Solar assist for a new boiler

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Old 05-05-16, 12:47 PM
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Solar assist for a new boiler

I am replacing my 20+ year old boiler with a new more efficient combi type that supplies DHW and baseboard heat.
I would like to be able to pre heat the water coming into the boiler (checked with NTI and that is not a problem) using the solar preheated water for the baseboard heat and the DHW.
I have done the research but canít find many examples of this type of system.
The solar installer seems this would just be good for the pre heat to the DHW but a $7k pre heater for an on demand DHW seems a bit much.
What Iím asking is should it be possible to utilize the solar hot water to heat (not to operating temp but enough to minimize the cycling of the boiler) in the baseboard heat AND the DHW?
From what I can see if anything I would need another/different water tank with 2 coils vs 1.
Any info, ideas, experience would be very helpful.

The major parts:
3 bed 2 bath 1400 sf rancher in central Colorado
2 adults

Apricus ETC30 Evacuated tube panel.
40.9kBtu/day
4 gpm max flow rate
Rheem Solaraide 81VR80U-T 80 Gal

NTI TX151 C modulating combi boiler

Thank you in advance
John
 
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Old 05-05-16, 04:44 PM
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Welcome to the forums.

This is not my forum and I'm not the pro here but just making some observations.

What I’m asking is should it be possible to utilize the solar hot water to heat (not to operating temp but enough to minimize the cycling of the boiler) in the baseboard heat AND the DHW?
Of course the solar heater will have an effect on the boiler cycling but the question is.. is it worth it.

When I think of Colorado I think of cold weather. That's a negative for solar heat. One thing you would definitely need is a lot of sun on the side of the house where the solar is located. Since you are in a cold climate.... your solar loop will need to be filled with anti freeze so it doesn't freeze at night. That means you can't use it directly for the DHW. It would have to be a loop incorporated in the boiler or you would need a water to water exchanger to preheat the domestic water before it gets to the boiler.

Others will stop by an add their opinions.
 
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Old 05-06-16, 05:49 AM
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I appreciate your reply and I agree that cost is a big factor and what I am hearing and reading it's not really worth the cost to benefit ratio.

Thank you
 
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Old 05-06-16, 06:24 AM
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Hi john,
In my limited exposure to solar heated water, it is an expensive way to save very little.
For domestic hot water it could preheat the incoming (very cold) water. But for the boiler, it is a closed system and more difficult to add heat to from a limited collection system. On some days you might only be able to collect 120į water which would be of limited benefit to a baseboard heating system.

But, there is a source of all things solar at builditsolar.com , he has a significant collection of DIY projects and advice.

From my perspective as an energy consultant, investments on the energy savings side are more productive. Combine them with the rapid advance of solar PV panels and you can approach a net zero home. Plus a lot easier to run wires as opposed to plumbing. The catch 22 kicks in as your home becomes more energy efficient, the SIR (savings to investment ratio) becomes more difficult to justify. Consider: A home using 1,000 gal of fuel oil replaces their boiler and reduces that number to 800 gal, saving 200 gal. But if they made significant energy improvements and reduced the 1,000 gal to just 500 gal, then the same boiler upgrade would only save 100 gal. I always recommend making all of the energy improvements first and then recalculating not only the size of a potential new boiler, but its projected savings. BTW, a 20 year old boiler is still young and modern high efficiency boilers rarely operate at their advertised level. Some of the boiler pros here could advise on your need for an upgrade.

Bud
 
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Old 05-06-16, 07:38 AM
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Twenty five years ago I installed a solar preheat system for DHW. After state and federal rebates my cost was around $1200. After a few years tank had to be replaced for $600. Then snow/ice buildup around panels caused that section of roof to deteriorate and had to re-roof for more $$$. Next, inner layer of clear plastic covering panels went. Finally gave up the ghost and sold the system on Craigslist.

Am holding off on photovoltaic systems which are now only around 10% efficient until it gets much higher. In labs they have done over 40%.

Eventually the power convertors now used on 120 VAC photovoltaic systems connected to grid will have be replaced by remotely controlled types. Germany now requires them because on sunny days solar systems input was making their grid difficult to control.

It may not be Moore's Law but changes in technology can obsolete things quickly.
 

Last edited by doughess; 05-06-16 at 10:18 AM.
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Old 05-07-16, 06:03 PM
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Solar used as a heat and/or DHW works real well especially with the huge rebates now. I installed a few of these systems back in the 90's. I had one job the entire basement and all the DHW was done by solar. If you install a mod/con boiler that will operated almost always at minimum fire due to warm water from the solar system.
I was on a job we did and the day we finished up the solar job it was 10f outside with a chill factor of -6. We were getting 137f from the panels. This happen to be a DHW only job but still a good temp even at that OD air temp.
Ground source heat pump and a mod/con boiler also works well. The boiler in both cases ia only used to boost the temperature if needed.
 
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Old 05-07-16, 07:34 PM
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rbeck... how many solar panels/size did you use on that job ?
 
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Old 05-08-16, 07:17 AM
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There were 3 panels 4 ft X 8 ft. Panels were hinged. Flat on roof at 30 deg in summer and raised to about 50 deg in winter.

Economically solar was not worth it. Now, during summer with oil fired 50 gal hot water heater use one gallon per week. Cost last year for 25 weeks X $2.00 gal oil was $50. In simple math the $1200 solar system cost would have a 24 year pay back,

In winter, boiler tankless coil and circulator pump keep 50 gal tank water hot. To raise water heater burner efficiency temp is controlled by PID at 3 deg deltaT during summer and and pump at 1 deg in winter. Recovery is fast in both modes.
 

Last edited by doughess; 05-08-16 at 07:36 AM.
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Old 05-09-16, 08:08 PM
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I am replacing my 20+ year old boiler with a new more efficient combi type that supplies DHW and baseboard heat.
Have heard stories of people paying lot's of money for new more efficient systems and finding it did not save much. The worst part is that new ones are very difficult to clean and keep efficiency up.

In the past 50 years since the introduction of the "retention head" oil burner there really has not been significant improvements in burner/boiler technology. During that period most buildings have made big improvements which greatly reduced heat load making the boiler often twice the capacity needed. By cutting firing rate with smaller nozzle it is possible to increase efficiency to better than 85%. Installing an Out Door Reset unit and electric stack damper also helps savings. I would not replace my 60 year old boiler unless it had unrepairable leaks.
 
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Old 05-11-16, 06:34 PM
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We normally did 3 4'x8 'panels set at a 45 degree angle for DHW only with a 50 gallon stone lined tank. When we did heating assist it would depend on the job.
We did jobs where the HO heated a finished basement with fanned convectors. These we did two 50 gallon stone lined tanks. One for DHW and one for heat. The basements were heated almost 100% from solar. The electric bills reduced by more than half and heating the basement was virtually free.
The efficiency of these panels have increased over the years and there are still great rebates.
To replace a boiler or furnace if properly sized should show a reduction of fuel by 25% to 50% especially if you include boiler ODR and direct vent with combustion air. I have seen more in a few cases.
Today almost all boilers are what we call cold start boilers and are most efficient due to greater thermal transfer. The greater the temperature difference the greater the thermal transfer. The hotter the water the lower the boiler efficiency.
DHW coils in boilers should be banned. NYC has tried twice but failed due to politics. We are the only country in the world that allows DHW coils in boilers except in special conditions and standard PVC venting.
 
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Old 05-11-16, 09:14 PM
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DHW coils in boilers should be banned.
I completely agree. A worse method of heating domestic water is almost impossible to conceive. It made some (very little) sense when boilers were huge, people used relatively small bursts of hot water and fuel was cheap. Today there is nothing to recommend a tankless coil.
 
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Old 05-12-16, 11:21 AM
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There are three subjects here solar, DHW and boiler size/efficiency. To simplify things I will use 80% burner efficiency. During summer my direct fired water heater uses 1 gallon oil per week @80% or 112,000 btu or 16,000 but/day. At $2.00 per gallon that was $100 in 2015, a very long payback for solar even if it may provide 100% of DHW on sunny days.

My typical split level home winter boiler oil use is 6 gallons/day for both heat and DHW tankless coil for 140,000 btu X 80%= 672,000 btu/day. Assuming winter DHW load is same as summer 16,000 btu that is less that 5% of total load. A very insignificant amount, almost a free ride on existing system.

What data is the basis for quote “... DHW coils in boilers should be banned. “ What is suggested as an alternative and more efficient winter heat DHW source?

Cold start systems are not the universal answer for all systems. I have never seen any data showing the cross over point between the two approaches. Then there is the increased recovery lag time with cold start. One of the advantages of ODR is a more even and comfortable heating system. The place seems the same on warm or cold days.

Many old boilers are very poorly set up and operated in very inefficient ways. Replacing them is often an expensive way to improve those systems, and might “.... show a reduction of fuel by 25% to 50%” That is like buying a new boat instead of fixing the leak. Leak fixes include smaller nozzles are $10. Insulating hot water pipes and boiler is cheap. Out-door-reset costs $150 and electric vent damper about the same. These may not be as profitable for dealers, but can be very effective increasing efficiency and lowering costs. Better than spending big $$$$ for a new boiler some of which come with modulating nozzles to vary firing rate!!.

As for properly sized boilers, design days are only 10% of the heating season. The other 90% of the time the boiler is over-sized. I see no advantage in being properly sized 10% of the time so on design days it runs longer. An oversized boiler just runs less hours to output same about of btu and has extra capacity when needed. Hurricane Sandy took off part of my house. With traps and plywood covering openings the heat load when up. The oversized boiler just had to run longer.
 

Last edited by doughess; 05-12-16 at 01:30 PM.
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Old 05-12-16, 01:16 PM
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There is a significant difference between burner efficiency and system efficiency. To fire a boiler containing several gallons of water to heat, via a tankless coil) a few gallons of domestic hot water will NOT come close to the efficiency of a stand-alone water heater no matter what fuel is used. This is even more pronounced during the non-heating months.

There is more but it has been discussed many times in the past.
 
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Old 05-12-16, 07:14 PM
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I have a stand alone 50 gallon single pass water heater. As a heat exchanger its efficiency does not come close to the triple pass boiler. Water heater stack temp is much higher.

The boiler/burner does not fire every time someone opens a faucet or a thermostat calls for heat. When the DHW PID/aquastat calls for heat it circulates water from the tankless coil. An ODR starts the boiler/burner in a different loop.

With DWH less that 5% of the winter load it is not much of an issue in terms of burner or system efficiency and less than $100 per year total cost! Whatever the efficiency numbers are it is a very economical setup.
 
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Old 05-13-16, 04:35 PM
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When I worked for the oil company we could see the numbers dependent on K-Factors. Whenever a tankless coil was removed and an Indirect water heater installed, boiler converted to cold start the summertime fuel use went down by 20% - 50% dependent on use.
As far as cold start boilers for heating they create more comfort in the home as the system efficiency goes way up. it will never meet the high limit if properly sized until it gets pretty cold outside. It only gets as hot as it needs to to satisfy the thermostat.
Another downside to a hot boiler all the time is the standby losses go way up. These are chimey losses and jacket losses.
It has been proven through the years that cold start is way more efficient. If you look around the world very few boiler manufacturers build a boiler to maintain temperature anymore due to the efficiency drop. The greater the temperature transfer happens at the greater temperature difference. As boiler temperature increases the efficiency goes down. As the boiler water temperature decreases the boiler efficiency goes up.
Boilers are tested at 120f return and 140f supply for AFUE. If the boiler operated higher than that the efficiency will be lower.
Flow through the system and boiler plays a large part of fuel savings.
As Furd stated, system efficiency varies with water temperature. Lower temp will increase system efficiency and comfort.
A DHW coil in a boiler and keeping the boiler hot is like keeping the tea kettle on the stove just in case somebody wants a cup of tea. Putting timers on water heaters save money and the only standby loss is jacket losses. No warm chimneys. Warm chimneys also pull more heat from the structure which requires more need for the boiler to run to replace the heat lost.
 
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Old 05-14-16, 08:45 AM
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Mixing and linking so many issues in a thread can can complicate things.

Hydronic systems are designed with a water temperature to deliver necessary btu's at outdoor design conditions. It cannot be always be dictated by boiler manufacturer or efficiency factors. DHW in most homes is a secondary issue which may or may not be linked to heating boiler.

Many threads are about existing systems which can define, constrict and limit what approaches to take. Each has cross over points where one becomes better than the other, i.e. cold start or hot start, short vs long cycling, etc..

Have a long background in electronics with experience in heat exchangers and specialized HVAC. Over 40+ years I have cut oil consumption by 75% on a 60 year boiler based system.

My approaches may not be heating industry dogma but they have been successful making the place very comfortable and economical to heat.
 
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Old 05-14-16, 08:20 PM
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Doug, if it works for you, great. But please do not post YOUR success as absolute fact for everyone. As you know, each system is a little bit different and therefore systems like yours that operate outside the "normal" characteristics are an anomaly and cannot be used as gospel on how ALL systems should, or even could, perform.

We respect your experiences in your field as well as your experiences with your particular heating system. The people who write the majority of the heating system responses to questions asked in the forum also have a great many years of experience in their respective fields. In my case the experience is not so much with residential heating systems but with commercial and industrial sized systems. I spent over thirty years doing it and while I am the first to admit there is a lot I DON'T know, what I DO know I know very well.
 
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Old 05-15-16, 07:58 AM
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My approach is to question things, dogma and others, not to state “absolute” facts. That is a basic part of scientific training, learning to question your own approach. The cold start and short cycle people sound more “absolute” than my looking for cross over points or suggesting the existing system may influence options.

The data on my system is normal. Suggesting that it is outside “normal” and an “abnormality” smacks of absolute thinking. What is the definition of a “normal heating system”?

My system is well instrumented and the data, along with degree days and oil consumption, used to monitor performance. From school labs to employers, performance was always checked. Engineering calculations have their place but the actual performance is often different.
 

Last edited by doughess; 05-15-16 at 08:23 AM.
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Old 05-15-16, 07:00 PM
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One thing I think you may be forgetting is we are talking to DIY'ers not seasoned contractors here and most do not have the test equipment or maybe even the understanding of electrical, controls or burner adjustment to understand what you are posting.
General and basic things like ODR, indirect water heaters etc works well and is easy in most cases. Your conversation of PID's etc is more for a contractor's website and many of them would get lost.
If in fact you have saved 75% which I doubt it would not be a normal savings. If you did to another boiler what you did to yours the savings would be different. In my 40 years in the heat industry in many capacities I can we I have saved one person 74% (Hershey, PA) and a couple around 50% and some only about 10%. Is a normal figure 20% to 30% yeah, that would be a more normal average.
 
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Old 05-15-16, 07:49 PM
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Although I thank everyone for their interest and replies in this thread I think we've satisfied the OP's question as he has not returned to comment further.
 
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Old 05-16-16, 09:12 AM
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I got into the heat system issues after bad advice and failures of the professionals. In the US "professionals" are those that make money from something. There are no standards to use that label so "buyers" should beware.

DYI posts from those claiming professional/industry experience often have similar bad advice. Bad advice cost the US consumers huge $$$. I have saved money based on DYI information and try to share my experiences.
 
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