Question:
Hi, all- Is there a general rule of thumb for sizing a hot water system for a house? We have six people living in the house, half shower takers and half bathtub users. The bathtubs will not be over-sized and the showers will all have 2.5GPM heads. The water will be solar heated. The cold water supply will be from an above-ground water storage tank. I’ve had a system with an 80 gallon tank recommended, but my gut feeling is that this won’t be large enough. Where we’re at now we have a 50 gallon gas fired water heater and it runs quite a bit in the evenings when baths and showers are taken. Would a 120 gallon system be ridiculously large? How about two 80 gallon systems? kickaha
Response:
– Hide quoted text — Show quoted text – > Hi, all- > Is there a general rule of thumb for sizing a hot water system for a house? > We have six people living in the house, half shower takers and half > bathtub users. > The bathtubs will not be over-sized and the showers will all have 2.5GPM > heads. > The water will be solar heated. The cold water supply will be from an > above-ground water storage tank. > I’ve had a system with an 80 gallon tank recommended, but my gut feeling > is that this won’t be large enough. > Where we’re at now we have a 50 gallon gas fired water heater and it > runs quite a bit in the evenings when baths and showers are taken. > Would a 120 gallon system be ridiculously large? How about two 80 > gallon systems? > kickaha
Well we used a 200 Gal (header tank) for the last 16 years or so most of the time for 4 people but now just two. Pumped water once a day to keep it full without long pump run times. It also gave up to 4 – 5 days use if pumping was not able to be done for whatever reason. George
Response:
>We have six people living in the house, half shower takers and half >bathtub users.
It’s also going to make a difference how long they shower for. IIRC showers take alot less water than baths, but as always, your milage may vary. Can you split bathing up between morning and evening? That’ll help with sizing and runtime, but solar of course changes everything… >The bathtubs will not be over-sized and the showers will all have 2.5GPM >heads.
My experience with low-flow showerheads is that I just shower longer, as rinsing takes a certain number of gallons, not a certain amount of time. I prefer normal showerheads with manual flow control. >I’ve had a system with an 80 gallon tank recommended, but my gut feeling >is that this won’t be large enough.
I’ve got an 80-gallon tank with 2 adults and 2 kids, and it’s barely enough for three showers in the morning (but my daughter and I are not into conserving hot water. 8*) >Would a 120 gallon system be ridiculously large? How about two 80 >gallon systems?
Solar systems want to have lots of storage capacity to cover showering at night _and_ in the morning, plus getting through rainy/cloudy days. I’d probably have a 120-gallon solar preheat with an 80-gallon main heater (what’s your backup heat source?) for six people… — William Smith ComputerSmiths Consulting, Inc. www.compusmiths.com
Response:
– Hide quoted text — Show quoted text -> Hi, all- > Is there a general rule of thumb for sizing a hot water system for a house? > We have six people living in the house, half shower takers and half > bathtub users. > The bathtubs will not be over-sized and the showers will all have 2.5GPM > heads. > The water will be solar heated. The cold water supply will be from an > above-ground water storage tank. > I’ve had a system with an 80 gallon tank recommended, but my gut feeling > is that this won’t be large enough. > Where we’re at now we have a 50 gallon gas fired water heater and it > runs quite a bit in the evenings when baths and showers are taken. > Would a 120 gallon system be ridiculously large? How about two 80 > gallon systems? > kickaha
I have an 80 with one 4×10 collector, year around I think I get something like 70-80 % of my hot water. You probably have forgotten about dish and clothes washing. My collector is flat on the roof and is not optimally aimed. If you want all of you heat from solar, again depending on where you live your going to need 4-6 collectors and I agree with your estimate of 120 gallons. Talk to some contractors and home owners in the area and see what works where you are.
Response:
– Hide quoted text — Show quoted text -> Hi, all- > Is there a general rule of thumb for sizing a hot water system for a house? > We have six people living in the house, half shower takers and half > bathtub users. > The bathtubs will not be over-sized and the showers will all have 2.5GPM > heads. > The water will be solar heated. The cold water supply will be from an > above-ground water storage tank. > I’ve had a system with an 80 gallon tank recommended, but my gut feeling > is that this won’t be large enough. > Where we’re at now we have a 50 gallon gas fired water heater and it > runs quite a bit in the evenings when baths and showers are taken. > Would a 120 gallon system be ridiculously large? How about two 80 > gallon systems?
One idea that I’ve mentioned on these NG’s before is a heat-exchanger to recover some of the heat from the shower drain water. I have one from www.endlessshower.com and it works well for us. By recovering some of the energy from the drain water, you use less hot water for the same comfort level in the shower. We have a 1.5 gpm shower head with pretty good spray flow (we have low water pressure in this area, I think it’s meant to be a 2.5 gpm head, but I measured it at 1.5). Anyway, a 50 gal tank set for 130F and four of us can shower without running out of hot water. I don’t use a lot of *hot*, but my wife and daughter sure do 
It only helps for continuous flow things like showers though. Won’t do much at all for the bathers. Search this or alt.solar.thermal for some discussions about using storage tanks and other setups to reclaim some heat from bathtub water. daestrom
Response:
– Hide quoted text — Show quoted text -> Hi, all- > Is there a general rule of thumb for sizing a hot water system for a > house? > We have six people living in the house, half shower takers and half > bathtub users. > The bathtubs will not be over-sized and the showers will all have 2.5GPM > heads. > The water will be solar heated. The cold water supply will be from an > above-ground water storage tank. > I’ve had a system with an 80 gallon tank recommended, but my gut feeling > is that this won’t be large enough. > Where we’re at now we have a 50 gallon gas fired water heater and it > runs quite a bit in the evenings when baths and showers are taken. > Would a 120 gallon system be ridiculously large? How about two 80 > gallon systems? > One idea that I’ve mentioned on these NG’s before is a heat-exchanger to > recover some of the heat from the shower drain water. I have one from > www.endlessshower.com and it works well for us. By recovering some of the > energy from the drain water, you use less hot water for the same comfort > level in the shower. We have a 1.5 gpm shower head with pretty good spray > flow (we have low water pressure in this area, I think it’s meant to be a > 2.5 gpm head, but I measured it at 1.5). Anyway, a 50 gal tank set for 130F > and four of us can shower without running out of hot water. I don’t use a > lot of *hot*, but my wife and daughter sure do > It only helps for continuous flow things like showers though. Won’t do much > at all for the bathers. Search this or alt.solar.thermal for some > discussions about using storage tanks and other setups to reclaim some heat > from bathtub water. > daestrom
Great….. Sounds like you’re heating the drain water. Good for septic systems.
Response:
I think you and I went over this awhile back. I’m actually thinking about installing three of their units, but am having trouble finding them…. kickaha
Response:
– Hide quoted text — Show quoted text -> Hi, all- > Is there a general rule of thumb for sizing a hot water system for a house? > We have six people living in the house, half shower takers and half > bathtub users. > The bathtubs will not be over-sized and the showers will all have 2.5GPM > heads. > The water will be solar heated. The cold water supply will be from an > above-ground water storage tank. > I’ve had a system with an 80 gallon tank recommended, but my gut feeling > is that this won’t be large enough.
Its not the size of the tank that is so crucial as the size of the burner (recovery rate issue). 50 gallons with a high enough recovery rate would work just fine… but since you are supplimenting with solar, heat storage capacity is an issue, so 80 gallons is better. If you wanted to go ultra high effiency you could get a 120 gallon tank with water heated by a heat pump (you can buy em)…thats SLOW…but with a big reserve it doesnt matter, Those will have gas or electric back up if you want as well…but at 120 gallons you should be running on the heat pump and solar almost all the time’ Wth the heat pump it puts out cold air that you can arrange to help cool the house…talk to the dealer on those details. Phil Scott – Hide quoted text — Show quoted text -> Where we’re at now we have a 50 gallon gas fired water heater and it > runs quite a bit in the evenings when baths and showers are taken. > Would a 120 gallon system be ridiculously large? How about two 80 > gallon systems? > kickaha
Response:
>If you wanted to go ultra high effiency you could get a 120 gallon tank with >water heated by a heat pump (you can buy em)…thats SLOW…but with a big >reserve it doesnt matter, Those will have gas or electric back up if you >want as well…but at 120 gallons you should be running on the heat pump and >solar almost all the time’ >Wth the heat pump it puts out cold air that you can arrange to help cool the >house…talk to the dealer on those details.
Where would I find more information about heat-pump style hot water heaters? Thanks! — William Smith ComputerSmiths Consulting, Inc. www.compusmiths.com
Response:
– Hide quoted text — Show quoted text ->If you wanted to go ultra high effiency you could get a 120 gallon tank with >water heated by a heat pump (you can buy em)…thats SLOW…but with a big >reserve it doesnt matter, Those will have gas or electric back up if you >want as well…but at 120 gallons you should be running on the heat pump and >solar almost all the time’ >Wth the heat pump it puts out cold air that you can arrange to help cool the >house…talk to the dealer on those details. > Where would I find more information about heat-pump style hot water > heaters? > Thanks!
The latest with cost of operation figures..but its from Japan. http://global.kyocera.com/news/2003/0301.html A US firm thats been around for a while, with a different approach. http://www.colmaccoil.com/hpwh.asp There were 2,000 other hits on "heat pump water heaters" You can learn to search google and open a whole new range of infomation for yourself..its better than the other search engines. go to www.gooogle.com highlight the ‘web’ tab just above the blank box. type yer search words or search sentence in " " in the box and press search. For any late breaking news on an issue, highlight the ‘news’ tab and do the same search. I used "Heat pump water heater" Phil Scott – Hide quoted text — Show quoted text -> — > William Smith > ComputerSmiths Consulting, Inc. www.compusmiths.com
Response:
– Hide quoted text — Show quoted text -> > Hi, all- > > Is there a general rule of thumb for sizing a hot water system for a > house? > > We have six people living in the house, half shower takers and half > > bathtub users. > > The bathtubs will not be over-sized and the showers will all have 2.5GPM > > heads. > > The water will be solar heated. The cold water supply will be from an > > above-ground water storage tank. > > I’ve had a system with an 80 gallon tank recommended, but my gut feeling > > is that this won’t be large enough. > > Where we’re at now we have a 50 gallon gas fired water heater and it > > runs quite a bit in the evenings when baths and showers are taken. > > Would a 120 gallon system be ridiculously large? How about two 80 > > gallon systems? > One idea that I’ve mentioned on these NG’s before is a heat-exchanger to > recover some of the heat from the shower drain water. I have one from > www.endlessshower.com and it works well for us. By recovering some of the > energy from the drain water, you use less hot water for the same comfort > level in the shower. We have a 1.5 gpm shower head with pretty good spray > flow (we have low water pressure in this area, I think it’s meant to be a > 2.5 gpm head, but I measured it at 1.5). Anyway, a 50 gal tank set for > 130F > and four of us can shower without running out of hot water. I don’t use a > lot of *hot*, but my wife and daughter sure do > It only helps for continuous flow things like showers though. Won’t do > much > at all for the bathers. Search this or alt.solar.thermal for some > discussions about using storage tanks and other setups to reclaim some > heat > from bathtub water. > daestrom > Great….. > Sounds like you’re heating the drain water.
No silly, look at how the thing is connected. Drain water of about 100F, incoming *COLD* water line of 35F (winter time). Outgoing drain water of 70F, outgoing *cold* water line to thewater heater of 65F. (my numbers, measured in December, YMMV) Water heater normally would have to heat the water from 35F to 130F (95F delta) and now only heats from 65F to 130 (only 65F delta) a reduction of 68% heating needs for the same flow rate. And since the *cold* water going to the shower is also warmed from 35F to 65F, you use more of this ‘cool’ water and less water from the water-heater. I don’t have any flow instruments in the hot and cold lines, so I don’t have exact measurements. But I have noticed that the mixing valve handle now leans more towards the ‘cold’ side for a comfortable shower (at about 1:30 position instead of the 11:00 position before I installed the thing). So you win in two ways, preheating water to heater, and using less heater-water to take a comfortable shower. daestrom
Response:
>No silly, look at how the thing is connected. Drain water of about 100F, >incoming *COLD* water line of 35F (winter time). Outgoing drain water of >70F, outgoing *cold* water line to thewater heater of 65F.
As I recall, the main point of the original patent was that these things work best if the preheated fresh water goes to the cold water supply for the shower AND the cold input to the water heater, because this makes the heat exchanger flow rates equal. The device would also work well if the water heater and shower temps were the same, but that isn’t the case in most houses. >…Water heater normally would have to heat the water from 35F to 130F >(95F delta) and now only heats from 65F to 130 (only 65F delta) a >reduction of 68% heating needs for the same flow rate.
You might do better if you took 130 F showers Nick
Response:
>No silly, look at how the thing is connected. Drain water of about 100F, >incoming *COLD* water line of 35F (winter time). Outgoing drain water of >70F, outgoing *cold* water line to thewater heater of 65F. > As I recall, the main point of the original patent was that these > things work best if the preheated fresh water goes to the cold water > supply for the shower AND the cold input to the water heater, because > this makes the heat exchanger flow rates equal. The device would also > work well if the water heater and shower temps were the same, but > that isn’t the case in most houses.
Not so much that the two flows are *equal*, just maximise the cold-water flow to have a higher Cmin for bettern performance. That much at least is standard HT technology. >…Water heater normally would have to heat the water from 35F to 130F >(95F delta) and now only heats from 65F to 130 (only 65F delta) a >reduction of 68% heating needs for the same flow rate. > You might do better if you took 130 F showers
Yeah, but ‘lobster red’ isn’t my best color 
daestrom
Response:
>Is there a general rule of thumb for sizing a hot water system for a house?
Maybe not. Family habits vary a lot, and non-solar sizing is usually based on recovery time, vs thermal storage tanks for solar water heating systems, which are often undersized because it’s more economical to have a smaller tank and use more backup fuel. >We have six people living in the house, half shower takers and half >bathtub users… the showers will all have 2.5GPM heads. The water >will be solar heated.
You might consider 1.25 gpm heads with 110 F water… 3 10 min 105 F 2.5 gpm showers and 3 30 gal baths per day would use 165 gallons per day. If incoming water is 60 F, that’s 165×8(105-60) = 59.4K Btu/day. If the water is heated to 120 F, that’s 59.4K/(8(120-60)) = 124 gallons per day, or 620 gallons of hot water for 5 cloudy days in a row, with a perfectly insulated storage tank. You might use a greywater heat exchanger. If water leaves a shower drain at 100 F (this might be closer to 105 F with a fully-enclosed shower), the exchanger below might heat 60 F water to about 60+0.97(100-60) = 98.8, so it only needs another 165×8(105-98.8) = 8184 Btu/day, ie 2.4 kWh/day worth about 24 cents, if it comes from electricity. For solar top-up, you might build another heat exchanger just like it. I bought a new $35 55 gallon lined steel drum with a strong removable lid (because the drum might end up under 2′ of greywater head, with the inlet and outlet above the lid) and a bolt ring and a 3/4" bung and a 2" bung with a 3/4" threaded knockout. I’ve collected 8 more used drums since then. They are unlined, so they need a plastic film drum liner and a layer of plastic film under the lid. So far, I’ve coiled two pieces of 100 psi/73.4 F pipe from PT Industries (800) 44 ENDOT inside the drum. Their PBJ10041010001 1"x300′100psi NSF-certified pipe is actually tested to 500 psi. True Value hardware stores sell it for $59.99. Lowes sells the rest of the hardware needed. It’s all installed through the lid, so the drum itself has no holes: Lowes total sales# qty price description 25775 1 $5.73 24′ of 1.25" sump pump hose (for greywater I/O) 105473 1 1.28 2 SS 1.75" hose clamps (for greywater hose) 54129 2 3.24 1.25" female adapter (greywater barb inlet and outlet) 23859 2 2.36 1.25×1.5" reducing male adapter (bulkhead fittings) 75912 1 0.51 2 1.25" conduit locknuts (bulkhead fittings) 28299 1 1.53 2 1.25" reducing conduit washers (") 22716 1 1.36 1.5" PVC street elbow (horizontal greywater inlet) 23830 1 2.98 10′ 1.5" PVC pipe (for 3′ greywater outlet dip tube) The parts above are greywater plumbing ($18.99.) 23766 4 1.28 3/4" CPVC male adapter (for 1" pipe barbs) 23766 2 0.64 3/4" CPVC male adapter (fresh water I/O) 42000 2 3.84 3/4" FIP to 3/4" male hose adapter 23813 1 1.39 10′ 3/4" CPVC pipe (for 1"x3/4" fresh water outlet) 23760 2 0.96 3/4" CPVC T (fresh water I/O) 22643 2 0.86 3/4" CPVC street elbow (fresh water I/O) 4 - 1" 3/4" CPVC pipes (fresh water I/0) 1 - 3′ 3/4" CPVC pipe (fresh water inlet) 22667 2 2.56 2 SS 1.125" hose clamps (fresh water I/O) 219980 1 4.87 10.1 oz DAP silicone ultra caulk (bulkhead fittings) 150887 1 3.94 4 oz primer and 4 oz PVC cement The parts above are fresh water plumbing. Subtotal $39.33. 26371 1 6.83 1500 W electric water heater element 22230 1 2.31 1" galvanized T ("nut" for heating element) 61294 1 11.76 single element thermostat with safety 136343 1 0.56 5 10-24×3/4" machine screws (mount thermostat with 3) 33368 1 0.37 5 #10 SS flat washers (mount thermostat with 3) 198806 1 1.38 10 #0 rubber faucet washers (mount thermostat with 3) 8763 1 0.67 5 10-24 SS nuts (mount thermostat with 3) The parts above would make it a standalone water heater. Grand total: $63.21. For 4 10 min showers per day and 20 minutes of dishwashing at 1.25 gpm we might heat 75 gallons of 55 F water to 110 with 8×75(110-55) = 33K Btu with about 10 kWh worth about $1/day at 10 cents/kWh. If the "heat capacity flow rate" Cmin = Cmax = 75gx8/24h = 25 Btu/h-F and the pipe coil has A = 300Pi/12 = 78.5 ft^2 of surface with U = 10 Btu/h-F-ft^2 (for an HDPE pipe wall with slow-moving warm dirty water outside and 8×300Pi(1/2/12)^2 = 13 gallons of fresh water inside), the "Number of heat Transfer Units" for this counterflow heat exchanger NTU = AU/Cmin = 78.5ft^2×10Btu/h-F-ft^2/25Btu/h-F = 31.4, so the "efficiency" E = NTU/(NTU+1) = 97% for hot water usage in bursts of less than 13 gallons. This works best with equal greywater and cold water flows, with either a 110 F water heater setting (preferable), or the heat exchanger output feeding the cold water shower inlet as well as the water heater. The Hazen-Williams equation says L’ of d" smooth pipe with G gpm flow has a 0.0004227LG^1.852d^-4.871 psi loss. At 1.25 gpm, the pressure drop for 2 150′ coils of 1" pipe might be 0.0004227×150x(1.25/2)^1.852×1^-4.871 = 0.03 psi. We might wrap the drum with 3.5" of fiberglass and a 4′x8′ piece of foil- foamboard with 7 4′ kerfs (knife cuts partially through the board) to make an octagon and aluminum foil tape to cover the kerfs and hold it closed. Nick
Response:
no, you preheat the cold water, which lets you cut down on the hot water. — Steve Spence Renewable energy and sustainable living http://www.green-trust.org Donate $30 or more to Green Trust, and receive a copy of Joshua Tickell’s "From the Fryer to the Fuel Tank", the premier documentary of biodiesel and vegetable oil powered diesels.
– Hide quoted text — Show quoted text -> Great….. > Sounds like you’re heating the drain water. > Good for septic systems.
Response:
www.econar.com — Steve Spence Renewable energy and sustainable living http://www.green-trust.org Donate $30 or more to Green Trust, and receive a copy of Joshua Tickell’s "From the Fryer to the Fuel Tank", the premier documentary of biodiesel and vegetable oil powered diesels.
– Hide quoted text — Show quoted text ->If you wanted to go ultra high effiency you could get a 120 gallon tank with >water heated by a heat pump (you can buy em)…thats SLOW…but with a big >reserve it doesnt matter, Those will have gas or electric back up if you >want as well…but at 120 gallons you should be running on the heat pump and >solar almost all the time’ >Wth the heat pump it puts out cold air that you can arrange to help cool the >house…talk to the dealer on those details. > Where would I find more information about heat-pump style hot water > heaters? > Thanks! > — > William Smith > ComputerSmiths Consulting, Inc. www.compusmiths.com
Response:
in the uk i used to use a combination boiler that runs on natural gas and doesn’t use a tank. it works on a flow detector when you turn a tap or shower a flow is detected and the boiler starts heating the water this continues until you turn off the tap when the boiler turns off again so you only heat water you use when you use it. it use to take about 20 secs to get hot but thats mostly the water in the pipe between the the tap and boiler. also to save money set the water heater temp to what ever your shower tempreture you like and then just run the hot as there is no point heting the water too high then cooling it by mixing in cold water hope this helps Mark
– Hide quoted text — Show quoted text -> Hi, all- > Is there a general rule of thumb for sizing a hot water system for a house? > We have six people living in the house, half shower takers and half > bathtub users. > The bathtubs will not be over-sized and the showers will all have 2.5GPM > heads. > The water will be solar heated. The cold water supply will be from an > above-ground water storage tank. > I’ve had a system with an 80 gallon tank recommended, but my gut feeling > is that this won’t be large enough. > Where we’re at now we have a 50 gallon gas fired water heater and it > runs quite a bit in the evenings when baths and showers are taken. > Would a 120 gallon system be ridiculously large? How about two 80 > gallon systems? > kickaha
Response:
Actually, you are removing heat from the drain water. The "endless shower" heat exchanger transfers heat from the shower drain water to the COLD water line that feeds the DHW heater. I visited their website when this topic arose a few months ago, and the concept appears sound to me. The biggest stumbling block for me to use the device is the fact that my shower is located directly above the staircase going to my basement, and the heat exchanger would become an obstruction to using the staircase. Gordon Richmond
Response:
> Actually, you are removing heat from the drain water. The "endless > shower" heat exchanger transfers heat from the shower drain water to > the COLD water line that feeds the DHW heater. > I visited their website when this topic arose a few months ago, and > the concept appears sound to me. The biggest stumbling block for me to > use the device is the fact that my shower is located directly above > the staircase going to my basement, and the heat exchanger would > become an obstruction to using the staircase.
That’s unfortunate. I put the heat-exchanger in the main drain line for the whole house. I happened to have a 6 ft section of vertical drain pipe in the basement from all the waste lines from the first and second floors. If I ever build another house, this is one relatively cheap investment that I’ll definitely repeat. daestrom
Response:
preheat the cold going to the shower, not the water heater. — Steve Spence Renewable energy and sustainable living http://www.green-trust.org Donate $30 or more to Green Trust, and receive a copy of Joshua Tickell’s "From the Fryer to the Fuel Tank", the premier documentary of biodiesel and vegetable oil powered diesels.
– Hide quoted text — Show quoted text -> Actually, you are removing heat from the drain water. The "endless > shower" heat exchanger transfers heat from the shower drain water to > the COLD water line that feeds the DHW heater. > I visited their website when this topic arose a few months ago, and > the concept appears sound to me. The biggest stumbling block for me to > use the device is the fact that my shower is located directly above > the staircase going to my basement, and the heat exchanger would > become an obstruction to using the staircase. > Gordon Richmond
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