Blending solar with condensing boiler

Greg Swob

Anyone familiar with using a blend of solar and condensing boiler in radiant system? I would appreciate any guidance in this area. Fortunately, this will be new construction, so retrofit problems are non existent. A development in our area requires a large % of heating to be obtained from solar. Due to water & energy conserving and other covenants, this development has not taken off well. Due to the attractive home sites and higher fuel costs, is looking more worthwhile to some customers.

We are considering a buffer tank heated by solar collectors that would be drawn from by secondary (heating) loop. A condensing boiler would act as night time/ back-up heating source for the buffer tank(s). How would one go about reasonably calculating storage sizing for the needed radiant? An indirect water heater is one option or simply an insulated storage tank with Heat Exchanger of sorts. I know there will be some efficiency loss at heat exchange, but that is not a concern of the covenant authors. Solar IS their concern.

Small heat loss/gain figures must be incorporated and proven in the home's design. This has to be verified by an architect. Energy Star ratings were not in existence when the covenants were drawn up and the development owners will make allowances for a max. E-Star rating.

In approaching the building committee, we are asking for a reduction or possible alleviation in the solar heating requirement. Looking at the home with a long-term cost/benefit approach and other means in exceeding the minimum energy use requirements is one approach. Extensive use of modern building techniques and materials (SIP, ICF, Radiant Hydronic, E-star rated HVAC and household appliances throughout, etc.). Hopefully they will offer allowances on reducing the solar heating requirement.

The reasons customer is opposed to solar: inconsistent energy production, space and cost requirements of storage needed, possible liability with panel reflections, to name a few. Another option I am suggesting is Ground Source Heat Pump. I feel this could be considered 100% solar technically, but GSHP brings up other opposition issues. Thanks - Greg

Mark Eatherton1

You have to be careful with solar...

or you'll get burned, in more ways than one.

The most prevelant problem with improper solar application is the oversizing of the array, and consequential boil out during non space heating months. I would suggest that a software progrma name F Chart be used to properly size the solar array based on theoretical load calculations and physical locations.

As for the mechanical, I'd not suggest the auxilliary heating system be used to heat the solar storage tank. It needs to be kept as cool as possible to allow for free solar btu input during the next period of solar availability.

You cold extract the solar heat from the storage tank using VSI technology, thereby enhancing the thermal stratification within the tank, maximizing solar btu usage and avoiding short cycling inefficiencies.

Depending upon whose condensing boiler you're using, I'd suggest it be piped as a secondary input to a primary loop. This will keep its' mass "off line" as much as possible.

At a minimum, you should ALWAYS provide a 70% fraction of the families DHW needs. This load is pretty constant year round, and shows the best economic ROI. DHW can be done using an immersed coil in the drain back solar tank to act as a preheat, and then a side arm for auxilliary back up connected to the condensing boiler.

As far as solar piping is concerned, assuming that you're using flat plate solar collectors, I prefer to use the Drain Back method. It avoids destroying the heat transfer fluid during the summer months.

If you MUST use closed loop. make sure you have a means of excess heat rejection (large fan coil in attic of house) built into the system or the glycols will pyrolize and become acidic in short order during the summer months.

I'm told that the Viessmann Vitosol evacuated tube solar collectors have a thermal limiting feature in them to avoid summer stagnation. I'm also told that they're so efficient that they can suck btu's out of passing clouds. Been there, seen that:-)

As it pertains to GSHP, if properly done by properly licensed contractors, there is little to no chance of an environmental disaster.

The state has regulations it follows in regards to grouting of wells to avoid trans aquifer migration, and the fluids that are used to transfer heat from the loop field to the heat pumps is generally consider low toxicity (alcohol/water combination).

As a possible thought, how about a combination solar/water source heat pump/condensing boiler. Using a large underground super insulated storage tank, you could pump every btu that ever fell on the face of the array into the tank. Upon a call for heat, you'd use the most efficient means of heat transfer/extraction (w.s.h.p.) to transfer and compress the low grade energy into the system. If the COP drops too low, you could use the condensor as your back up.

Just thinking outside of the box here...

Good Luck with your project. It does my heart good to see projects like this coming back to life. It's about time we woke up again... Let me know if you need any design assisitance.

ME

hr

The best bang for your solar bucks

is DHW. This is more of a year round need. Maybe time to lobby the developer to require just SDHW, instead of a sometimes complicated and expensive solar/ radiant program.

Don't get me wrong I love and embrace solar energy, but it has to pencil out for the owner/builder.

Possibly some passive or PV trade off credits?

What's the sense of a hip "solar subdivision" if no one builds there

petition to lossen the requirements, and let the sun go to work!

hot rod

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Greg Swob

Thank you, Hot Rod and Mark. We (homeowner, gen. contractor, development owner and I) need to meet to discuss options and feel out how staunchly set on solar the property owner is. This project is barely in the planning stages so lots of time yet. He did permit a home with less than the min. required solar heat (basically had many large windows facing South) a few years back. This particular home has conventional DHW, 80%+ AFUE boiler coupled with HRH in light weight concrete system. Their fuel consumption is paltry due to tight construction and the highest R windows available then.

I believe we can convince this gentleman to allow a substitution of: Energy Star boiler, RHR and indirect DWH, as many household E-star appliances as possible, LED, florescent or other energy-saving lighting and meet his conservation objectives. I agree with Hot Rod- a couple dozen lots, but only four homes built since the development was conceived 22 yrs. ago. He is not in it for the money, he is highly principled. But the street & other improvement costs he made still have not been recovered. He's 69 yrs. young & commutes to his jewelry store daily via bicycle except in harsh weather!

Mark, I may send you a rough schematic of my HVAC & DWH ideas and get your input when we get that far. Greg

Greg Swob

Thank you, Hot Rod and Mark. We (homeowner, gen. contractor, development owner and I) need to meet to discuss options and feel out how staunchly set on solar he is. This project is barely in the planning stages so lots of time yet. He did permit a home with less than the min. required solar heat (basically only passive with many large windows facing South) a few years back. This particular home has conventional DHW, 80%+ AFUE boiler coupled with HRH. Their fuel consumption is paltry due to tight construction and the high R windows used.

I believe we can convince this gentleman to allow a substitution of: Energy Star boiler, RHR and indirect DWH, as many household E-star appliances as possible, LED, florescent or other energy-saving lighting and meet his conservation objectives. I agree with Hot Rod- a couple dozen lots, but only four homes built since the development was conceived 22 yrs. ago. He is not in it for the money, he is highly principled. But, the street & other improvement costs he made still have not been recovered. He's 69 yrs. young & commutes to his jewelry store daily via bicycle except in harsh weather!

Mark, I may send you a rough schematic of my HVAC & DWH concepts for your input when we get that far. Greg

Kevin McAfee

VSI Technology

What is VSI Technology?

Mark Eatherton1

Variable Speed Injection (NM)

ME

Dale Pickard

Solar

Hello Greg,

If you are interested, we have a great deal of experience with solar thermal and photovoltaic applications and we provide professional solar engineering services. We can provide objective consulting including evaluation of annual performance and the energy economics for your situation. We can also provide design/spec services suitable for bid documents or to aid contractors or do it yourselfers. We are not experimenters.

Dale Pickard
Radiant Engineering Inc.
pick@radiantengineering.com