Integrating Solar

Gordon_3

Hello,
I've been lurking here for a while as I have been trying to decide on the best boiler or water heater for my application (new, larger home in Northern California). I'm hoping someone here has experience with solar. I would like to integrate solar collectors, a storage tank, and boiler for the radiant heat, DHW, and possible future swimming pool. I was wondering if any certain boilers or heaters are better suited for incorporating solar than others? I was visiting the Viessmann web site and noticed they seem to manufacture all of the solar components, but I do not know of anyone that has used their solar products. Their solar collectors are not the standard flat plate collectors, but rather the newer evacuated tube type similar to those made by Thermomax. Has anyone here had experience incorporating solar with a boiler (or heater) for all the other water needs mentioned above? If so, any particular products stand out as best for this situation? Anyone have experience with the evacuated tube type collector? Any comments would be appreciated.
Thanks much,
Gordon

Paul Pollets

Solar

Gordon, give me a call at 206-783-4315. I'm based in Seattle and can help you.
Paul

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chris_24

where

in northern CA are you located?

send an email chrisb@mcn.org

as Paul said, there is help available!

Chris

Alan(CaliforniaRadiant)Forbes

I prefer

to keep it simple. If you want solar, dedicate it to making domestic hot water since most of the time you want to heat your house, there will be very little solar exposure.

I have yet to see an elegant control that incorporates solar, DHW and radiant.

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[Deleted User]

Solar

Lots of storage in a open tank non pressurized drain back prefered. Heatexchangers to each application. Works great if you have the sun and $$$$

Mark Eatherton1

tekmar 363

comes close. VSI from two different sources with one prioritized over the other.

ME

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Duncan_13

Agreed

I've been involved with solar since the 80's, and plain old drainback systems survive long after others have gone by the wayside. A lot of equipment and design strategies came and went.

It seems the reason for this is the inherent simplicity of the equipment. Not to say more sophisticated equipment doesn't survive the long haul, but the simple stuff definitely does.

I haven't seen the Viessmann evac tubes, but their equipment is generally top notch. Evacs are more efficient and compact, but usually pricier than flat plate. Less efficient equipment costs less than more efficient equipment, what else is new? If I had lots of roof space, or was doing a remote installation, I'd investigate whether I could get more btus from flate plates for $ spent. If I had limited roof space and/or a generous budget, the better efficiency of evacuated tube appeals to me, especially in cloudy climates.

Simple copper coils immersed in a solar storage tank are effective, simple, inexpensive heat exchangers. Preheater coil for DHW, first stage of two-stage heating for space heating (boiler as second stage) can both be easily done.

Jack_23

Back in the day

I did a lot of solar systems in N CA. For your purposes, a good flat plate is fine. You can buy evac tubes but they are big money. I used Heliodyne collectors and I believe they are still in business in the bay area. Excellent product back then. I agree with the open loop drain back system, unless you are in an area that sees a lot of freezing temps (mtns). A domestic hot wate system is the best app. for solar. If you decide to go with heat the non-pressurized tank with a ratio of 1.5-1.8gal/sq ft of collector was the way I used to size it. Keep your use temps low and solar works efficiently. Having said that you need to get some info from the CA Energy Comm. and have someone put a good eyeball on the plans. To much money to make a mistake!!! Good luck!

Jimmy Gillies

Solar.

After reading all the good advice you guys posted. Can you please help me? We have installed quite a few solar systems here in N.W. Scotland, they are mostly drain-back systems and work very well. However, do you think a twin coil hot water tank is better than two seperate tanks - with the solar tank feeding the indirect tank.
We have fitted both, but our panel supplier favors the twin coil, what do you think?
There is a 30% Grant available for this type of system here in the UK, so I'm sure we will be doing a lot more.
The last one we bid for was on the roof of 8 Flat block(new build). I love this stuff, free hot water in Scotland!!!
Kind regards.
Jimmy Gillies.

Duncan_13

Drainback and freezing temperatures.

"I agree with the open loop drain back system, unless you are in an area that sees a lot of freezing temps..."

Jack, I can tell by your post you know what you're talking about when it comes to solar heating. But consider this: All my experience in solar is in an area with LOTS of freezing temperatures. The systems I see that survive in Colorado are open loop drainback, regardless of freezing temperatures.

If the panels are designed and sloped to drain correctly, and the panel temperature sensor is located properly, chances are slim to none that you will have a freeze. This assumes a proper slope to drain and a reliable differential controller.

We used to see thermistors strapped to the return from the collectors, and on cloudy days this could cause problems with stops and starts - possibly leaving some water in the panels which occasionally caused freeze problems. Mounting the sensors on the back inside of the panels cured this problem.

The other common cause of freezing was small diameter risers. If the panel design was such that risers were small enough to hold a slug of water by capillary action after drainback, a freeze could occur. Pretty rare situation.

Oh yeah, one more... If the return tube from the collectors was submerged in the storage tank, draining problems could occur. The usual cause for this was an over-filled storage tank. The easy fix to this was to drill a relief hole in the return dip tube above any possible water line.

I agree about flat plates being the way to go for survivability: experience and the test of time bears this out. Absolutely agree that the design needs to be eyeballed by someone with experience in solar heating design; for the reasons above, if nothing else.

We saw a lot less problems with drainback than closed loop glycol. If the solar loop pump fails and the glycol cooks, you have a mess. If the solar loop pump fails on a drainback, no big deal. I'd have to say closed loop glycol comes in second in terms of survivability.

I used to think that drainDOWN was a good idea in theory, but the problems we encountered with the old Sunspool valves cured me of that idea.

Duncan_13

Drainback and freezing temperatures are OK.

"I agree with the open loop drain back system, unless you are in an area that sees a lot of freezing temps..."

Jack, I can tell by your post you know what you're talking about when it comes to solar heating. But consider this: All my experience in solar is in an area with LOTS of freezing temperatures. The systems I see that survive in Colorado are open loop drainback, regardless of freezing temperatures.

If the panels are designed and sloped to drain correctly, and the panel temperature sensor is located properly, chances are slim to none that you will have a freeze. This also assumes a reliable differential controller.

There were a couple common causes of freezeups, and once these were addressed, years of flawless operation followed.

We used to see thermistors strapped to the return from the collectors, and on partly cloudy days this could cause problems with stops and starts - possibly leaving some water in the panels which occasionally caused freeze problems. Mounting the sensors on the back inside of the panels cured this problem.

The other common cause of freezing was small diameter risers. If the panel design was such that risers were small enough to hold a slug of water by capillary action after drainback, a freeze could occur. Pretty rare situation.

Oh yeah, one more... If the return tube from the collectors was submerged in the storage tank, draining problems could occur. The usual cause for this was an over-filled storage tank. The easy fix to this was to drill a relief hole in the return dip tube above any possible water line.

I agree about flat plates being the way to go for survivability: experience and the test of time bears this out. Absolutely agree that the design needs to be eyeballed by someone with experience in solar heating design; for the reasons above, if nothing else.

We saw a lot less problems with drainback than closed loop glycol. If the solar loop pump fails and the glycol cooks, you have an expensive mess. If the solar loop pump fails on a drainback: new pump, no big deal. I'd have to say closed loop glycol comes in second in terms of survivability, but needs more attention.

I used to think that drainDOWN was a good idea in theory, but the problems we encountered with the old Sunspool valves cured me of that idea.

DaveGateway

solar

Duncan,
Good solar info, thanks.
My solar is not operational yet. This summer I hope!
Solar will be used to heat my 23' x 47' basement. Radiant is in & working in upper 2 levels of home.

What do you think of this design?
Overview: Solar panels heat a 1200gal. tank of water via a copper HX (quick efficent transfer 0-8hrs/day). The heat is then distributed to about 75 tons of sand & radiant concrete slab (slow transfer 12-24hrs/day)

5' (yes feet) below basement slab is the top of a 1200gal. plastic tank with 3x2' manhole extensions, insulated it with 6" of blueboard, filled it with water & backfilled with sand. At about 30" below slab, I put down 4-1/2" of blueboard everywhere, except manhole, including perimeter walls. Then I put in & leveled about 12" of sand. Then I put down 1000' of 1" thick wall black plastic pipe, no joints or overlap, it ends up about 6-12" spacing. Then covered the pipe with another 12" of sand & then a 4" slab.

This winter I built & installed a copper HX into the tank. The HX has 1" headers, reverse flow return, 4x equal length loops of 1/2" copper, 80' each, for a total of 320' of 1/2".

I was given 5x 4'x8' double glazed good quality copper flat plate collector panels. I will mount the panels 45deg facing south, on a ground rack of substancial strength, due to wind here. Behind & hidden by house.
I will slope panels & pipe (about 50' each way, of insulated 1" copper) so as to drain-back into a 40gal. insulated tank in the basement, air head of about 15gal.

A Taco 0011 pump will circulate from the 40gal. tank, through the HX, & out through the panels. This would be a closed loop. A Goldline GL-30 with a thermocouple in the 1200gal. tank & a thermocouple in the back of a solar panel, will control the Taco 0011 pump.
The slow distribution to the large thermal mass/radiant slab will have an airstat in series with another Goldline GL-30 & a 3gpm bronze DC pump, this would be an open loop.
Whats right & whats wrong with this system?
Thanks fo any input! BP

jim lockard

Photo voltic

Its the Electrican in me but are there not some photo voltic panels that generate an electric current? That power is used to constantly heat water? J.Lockard

Duncan_13

WOW!

That's a lot to digest, Bill! Lots of things to consider. 1) Did you run any numbers? I'm real curious about how and why you came up with this particular design. 2) Do you have any drawings?

It sounds like the huge thermos you have under your slab will store any amount of heat you can generate with five 4x8s, and there will be a large bubble of gentle heat under your slab once it gets up to heat, instead of 50°F soil. As you figure out the operating characteristics, I'm guessing you'll be able to add more panels over time, up to the capacity of the heat exchanger.

I'd say 1200 gallons is a lot of water to heat, and you'll end up with warm, not hot water. That's a good thing. Plastic tanks don't take high heat well, they get brittle and eventually crack. Since it doesn't sound like the tank is accessible for repairs, I'd keep the temps low... below whatever the maximum rating is for that plastic tank.

There's a HUGE amount of capacitance in your system, so I'm hoping the basement you're heating doesn't have any solar gain. It would be even better if it was a stable load, like all below grade.

I can't even begin to figure out the rate of heat exchange through the sand to the slab without a lot of number crunching. I'm guessing it will take a lot of trial and error to figure how much heat to put to that 2' of sand and 4" slab. Folks with four inch slabs heating their entire home have told me it took a while to dial it in for comfort, took some re-thinking and adjusting to get it right. And your floor has a lot more capacitance than that! Probably end up mixing to a fixed temperature and letting it cruise. Turn the heat on around July so it will be warm by September. Just kidding, but you get the idea. ;-)

Sounds like a very cool project you have going on! 3) Please stay in touch with me and let me know how it goes. I love this stuff! 4) Where are you located?

5) I find myself wondering what you capped the tank with, and how you compacted the sand over it?

Email me... duncanw at mindspring dot com

Duncan_13

Jimmy...

Could you explain how the twin coil hot water tank works ? What coil does what? One coil from the panels and one coil to the house?

The systems I've seen here are of two types...

One -the most common- uses a large capacity DHW-filled 'preheat' coil immersed in the hot solar storage tank. The water in the solar tank surrounding the coil preheats the DHW stored in the coil before it feeds the DHW into a standard tank-type water heater. The DHW flows under pressure from the street or well into the immersed coil, then into the water heater.

In the States, storage type water heaters are the normal way to go. 40 gallons of hot water in a tank with a burner under it. If the solar preheat can't meet the hot water demand, the burner fires up and acts as a regular water heater.

The other approach I've seen has a coil immersed in the solar storage directly coupled to the DHW tank water with a supply and return. A bronze or stainless circulator circulates through the immersed HX directly into the DHW untill the DHW storage heater is up to temperature.

Constantin

Very inefficient to use PV to heat water

If you consider the cost of a Pv system, its conversion efficiency, etc., it seems like a waste to heat water with it. Instead, take advantage of the much higher efficiency that direct heat of water allows via large HX's.

Where I have seen PVs used in conjunction with solar heat is to power pumps or fans to move water/air.

JackFre

With you all the way

A "drain back" system is absolutely the way to go, if you have the right guy doing the work. Once again...experience counts.

hr

I like the dual tank

approch best. The first feeds through to the fired tank. HTP Super stor makes dual coil tanks in several sizes. Dale from Radiant Engineering has dual coil tanks built by the folks from Crown Boiler, for some of his solar jobs. Sounds like they will customize indirects for you.

hot rod

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Duncan_15

OK

So you're saying one coil of the 2-coil tank is from the solar panels and heats the contents of the 2-coil tank, and the other coil extracts the heat as a preheat for the DHW ?

Why do you like that approach best, hot rod?

Duncan_15

OK

So you're saying one coil of the 2-coil tank is from the solar panels and heats the contents of the 2-coil tank, and the other coil extracts the heat as a preheat for the DHW ?

Why do you like that approach best, hot rod?

Jimmy Gillies

UK Solar.

Sorry Duncan, I did not make it clear what I was meaning.

The twin coil system is one hot water tank with two indirect coils, one heated by a boiler and the other heated by the solar panel, this tank being feed by mains water pressure and unvented.

The two tank system is a vented system with a header tank(in the attic)feeding both tanks. The solar tank has an indirect coil heated by the panel, with that tank providing pre-heated water to the second tank. The second tank also has an indirect coil that is heated by a boiler. With this system being vented the hot water is at a low pressure.

Both systems can have a sealed or drainback solar system.

If that's a bit clearer, what do you think is the best system?

Kind regards.
Jimmy Gillies Scotland.
P.S. Duncan, gas direct fired water heaters are not common here. I think it's due to fact that most houses having wet central heating radiator systems.

DaveGateway

solar

hey Duncan, sorry about the delay!

Where in Colorado are you? I'm down south by Stonewall, west of Trinidad. 8,500' elevation & alot of sunshine.

I have no numbers or drawings.

About 5 yrs ago I read a book about 25 solar systems that didn't work & why. Real informative vs. the gushy over-hyped normal books on solar! A common failure was not enough storage.
Then I read about somewhere in Europe where they filled entire insulated basements with earth for summer heat storage for winter needs. I compromised on about 1/4 of that.
It's a full basement, below grade, with no solar gain.

The sand over the plastic tank packed easy.

I hope to get the thermal mass just hot enough to keep the basement at 60-70* in the winter. I figure on several years of tinkering around with it, maby many years!

About how many ft2 of hot water panels do you think I will need?

I plan to add a coil of 3/4 copper into the tank someday to pre-heat for DHW.

Regards Bill Patrick

Brad_9

Number of tanks

A two tank system is a solar tank heated by the solar collectors using either an internal or external heat exchanger with the solar fluid in the coil, heating the domestic water in the tank. When hot water is drawn, the pre-heated water then goes into the backup tank, which can be any type of water heater. The disadvantage of a two tank system is that standby losses in the backup tank are not made up from the solar tank when no water is drawn. So even if you have lots of hot water in the solar tank, if no hot water is being used, the backup tank is still using fuel or electricity to keep the water heated. This is especially bad with standard gas water heaters with their high standby losses. This is better than the common single tank system and is the most commonly used design by far.

In a single tank system where the solar is heating the water in the regular heater, you have a situation where the backup heat is keeping the water in the tank at temperature and the solar does not have a chance to add it's heat to the water first, so is not nearly as effective. One way around this in some cases is to put a timer on the electric elements so that the solar has a chance to heat water during the day and then if it doesn't heat it all the way, the backup comes on in time for evening hot water use. This only works if no hot water is needed during the day.

A single, dual exchanger tank, to me is the best of both worlds, and I gather is what is used in Europe. This is a tank (like the Vitocell B-300) that has a heat exchanger in the lower half of the tank for solar heating and another exchanger in the upper half of the tank for the indirect heating by a boiler. It's like two tanks because the boiler never heats the lower half of the tank, but the boiler exchanger always keeps the upper half of the tank hot if the solar doesn't heat it all the way. The best part is that if no hot water is used, the solar will heat the lower half of the tank, then keep heating and will keep the upper half heated and replace the standby losses. The biggest problem with this is that the tanks are either very expensive, the heat exchangers are not placed right, or not insulated well (that covers all 4 tanks I know of). I don't know of any tank like this configured for electric backup.

None of these methods are used for space heating, only for domestic water heating. It may be possible to integrate space heating in with this, but that is another ball game entirely. Up here in cloudy western Washington, active solar heating doesn't pay at all, while solar domestic water heating in the sunny months works great.

Brad

Jimmy Gillies

Thanks to all.

Thanks to all who posted a reply to my question, it was most informative. The upshot being one tank with twin coils are best.
If it's any help, we also only use Solar heat to provide hot water, and it works great - only called back for service work.
KInd regards.
Jimmy Gillies.

Duncan_13

Excellent description, Brad.

Thank you Brad, got it. Excellent explanation of the plusses and minuses of each system, too.

The bottom line with solar is: Ya gotta make hay while the sun is shining. You really brought it home that different approaches in different areas makes sense. In the sunny semi-arid western states, you can make a lot of hay -there's no shortage of solar. But you need storage to use the heat.

That's one reason I like an open tank system for this area. If storage is large enough, you can take advantage of collecting and storing as much heat as you can get.

The other reason is its versatility. You can immerse as many heat exchangers as you want, to use the heat however you want to. Hot tub preheat, baseboard, radiant, fan coils, DHW.

By circulating water from the solar collectors directly into the open storage tank, you take one heat exchanger and its associated inefficiency out of the loop.

The reason I don't like them is possible humidity problems, and the need to keep an eye on water quality and (if there is no auto-fill), water level. They really need owner involvement. Not so desireable for a lot of people.

To get around those drawbacks, a lot of systems did use closed, pressurized drainback tanks in combination with a heat exchanger and separate storage. The drainback tank held water that drained back fron the collectors during the off cycle, and there'd be a separate storage tank before the DHW. Maybe that'd be a 2½ tank system [G].

The two tank system is the most common I've seen around here. The single tank, two coil system seems to make sense for your area, especially for DHW only. Maybe for Jimmy's area too, I dunno.

Duncan_13

Smack dab in the middle.

I'm in Florissant, west of Colorado Springs, about 9,000 feet in my back yard.

That's gotta be a BEAUtiful area you're in, absolutely gorgeous. I need to get down there and explore around a bit. Seriously.

Yeah, there's some groups on usenet that go for the year-round approach to solar, zero-energy homes, you might want to check that out. It's kind of a whole different school of thought than I'm used to.

All my work was in higher temperature stuff, and we usually sized storage 1½ to 2 gallons per square foot of collector, just like Jack said. But we were shooting for (and getting) storage temperatures around 160° to 180°F. So I'd be guessing at a setup like yours. I'm very interested in that approach, I hope you stay in touch!

If there's one thing I can't stress enough, one thing I'd hope you get out of this whole thing, is that you need to keep your temperatures in that plastic tank LOW. I'm thinking that kind of polyethylene doesn't want to see anything over 160°F.

Hey, the tinkering approach should serve you well with your system, Bill! That's what I'd do, it's easier and more fun than doing the math. ;-) My gut feeling is that during the long hot summers, you'll probably be able to charge up the tank pretty well. And if not, add a panel or two at a time. Keep an eye on the Denver Post for used flat plates. I'd be patient, keep looking, and try not to pay more than $100.00 each for a 4x8.

Bob Gagnon plumbing and heating

Duncan

I have a solar system like you describe with a 1500 gal. non-pressurized storage tank. i'm going to add 5- 100' coils of type k roll copper 2- 1" coils and 3- 3/4" coils. to be used for domestic, radiant heating and a new collector. do those coils sound right?

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Duncan_13

More info needed

Bob, the storage tanks I've worked with have all been quite a bit smaller than that, but coils I've seen in 250 gallon storage tanks with 4 or 5 collectors are usually smaller than you describe. Which is fine for you, more HX area is better than not enough. I think with some delta t and flow rate information, you could figure out how many btus the copper coils will pick up.

But it's really hard to tell without knowing the number of, size and type of your collectors, and what everything's hooked to. Sounds like you've got plenty of HX area in those coils. What's your situation? Could you describe the system a little more?

hr

Build a HX more like

the top drawing. I have not had great luck with 100 foot coils of copper. I had 3 100 footers in my 500 gallon tank. The short loop, with headers, which mimick the solar panel perform better.

Ever see the HX Tarm sells with their wood boiler 500 gallon storage. Those transfer real well.

Ergomax indirect tanks are another good example of excellent HX design.

If it were me I'd contact Wolverine. Apparantly they sell finned copper tube, possibly with the "rifleing inside for better transfer. The Radiant Enginnering team recently built some custom solar HX's for a wool washing process. Call them for ideas.

I saw huge rolls of finned tube at HTP a few years back. Guess they still manufacture most of the indirect DHW coils that go into boilers. Sure like to get a roll of finned tube for hair brained projects I have

I have a bunch of these copper shell and tube HX's from some old solar DHW systems. I have a dual one hooked up to a 4X10 panel on my shop. I saw 180° degrees from it yesterday in 67° weather. No load other than the HX itself, but that temperature surprised me!

hot rod

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David Woycio

Solar heating

As with any system do a heat loss. Then size your system. In Colorado we will take 7 months(sept-march) of average low temperatures for our design temp. for solar heating. Trying to size a solar heating system for the coldest day of the year will result in a very costly system. Calc. the size of system for 2 days of no sun and your in business. We use stainless steel storage tanks, flat plate solar collectors. Every system is a drain-back water system. If set up properly the only maintenance is keeping the water level in the storage tank.

We have at our shop in Denver over 100 used and rebuilt flat plate solar collectors for sale. Get a hold of me if your interested.

Bob Gagnon plumbing and heating

Hot Rod

what problem did you have with the 100' coils, not enough heat transfer or too much resistance? how long would you make the coils if they are manifold together? i'm thinking of adding 4 sets of thermomax evac. tubes so i will need a lot of transfer. i was planning on piping 2- 3/4" 100' coils for the domestic but i didn't think i would need to for the collectors. i'll call wolverine. thanks for your help, and thanks for your help a few years ago on the rubber lined tank, it worked out great. bob

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David Woycio

Metro Man

David Woycio

When you don't have to worry about a double wall heat exchanger,
we use 3-4 50' coils of refer tubing plumbed with a 3/4" manifold on the feed and a 1/2" manifold on the return. Plumb the coils in reverse return. This set-up works very well. Last one we tested pulled within 1 degree of the storage tank.



Metro Man

David Woycio

DHW/Solar heat exchanger

When you don't have to worry about a double wall heat exchanger,
we use 3-4 50' coils (spun about 15" diam.)of 1/2" refrigerator tubing plumbed with a 3/4" manifold on the feed and a 1/2" manifold on the return. Plumb (braze) the coils in reverse return. This set-up works very well. Last one we tested pulled within 1 degree of the storage tank @ 3gpm.
Place the coils in the top 1/3 of the storage tank and plumb in series with the hot water heater. As hot water is used, cold water from the street 1st runs through the coils, then to the normally cold inlet to the water heater. This heat exchanger will heat 54 deg. water to within 5 degrees of whatever the storage tank temperature is. Very simple with no controls or pumps.

The more you can stretch the coils apart the better they work. Also be careful to anchor the coils and not allow them to touch the side of the storage tank. An expansion tank plumbed on the output and
before the water heater helps to reduce coil movement when hot water is turned off and on.

DaveGateway

?

Metro Man,

your: "and a 1/2" manifold on the return" why?
I would think 3/4 on both ends!

Bill Patrick

David Woycio

We like to restrict the outputs of the coils so they stay balanced. After going to 1/2" manifold we go back to 3/4".

hr

Bob, flow rate and HX

The faster the fluid mover through the HX the better the heat exchange. Notice a plate solar collector is built with large headers, generally 1" or larger. Then short tubes between. Surely it would be cheaper and easier to build a collector with a single loop of 1" pipe, but you are looking for the highest average temperature, in the collector or HX to move the most heat. Use this baseboard example on my drawing.


Same with flow rate.
Use the "most important formula"

Q= flow (gpm) X 490 X delta T

1 gpm X 490 X 20 = 9800 BTU/ hr.

4 gpm X 490 X 20 = 39,000 BTU/hr.

If you look at baseboard or fan coil data sheets they often indicate output at 1 and 4 gpm flows.

hot rod



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David Woycio

solar dhw interface

If you increase your flow rate through a solar collector your delta T will drop. If the drop goes to 5 degrees or less, with the controls that we use, the system shuts down. Running the system too fast will cause it to short cycle all day long. Its better to have the system come on and stay on.

Most collectors manufactured today have a 1" header because you can install up to 6 (4' x 8") together without having to plumb feed and return manifolds with larger diameter piping. The more piping you have on the exterior the greater your heat loss. This heat loss screws up you delta T. We have installed more collectors together but spec. larger headers. 1-1/4" and 1-1/2" depending on the size of the array. Most collector manufactuers spec. .75 gpm/ collector (4' x 8'). If you were only installing 2 collectors, you could get away with 3/4" headers.

When installing a drain-back water system it is VERY important to "throttle back" the return. On a 4 panel system for example we typically plumb a 1" feed line with a 3/4" return. To overcome the head of the system (collectors are usually installed on the roof) the pump for a drain-back system needs to sized correctly. We use alot of 009 Taco's. After the water reaches to top of the collector array and returns to the tank the flow rate increases. If left unchecked this flow rate would be too high and air would be introduced in the line causing unwanted gurgling noises. The reduced return line helps to slow the flow rate and create some needed back pressure. We also will install a balancing valve on the return near the solar storage tank to further dial in the flow rate. A 15-20 degree delta T is what we shoot for between feed and return temps. I have an animated example of this on our web site (www.metro-solar.com) under "system types". This site is still in the development stages and incomplete. (didn't realize how much work it was putting one of these together) Notice the balancing valve at the return.

Back in the day we used to have proportional delta T controllers that would change the flow rates according to the change of delta T's. As long as there was some sun the systems stayed on most of the day collecting heat. These would only work with a closed lood glycol system but where very slick. I would love to install one on a drain-back system with some control modifications to see how it performed.

Duncan_2

half inch manifold

> We like to restrict the outputs of the coils so

> they stay balanced. After going to 1/2" manifold

> we go back to 3/4".




Not sure I follow you on that. How does a 1/2" manifold help it to stay balanced?

I liked your explanation on the 3/4" returns on 1" drainback panel arrays. We found it made all the difference with problems of panels flashing to steam on some systems.

David Woycio

Duncan

Are you the same guy who used to work with Tom Olds?
The coils that I described (maybe poorly) are 1/2" refer tubing @50' lenghts. When you coil them, sometimes you get a kink, flat spot or other imperfection. Feeding with a 3/4" manifold and having all the returns run to a 1/2 manifold, we have found that we ensure all the coils keep the same flow rates. Again this is only if you don't need a double wall heat ex.


Metro Man

hr

Thanks M man for the shine bright

wisdom.

I tinker with closed loop systems mainly, Glycoled with double walled HX's. I always went with the .02 gpm per square foot of collector or 8 gpm for a 4X10 panel?? So mine are piped with 1".

I have never seen the proportional valve you mentioned. Mine just cycle the circ with a Goldline diff. control set a 8°. The valve you mentioned changes flow based on the differental temperature? I'd like to get one to mess with if you still have a source.

It's a sunny, windy, 45° here today but my solar systems are cranking out the BTUs. It always brings a smile to feel that energy flowing into the storage.

I use scrap InsulTarp to further wrap my storage tank. seems to retain heat better during no sun days.

hot rod

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