Options for an old system - what am I missing?

[svntn]

Much thanks to all those posting their experiences, installs, and calculations. It provides inspiration that you are taking action, I tend to read too much and act too little.

I am considering options... I'm a little stuck wanting to keep the system, but struggling to see it make financial sense, so any input would be appreciated.

Current System:
Sun belt system, installed around 1984
3 rooftop collectors, Internals in good condition.
Silicone fluid heats water via heat exchanger. Fluid was spec'd to essentially last a lifetime.
2 x 120 gallon storage tanks + 50 gallon backup
(these are insulated storage tanks, the heat exchanger is external to the tanks)
The system was sized so large because it originally had a hot air exchanger that heated a room, but after a couple of room remodels, the air system is no longer in service.

Issues:
Storage tanks are corroding. Small leak has started from drain valve. Replacement costs are estimated at $3k per tank including labor. Backup water heater also should be replaced given its age.
Lots of sediment in water - clogging up shower heads (likely due to sediment in tanks - we haven't flushed them since being the house 4 years ago, and because of age, was advised not to flush as it might cause a leak)
Expansion valve needs some maintenance - estimating $450 to replace, add 1-2 gallons of fluid and re-pressurize system.

Needs:
6 people in household, estimating 105 gallons as peak need in a single hour.

Desire:
Most economical route. Shutting it down seems the cheapest option, but I am having a hard time letting this system go to waste. If there are options I'm not considering, I'd love to know.

Options:
1. Service and keep system. Est cost: $7250 ($6k for 2 tanks, $450 servicing, $800 backup tank). A solar servicer thought the system could last another 20-30 years with proper maintenance.

2. (if possible) Downsize system to single tank. est cost $4250 ($3k for single tank, $450 servicing, $800 backup 50 gallon tank)
- Open question: Can I downsize the tanks to a single storage tank (single 80 or 120 gallon), or would this cause the system to overheat (as it can't offload heat to 240 gallons of water)?

3. Shut down solar system. Replace with 75 gallon, 75k btu gas water heater. Est cost: $1500. Introduce water heating costs of $30-50/month.
Drain storage tanks, shut off pumps, drain silicone (if required).
- Open question: If I do this, do I have to drain the silicone from the system? My concern is that if I drain the system, with air and age, it will just begin to corrode and rot, and wouldn't be serviceable in the future. If I didn't have to drain the system, my preference would be to have the option to replace the tank(s), and restart the solar at a future date. I have the same question as with #2, which is whether the system can survive without water to offload heat to, although with this option, I wouldn't be circulating the silicone fluid.

Would love to hear any thoughts/opinions. TIA.

[inetdog]

To prevent overheating of the panels, with or without fluid in them, you should probably plan to cover them with something to keep them from direct sunlight.

[svntn]

Interesting. I hadn't thought about the stress on the panels, but that would make sense.

I did find info on the fluid - syntherm 444. Boiling point of 314 C, so I think it should be fine. But you're right about heat and stress on the collectors.

if I were to downsize to a single storage tank your comment makes me wonder if could cover 2 of the panels to avoid overheating.

Is is there much of a market for used collector panels? Because of the age I am doubting these would compare favorably to what's available today, but haven't seen panels these old out there.

[J.P.M.]

Lots to consider. Start with a look at what you want to accomplish. Heat domestic water ? If so, determine a water heating load that meets the goals you set.

FWIW, my house uses about 12,000 BTU/day per person for a DHW load but we're lower users of hot water than most by maybe 50% or so. Standby losses are something you'll need to figure on your own. Ways to calc. line/tank/other losses and examples abound in the literature.

Before you start figuring ways to supply energy to a load, know that it's MUCH more cost effective to start by finding ways to reduce that load. Reducing your DHW load is the easiest, most cost effective and most sensible place top start a system analysis/redesign. Begin there.

Quite honestly, given the relative sophistication and what I'd imagine the components you have, and as you suspect, it may be more sensible to replace (scrap out) the whole thing rather than fix it piece by piece as it slowly disintegrates.

At this time, the system may have given good service, but it's getting pretty long in the tooth. Things like the heat exchanger and all or most of the valves will be in need of replacement. One component in what's probably a series of components will impact performance. So, you'll be faced with replacing the system piecemeal over time or all at once. All at once will be less expensive than piecemeal and less disruptive, but entire replacement in kind is probably not cost effective compared to other methods of heating water.

The question then becomes replace it with what ?

Depending on your location and climate, solar thermal is one possibility. In warm (and definitely non freezing) climates, a direct system (direct meaning potable H2O circulates directly through the collectors and so eliminates the need for a heat exchanger and a lot of piping) will be OK and less involved (and less $$) that an indirect system of the type you have now that involves a separate coolant loop.

However, most of the world experiences freezing conditions 1X in a while and that's all it takes to mandate an indirect system.

A possibly cost effective alternative to solar thermal is a PV system (or an addition to an existing PV system) with the (additional) PV output supplying some or all of the power to a heat pump hot water system. Such arrangements are about as or more cost effective than a direct solar thermal system, but cost effectiveness of such systems tends to decrease as the climate gets colder. However, the freezing problem goes away. The down side is that heat pump DHW heating systems are still somewhat new and all the bugs ain't out yet.

Whatever you do, I'd suggest analyzing/deciding what you want to accomplish, deduce your loads through pipe and tank insulation and other conservation measures like flow rate reduction and water conserving or low use appliances before you think about new equipment.

I've had a solar water heater for many years and solar thermal is one of the big reason why I'm an engineer today. But if I'm honest, I'd say that as an opinion, you're better off limping along with what you have until you can get a handle on your loads, reduce them as much as possible and then consider alternatives - a couple of which I've skeleton described.

I like solar thermal and think I might know a bit about it, but as much as I like the system I designed, if I was replacing my solar thermal water heater today, it would probably be with a heat pump water heater and additional PV.

Take what you want of the above. Scrap the rest.

[LucMan]

Just several questions for you. What state are you in, what size are your panels, do you heat with a boiler? Normally 3 (4x8) panels are good for 120 gallons domestic HW perfect for 6 six people.
As you state the silicon fluid is forever. I haven't seen it used much since the the first system I helped install back in the late seventies. Converting the system over to a single stainless steel tank with integral HX 's should give best bang for the buck.
Check this site http://www.htproducts.com/literature/SolarBrochure.pdf