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Community and Q&A

Efficient hot water delivery for group lodging?

Ben Blue | Posted in Energy Efficiency and Durability on

While most advice on Green Building Advisor focuses on the case of the single-family home, I’m hoping for some guidance about making hot water for a group lodging:

48 beds, common bathrooms with 8 showers and 12 sinks total.
We have propane service, typically somewhere between $1.50-2.50/gal.
Electricity just crept up again to 19 cents/kwh.
Lodging is unoccupied most of July (summer camp), and for the rest of the year varies between groups staying for just 2 days a week and groups that stay for 7 days.

The lodging in question is located at a camp/retreat center in eastern San Diego County, California. It’s about 30 years old and was designed with a south-facing roof to accommodate solar. For many years the hot water needs were served by a 5 collector open-loop solar hot water system with 240 gallons of storage and a 100 gallon gas heater for backup. We’re at over 4000 feet and last winter during an extremely cold week, the circulation system failed/was overwhelmed and the collectors were ruined. Basically, the entire hot water system needs to be replaced as the gas heater is also failing. Interestingly, since the freeze, we’ve only had the aging 100 gallon LP hwh servicing the building and it’s met the demands of the guests with almost no complaints of hot water shortage.

In various other (newer) buildings we have either a pumped closed loop glycol system or a passive, roof-mounted integrated thermosiphon system feeding a propane on-demand heater. This makes sense in a lot of ways, since many weeks the lodges are only occupied 2-3 days out of 7, and when there is demand it’s typically high since it’s multiple showers running at once.

On the other hand, particularly in the case of the roof-mounted thermosiphon tank, it doesn’t make sense to me to store solar hot water overnight on the roof when the majority is used first thing in the morning. The company we’ve worked with in the past is pushing for two new thermosiphon units with a LP tank backup.

I’m open to exploring all options here. For less the install price of solar, we could install a high-efficiency condensing hwh that would meet all our demands. I’m wary of the solar installer’s advice since I get the feeling they push the thermosiphon units in almost all situations and totally dismissed me when I inquired about installing a drainback system utilizing a storage tank in our heated mechanical room with: “yeah, well, those sound all fancy and high-tech but they’ll freeze on you.”

Any help navigating this situation would be greatly appreciated. If I can provide additional helpful information, please let me know. Efficiency and conservation are an important part of the camp’s mission and program.

Thanks for your expertise.

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  1. Nick Welch | | #1

    If there is vertical space available (even just a few feet) below the showers, I would definitely look at augmenting any water heating system with a drain water heat recovery unit. It could easily halve your water heating load.

  2. Expert Member
    Dana Dorsett | | #2

    The problem with drainwater heat recovery and a remote hot water tank is the lengths of the runs between the heat exchanger and the water heater, or the distances from the drains to the hot water heater. It might work here, but it's not a no-brainer, and it's more than just about vertical drain lengths.

    But if it works, it pays back pretty quickly with propane, and you could down-size the storage &/or burner capacity on the hot water heater. It may be possible to do the whole building with just a few drainwater heat exchangers paralleled up near the main drain to the building: (Lowest internet price, but not all models available. The manufacturer sells direct, and bigger is always better, both length & diameter.)

    The pressure drop across a single PowerPipe or similar would be too much with 8 showers running, but it can easily handle 2, maybe 3. By returning 50% or more of the heat going down the drain to the incoming water stream you're now feeding the hot water heater and the cold sides of the shower with ~70-80F water instead of 35-50F water- extending the apparent capacity of the tank by about double. Given that the 100 gallon heater is handling the load, with drainwater heat recovery you could handle the whole thing with the 50 gallon 199KBTU/hr condensing Polaris:

    That would cut propane consumption for showering by at least half or more, since when paralleled up with equal flow the return efficiencies during a single or pair of simultaneous showers would be well above the NRCAN tested ratings:

    These things are dumber than a box of rocks, with no moving parts to wear out, and you'd get a good 40-50 years of service out of them as long as you don't dump acidic cleaners down the drains on a regular basis. It's basically copper drain pipe with a tight slinky of potable copper piping wrapped around the exterior.:

    Hopefully the drain & potable piping geometry works for taking this approach. A single 2gpm shower at 35incoming water temps (likely, in winter at 4000') takes about 70,000BTU/hr. With 50 gallons of storage and 35,000BTU/hr or more of heat recovery per shower you'd be able to support 8 simultaneous showers with the 199,000BTU/hr Polaris for several minutes, with a very short recovery time on the water heater, and it would use only half the propane. Since showering is likely to be by far the lions-share of hot water use it should be easily measurable in the fuel use numbers.

  3. Nick Welch | | #3

    Dana, thanks for the pricing link to EFI. Those prices are extremely good. The only prices I'd looked at were Home Depot and the manufacturer's list prices, and they were both pretty close to each other -- and way higher than EFI.

  4. Ben Blue | | #4

    I appreciate the thought of drainwater heat recovery--I love the concept and the conservation it offers. Unfortunately, in this building, all the (tiled-in) showers are on the bottom floor and share a slab with the mechanical room. Retrofitting for waste heat recovery would be a big project and I'm not sure we'd even be able to get it done in the time frame I'm going to have to replace the hwh. I'll look into it a little further though.

    The Polaris units seem to be well regarded. Would it make sense as an option even without drainwater heat recovery?

  5. Expert Member
    Dana Dorsett | | #5

    The if all the shower drains are below the slab & mechanical room, you're kind of stuck.

    The Polaris can manage 3 simultaneous showers all day & night until the propane tank runs out. It has comparable first-hour gallons performance to a 100 gallon standard water heater, but if all 8 showers are in use it's not the first hour performance that counts, it's the first 10 minutes that counts. With 1.5gpm low-flow shower heads (measure what you have with a bucket and stopwatch to see what gpm you're really running) you'd be able to run about 4 showers nearly forever, but with all 8 going you'd better be done in 5 minutes, and be willing to wait a few minutes before sending in the next batch of 8 showerers.

    If you've been doing OK with the 100 gallon tank, how big is the burner? If it's 100KBTU/hr or less and it's been meeting the load, the Polaris will be enough. You may have to upgrade the propane supply or regulator to handle the larger burner (depends on what else you have going), but if the regulator is at the hot water heater rather than further upstream on the propane supply you'd probably not have to deal with that.

    Nick- you need to open an account with EFI to get that price (do-able over the phone with a credit card), but they're willing to sell them onesie-twosie to 1-person shops (or individuals), but they try to keep their commercial side somewhat separate from their retail web-store type customers. They're a good outfit, they don' t mark up shipping communicate well, and are organized to ship orders quickly. As long as you don't demand hand-holding or habitually return stuff, or demand credit terms, small orders on their commercial side aren't a problem for them. When I last ordered a PowerPipe from them it shipped from their Wisconsin facility, and landed at my door in under a week, all transacted over the phone, paid via debit card.

  6. GBA Editor
    Martin Holladay | | #6

    As PV costs have dropped, the cost of solar thermal collectors has risen. These days, if you want to use a south-facing roof to lower your energy bills, it makes a lot more sense to install a PV array than it does to install solar thermal collectors. If you are paying 19 cents per kWh, you are a good candidate for PV. The economics are explained in my article, Solar Thermal is Dead.

    Of course, there is no need to make a decision about PV just because you have a hot water crisis. For the time being, buy a propane-fired water heater that meets your needs. You can add the PV at any time.

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