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Community and Q&A

Solar Thermal Retrofit

Robert Post S.E. Pennsylvania Zone 4a | Posted in Green Building Techniques on

We are about to start some work on our 1950s Cape Cod here in Pennsylvania. The work includes a new roof and a solar thermal system for domestic hot water. Currently we have a high-efficiency furnace vented out the side of the house via PVC. Our natural gas hot water heater is orphaned in the chimney. It is sluggish to establish draft when firing up. I have installed a CO detector for that reason. We do not have a fireplace or anything else in this chimney. My questions are as follows:

1) If I want to demolish the chimney, what would be the best choice for an efficient, primary hot water heater? We have natural gas and electric available. We also have a family of six and use a fair amount of hot water.

2) The solar thermal system will most likely consists of 2 to 3 flat plate collectors and a tank. We have been using Sun Earth products for solar thermal installs and I will most likely use them on my own home. We plan on a one-story addition in the next year or two that would be approximately 200 ft.². With the due South exposure we plan on a passive solar design. This question is whether to include radiant heat in the addition using the solar thermal system and if so how would this change the installation that we plan to do next month. Another consideration is whether this makes the solar thermal system too complicated or just not really efficient for winter heating. Of course there is a budget and while I am an energy auditor & home performance contractor intrigued with the various green tactics, I really don’t want to unnecessarily over-invest in this house or employ technologies that, at the end of the day, are not that efficient.

Thanks in advance for all your wonderful input.

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  1. GBA Editor
    Martin Holladay | | #1

    Robert Post,
    For domestic hot water, I would choose a natural gas heater over an electric heater, because of the lower fuel cost. I think tank-style heaters make more sense than instantaneous heaters.

    I don't recommend active solar thermal systems for space heating in the Northeast. In general, days are too short, and weather is too cloudy, for the investment to pay off. The days when space heat is most needed are the same days when the path of the sun is low in the sky, when sunrise comes late, when sunset comes early, and when clouds are thick.

    The small amount of heat needed to keep a 200-square-foot room heated isn't enough to justify the investment in an active solar space heating system. The best way to keep your heating cost low is to invest in good envelope details: high performance windows, thick insulation, and good air sealing details.

  2. Steve El | | #2

    Hi Robert, I'm just a interested homeowner, so give my comment due weight.

    Assuming your high efficiency furnace and its blower can can handle your finished design, then for your new addition:

    Option A seems to be a new supply and maybe a return duct

    Option B is "something else" be it radiant floor or whatever. Option B will require a significant amount of "other stuff".

    If you've detailed for efficiency and passive solar, its my guess that Option A makes a lot more financial sense as well as environmental sense over any alternative in Option B. A few feet of metal ductwork attached to your high efficiency furnace would be cheap, recyclable, relatively easy to install, and the increased gas consumption placed on your high efficiency furnace would have to be mighty huge to begin to compare to the energy inputs of manufacturing all that "other stuff" for Option B.


    For what its worth, I'm also in Pennsylvania. We are not doing a solar refit, but we are abandoning a chimney. The chimney will remain standing until we replace the roof. In the meantime it will serve as a chase for the PVC for our new furnace. When we re-roof the chimney will come down and the PVC will get some properly flashed boots.

    After much reading we settled on a small direct vent Rinnai tankless water heater (model R75LSi). A local supply house will sell direct but they are on amazon also. They come in bigger sizes. We chose that option because its efficient, it has sealed combustion using outside air, there are service people in the area, its reasonably priced (though not the cheapest), customer reviews seem to be very strong, and it qualifies for the tax credit if you hurry up and get it in by Jan 1.

    Have fun,
    Steve El

  3. Robert Post S.E. Pennsylvania Zone 4a | | #3

    Thank you Martin and Steve.

    I never considered using a tankless HWH in conjunction with the solar mostly due to the capacity constraints. I have a wife, 2 daughters, 2 sons, lots of dishes and laundry. My sense is that a tankless would not be up to the challenge. I was considering a storage type hybrid (heat pump- elec.) HWH or looking for information on the most efficient direct vent gas fired tank HWH. Can anyone recommend one. I really want to abandon the chimney and get rid of it.

    I appreciate your input regarding the space heating, Martin and Steve, and I will not pursue this. My furnace-AC, coupled with the passive solar design will meet the needs. This addition will need zonal control, however, because it's thermal characteristics will be very different fro the rest of the house. Any input on how to best control this new zone using my existing system?

    Many Thanks!

  4. steve El | | #4

    I spent all of 10 seconds reading about powered dampers in a branching main duct close to the plenum.... said something about increasing duct sizes so each zone can accomodate the CFM from the blower without too much back pressure. I quit reading at that point since most of my ducts are already installed and sized for a single zone. If your new room is so different from the rest of the house, maybe that's a reason to go for "option B". Maybe a heat pump?

  5. Riversong | | #5

    Robert Post,

    I agree that solar thermal doesn't make much sense in this application and also that a tank-type water heater will serve you better.

    I haven't used them myself, but I know other builders who have been impressed with the Polaris gas, direct-vent 95%+ water heaters (some use them for space heat as well). They make some of the most durable and reliable units on the market, but expect to pay a hefty price.

  6. Robert Post S.E. Pennsylvania Zone 4a | | #6


    That is GREAT advice and just the product I am looking for.

    Thanks to all!

    Anyone want to help me tear down a chimney?

  7. steve bumpus | | #7

    Robt R./ Martin
    Thought I was mostly sold on an instantaneous hot water for my future well insulated home, but you both do not seem keen on them over either a tank unit or a direct vent polaris unit. Care to elaborate/ thots ?? I value both of opinions and appreciate your shared input.

  8. Riversong | | #8

    A Polaris is a tank heater.

    Instantaneous water heaters have burners often three times the output of a tank heater because they have to transfer heat so quickly to the water. They all have output limits and most have minimum flow rates before the burner can ignite so very low flow fixtures won't activate them.

    Because the heat transfer is instantaneous, any mineral deposition on the coils dramatically slows down the transfer rate and the efficiency, and the very high burn rates also means higher exhaust temperatures and lower efficiency.

    The new generation of tank heaters are very well insulated and so lose little heat and the direct vent or power-vent units have very little stand-by flue loss, so the overall efficiency can be quite high and they have storage capacity which gives much higher first hour hot water delivery. The burner doesn't have to cycle as often, which also increases operational efficiency. And a decent tank heater can cost half the price of an instant heater.

    Tank heaters are also available with additional internal heat exchanger coils, so they can be used with solar thermal systems or with radiant floor heating systems.

    The only advantage to an instantaneous heater is that they take up less space. For low use hot water, they can be OK. Otherwise a gas-fired, direct-vent tank heater can be more versatile, at least as efficient, and less costly (except for the top of the line Polaris).

  9. Kevin Dickson, MSME, P.E. | | #9

    But do keep the solar DHW sytem. A 2-3 panel system, however, won't have any extra BTUs in the winter for space heat, especially with your 6 person family. 4 panels would be even better since the marginal cost of the 4th panel is low. It still won't have extra heat in the winter, and won't be oversized in summer.

  10. GBA Editor
    Martin Holladay | | #10

    Because of their high price, it's hard to justify the incremental cost of an instantaneous water heater. Most families won't see enough energy savings to justify the incremental cost.

    Many users are irritated by the quirks of instantaneous water heaters, which take some getting used to. If the flow rate is too slow, the heaters' burners don't come on. If you are running warm water -- a mix of hot and cold -- and you adjust the temperature by turning down the hot tap instead of turning up the cold tap, you can end up with no hot flow at all.

    If you sometimes need just a trickle of hot water, you can't get that with an instantaneous heater.

    Repairs are likely to be more frequent and more expensive than with a simple tank-style heater.

  11. steve bumpus | | #11

    As the saying goes, " you just saved me a ton of money"!!!! thank you both in my decision making!!!!


  12. Steve El | | #12

    Steve B, in making your choice beware of creating an "orphaned hot water heater" and factor in cost of chimney lining if needed (our contractor estimated that cost for our situation at $600)

  13. steve bumpus | | #13

    thanx steve----no chimney for vent hwh AND minisplit!!!!

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