand your question correctly, but if I do, you don't understand my answer. I've got systems that do exactly what you describe and the solar controls operate both pumps, and more, accordingly. The injection pump does not operate independently or simply upon a heat demand call. The zone pump(s) do that. Anything that is a part of the solar heating contribution is controlled and co-ordinated by the solar control(s). I'm referring specifically to getting heat out of the tank and injected into the primary heating loop.



From there the zone circ(s) pick up the heat and take it where it's needed. If the charge in the solar tank becomes too low, the solar controls slow down or turn off the injection pump and the boiler begins to take over.

Grappa

I think it's me,

Thanks Ironman,   Yes, it very well could be that I don't understand your answer, or even my question.  I do appreciate the feedback, please feel free to set me straight on anything/everything.



I see that flow rate and temperature interact.  I'm electronics and software by trade, so the heat & water flow is not intuitive yet.  Flow and pressure equate to electron flow, the heat transfer with flow still throws me. 



My system (on paper) has several radiant zone valves w/ 1 pump on the primary loop.  The secondaries are the solar tank with a submerged HX and pump, and a modcon boiler with built in pump. Both secondary pumps are controlled by a 2 stage boiler control.  The solar injection pump would have an additional "disable" when the primary loop temperature is greater than the tank temp. My target terminal units would like to be low mass floor radiant, all using the same temperature.



For now it's just at a high level so I'm ignoring the input to the tank. I'd like to try to get a feel for how this might work, and what happens with flow through the solar tank heat exchanger. 



1) Is a 2 stage control the proper way to do this type of system? 

I've had a lot of guidance from Tekmar that point me that way. I do like the features they provide, but I'm open.



2) Does the solar HX act like a little on-demand water heater, where the

output temperature rise is dependent on flow rate?  Or does that just mean my

HX is too small for the job?  If the output of the HX is fairly constant over a usable range of flow, the pump modulation idea should work?  Maybe I need to read about the Pri/Sec flow again.





3) I'm under the assumption that with full outdoor reset, that the zone water temperature NEEDS to be low and flow often?  With that, I assumed that the solar needed to be modulated in some way? Can I inject full temp solar water less often and get the desired results?



Basically, should I control the solar output temperature by modulating flow, or run a wide open temperature and let it duty cycle?



Thanks again,



Paul

NRT_Rob

I strongly prefer

a technique called "return water reheat".



run the system with a boiler or whatever. use a 3 way switcher on the return from the heating system. this is the coldest water in the heating system.



compare that temp to the tank extraction coil temperature. If return water is X degrees cooler, switch return flow to the tank coil.



it will pick up however many BTUs it can, and it will keep the tank as cold as possible while using every BTU possible.



I don't see many arguments in favor of other methods, really.

meplumber

Take a look at the controls offered by Steca.

They have a line of controls that can do what you want in a plug and play format.



Also, for piping.  What used to be HS Tarm NA (maybe BioHeat NA now), has some great piping diagrams for piping out of storage tanks. 



One other source is a company called STSS (I think, I will check my card file to confirm).  They are a tank manufacturer that also has controls available.



I have a couple of similar systems in operation.  One of them uses solar, wood, and a Viessmann Vitodens all in concert.  Works amazing.  The solar and wood feed the storage tank with 4 copper HX coils inside.  The 4 coils feed different parts of the system and the Vitodens makes up the difference via the boiler sensor and outdoor reset curve.



Good Luck. 

Grappa

Thanks,

Thanks MePlumber,



I checked out Bioheat USA, (redirects to woodboiler.com).  The open storage and the heat exchanger is what I was thinking of using, but I couldn't find any piping details. 



I would like to set up a system that uses either the solar tank or the boiler, but maintain separation.  I'm hopeful that I won't need a buffer tank for the boiler.

NRT_Rob

we're using another

brand of open tanks with great insulation and coil options, and I like the idea. I'm very intrigued with unpressurized drainback, and reducing coil counts for the remainder of the system... shared coils for multiple purposes and the like.



Keep costs and complexity down that way.

Grappa

Not sure

Thanks Rob,

Let me see if I understand what's happening.  The warmer zone return water goes straight back to the boiler and the cooler return water gets routed through the solar tank , heated, then back to the boiler?

NRT_Rob

not AND

OR... the water coming back from the zone is either hotter than the tank sensor, or it isn't.



if it is hotter, it just goes straight back to the boiler. No solar contribution possible until zone return cools down, or tank heats up. No heating of the tank by the boiler possible.



If it is cooler, the 3-way valve flips and it goes to the tank coil to pick up whatever heat it can.

Grappa

Thanks

Rob, at this point I'm not so concerned about brands of tank, or controls, but the method of heat extraction and how it is piped into the system.



So far I've seen 3 methods to add open solar including what you posted above. 

A separate tank with a heat exchanger, injected as a first stage

A community tank where solar, boiler, wood, etc add heat

And the return preheat method (if I understood it) described above.



If there is any more, I'd like to know.



I don't really like the idea of a community tank.  That makes the tank mandatory for the system to run, and I won't have the tank area built until the second year of the project.



If I understand the return preheat method, I could see that extracting the most amount from the tank when there is low return temps, but doesn't that screw with boiler condensing?  And if the tank is hotter than the return, it goes unused other that maybe DHW preheat in a separate HX?

NRT_Rob

water goes through the boiler

whether or not it goes through the solar tank.



if for some reason it made the difference between condensing or not on a condensing boiler, yet still required the boiler to run, you're talking a few percent efficiency on a fractional load vs the "free heat" you picked up in the solar tank.



if you couple this with low temp design... as you should, if you are trying to use solar to heat, which is very hard... your solar tank could be kept at 80-90 degrees for optimal solar collection as well... getting more BTUs, and using them better.



only downside is solar gain might shut down your zones. if that is very likely, you might need an "indoor heat dump" strategy.

Grappa

So with

that method the boiler always comes on with every heat call or the water just circulates through the jacket?  



I don't have house plans yet, so I don't have any real data.   What I have noticed is that it seems the DHW loads drive the boiler size in an efficient home?  It seems I find quite a few threads on modcons short cycling, and I'd like to head that off.



While searching for low temp radiant install methods, I stumbled across DIY solar and thought it might be worth a look.



I do agree it's easier to get plenty of lower temp water in Michigan, but I'm still unclear how best to use it.



Thanks for the feedback.

NRT_Rob

no

the boiler only fires if the water temp is too low. otherwise it just circulates through.



DHW load should drive indirect sizing, not boiler sizing, unless it's a huge DHW load.



just be aware that solar thermal space heating is very rarely worthy any significant investment from an economic standpoint. You need very large arrays to make any dent in fuel usage and it is almost never cost effective. But I'm not familiar with the economics of DIY solar so I dunno how that stacks up.

Grappa

Okay,

I figured that was what you meant.



I've seen it both way, some say sized the same as "normal" some say much smaller? I don't have house plans yet, so I wouldn't even guess at the BTU requirements yet. (hopefully small)



Yea, I know that but I like solar. (NO, I don't drive a hybrid either) It sure would be cool to use that energy somehow.   builditsolar.com has a lot of DIY solar ideas.  Some good, some not so. 



I still think my first stage fake boiler idea could work.

NRT_Rob

it can work.

but, it's more complicated, more expensive, and you can only use the solar when it's hot enough to satisfy the demand by itself. This means you use solar less often, and your tank is hotter, so you collect less solar as well.



that's why I say I don't see much of an argument for other methods.

Grappa

Yea, I think I tend to agree

Thanks Rob,

but until I get the house prints back, get some heat load data and a solar site survey I'm guessing as best.



I think I still want to do a low temp radiant system, but I waffle over low vs. high mass. Any comments on that?

NRT_Rob

depends

low mass is easiest to control, high mass adds more buffer capacity and passive solar storage capacity... and room temp overshoot in solar gain situations.



both have pros and cons. we do mostly low mass these days but they can both be great and if you did concrete with a polish surface that's about as ideal of a radiant emitter as is possible.

Grappa

I like

the look of polished concrete, but my wife likes hardwood.  So far I'm leaning toward low mass on the top side of the subfloor with engineered hardwood over that.  I am concerned about squeaks.