solar / radiant heat interface?

Is_solar · December 2011

I have an existing radiant system consisting of an oil fired boiler and a Wirsbo propanel 311-9.

I would like to add the solar heat from my 600 gal unpressurized tank. There are copper coils in the tank waiting to pull heat out of the tank.

I want to add the simplest, most elegant piping, control system.

One idea was to set up a primary loop from the solar tank and inject heat into the radiant loop with an injection pump. Another idea would be to keep the radiant system and the solar system seperate and interface them with a flat plate heat exchanger cut into the radiant system.

Any ideas or suggestions about piping or control would be appreciated.

NRT_Rob · December 2011

In my mind

there are only 2 ways to tie radiant and solar together that are worthwhile.

Simple: Delta-T control watches return water from radiant and tank coil temp. If return is colder than coil, divert with 3 way valve through coil (or inject with pump for high flow applications). Especially rocks if you're doing higher delta-t radiant or low temp radiant so return temps are close to room temp. will need a "boost heater" for DHW in that case as with any luck you'll pull that tank down to close to room temp most of the time.

More involved: Use the radiant as a "dump zone" with a high limit thermostat defining how hot you are willing to let the room go. that thermostat will open a zone/run the pump only when it is not too hot, and the delta-T control contact is closed indicating that the return return is cooler than the tank. This is nice, because when you have solar gain you might not have heat demands, so this allows you to dump heat into the house even if you are at "regular" room temp and 'charge' the house before allowing the tank temp to rise.

these are nice, fairly simple things to do. they also minimize tank temp which maximizes collection.

hot_rod · December 2011

temperature requirements

do you know what temperature your zones require? It looks like you have one mixed down temperature zone. Depending on what type of collector, and how many you have possibly all you will be able to supply is that mixed down temperature zone.

Your best use of the solar is to cover DHW loads first. IF you have enough collector array to cover that and have some additional energy, only then would I consider tying into that low temperature zone.

No sense spending a lot of money to tie into the heating load if you don't have enough HP to power it.

So

How much DHW do you use per day? What is the heating load and required temperatures?

How many collectors? Evac tube of flat plate?

Where are you located?

hr

Fortunat · December 2011

Lots of options

I like both of the options that Rob (NRT) suggests, but I don't know that I agree that those are the ONLY choices which make sense. Generally I prefer simpler tie in where possible, and as rob says, the lower you can pull the tank temperature, the better.

Another design we've used in the situation you describe is to use a dual injection strategy from the tank and from the boiler (see attached sketch). If I remember right we controlled it with a Tekmar 363 which worked pretty nicely.

~Fortunat

www.revisionenergy.com

NRT_Rob · December 2011

we've done that too

but in that case you can only use the solar if it satisfies the load (higher temp) and it's a much more expensive integration (363 is a very expensive controller).

Pay more, for less solar contribution... I don't see the additional benefit. I know the "it saves a boiler cycle" argument during the shoulder seasons, but my stance that anything that puts heat in the envelope saves boiler cycles over time, as it is heat loss over time that causes heat demands and then boiler demands.

Interceptor · January 2012

tekmar 363

I'm using the Tekmar 363. It seems like the simplest way to get integrated boiler control and solar control from one box. I don't have my collectors running yet, so I can't say how well it actually works.

Here's the problem I have with it. I have one zone with light radiant floor, one zone with heavy radiant floor, and I want to add a third zone with an oversized cast iron radiator. I'm using the 369 zone control. You can only use a zone control with the dip switch on the 363 set to "characterized heating curve". This means you need to select a heating terminal unit. This is fine if all of your zones use the same type of terminal, but mine don't. There's no way to select a simple reset curve when using a zone control, and no way to select separate terminal curves for each zone. The two radiant floors are close enough that they work fine on the same curve, but I don't know how the radiator will work on a floor curve. And if I put the radiator as a boiler zone it doesn't get any solar supplement.

I'm still happy with the 363 at this point, but if I'd known about these limitations I may have done something different. I would like to use my slab as a "dump zone" for excess solar, but I'm too deeply invested in the 363/369 to drop them now, and I don't know how to make it work with what I've got.

NRT_Rob · January 2012

too bad

but not crippling. the 363 won't do "dump zone" control though, you'll need your solar controller to handle that.

use the high mass radiant curve for the radiant zones. Radiator will work fine as long as your design water temperatures are similar (my Rule of thumb is 20 degree spread per curve in design water temp requirements).

sure would have been simpler to do return reheat though

Interceptor · January 2012

Thanks

Thanks Rob. I'm sure it will work, it's just a little disappointing after spending so much money to find out that it's not "perfect".

Is_solar · January 2012

closely spaced tees?

With everyone's help im ready to do the solar tie in.

my last question is wheter to use closely spaced tees or not.

Ive tried to show, in the diagrams, my two ideas. One is to use closely space tee as show on the top.

Im thinking that the diagram on the bottom will get the cooler return temps to the solar tank and be more efficient.

Thoughts?

Fortunat · January 2012

none of the above

My suggestion would be to use the closely spaced T's, but to put the solar injection upstream of the boiler injection so that, as you note, the solar coil is always getting a crack at the lowest temperature heating.

In the case of your other drawing ( bottom drawing), you haven't achieved effective hydraulic separation between the loops so you'll be fighting ghost flow in the solar tank coil and uneven flow in distribution depending on which pumps are running.

good luck.

~Fortunat

www.revisionenergy.com

Is_solar · January 2012

Wirsbo panel

Fortunat,

I'd like to do what you suggest but look at the picture of the pre built Wirsbo system Im trying to deal with. Im not enthusiastic about cutting into it. Maybe i should just toughen up and do it.

Karl_Northwind · January 2012

preheat

I agree with Fortunat, put the CST's upstream of the boiler injection point.

how many SF of panel are heating this 600 gallon tank? I'd just like to hear more about the system as it's built.

I have something similar although a 200 G tank, but the system's low flow enough that I just run all the return water thru a diverting zone valve and control that with the auxillary delta T control on the Caleffi BX (BTW, I love that controller. even using the timed aux relay to control my homebuilt off-peak heating system) I'm able to pull the tank down to about 75F every night in cold weather (think 20 degrees or so) in colder weather I only pull it down to about 90, because the return temps are in that range.

let us know how it works out.

cheers,

karl

solar / radiant heat interface?

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