We are brainstorming about the 'dream home' we want to build, our main focus being trying to keep it small but comfortable living (20' x 40' footprint). What if we plumbed the house using point of use instant hot water rather than whole house instant? Is anyone doing this and is it an efficient option? I was thinking we'd save on plumbing, only having to plumb for cold, little heat waste (no hot water left in pipes) no need for those re-circulating units... We found a Bosh unit at Home Depot a flow rate of 2Gallon/minute: PowerStar AE 12 Tankless Point-of-use Water Heater $200. The way the house is designed the kitchen sink shares a wall w/the powder room sink w/the dishwasher right next to the kitchen sink. Upstairs the shower/tub and washing machine are on a shared wall (directly over the wet wall downstairs) and the single sink is next to the shower/tub. My biggest concern is the flow rate for the bathtub and looking at this factor it seems that the size unit you need just to fill a tub might be big enough to service the whole house?? FYI, we plan on a Wood Ofuro tub (standard capacity of 36 gallons) which would fill in 12 min. at a flow rate of 3gpm, and the front load washer we're looking at heats its own water. Maybe we should put a larger unit upstairs for the tub/washing machine/and sink, and a smaller unit (like the Bosh above) for the downstairs (kitchen sink, powder room sink & dishwasher)?

For showering and tub filling the tankless heaters do need to be really big units. You have to look closely at what each unit can handle, because the price seems to go up dramatically with it's ability.

Efficiency-wise you'd save quite a bit on distribution losses (they'd go to zero, whereas distribution losses are typically 10-25% of the total energy used in hot water). But the size of your electric service may need a large increase, since the power distribution has to assume the possibility of all of these several 50A 240Vloads being operated simulaneously at full modulation.

Your peak power is going to be on tub fills & showers, not kitchen stuff. You can cut the power required for showers in half with a drainwater heat recovery heat exchanger (search the forum for details), so the Bosch could cut it for multi-use in the bathroom. Tub fills would be at fairly low flow though- possibly sub-2gpm in winter to keep the temp up. (You typically need 110-115F in a tub fill, but only 105-110F for showers.) If you had a tempering-tank in line with the cold feed to the bathroom tub fills would be faster, since you'd be starting with 55-70F water, not 35-50F. You may still want to avoid running a hot/warm-water laundry load when someone is in the shower though- particularly in winter. You can get away with smaller unit for the kitchen.

How are you heating this place, in your dreams? (Combi heat/HW tends to increase the efficiency of both when done right.)

wow thanks Dana1 for the great info! To answer your question, we will heat the house with one wood stove, no other form of heat is necessary. We currently have a 3,000 square foot home that heats great with one little Lopi :) Plus IMHO hot water radiant heat works well in areas of consistant sunshine (like in CO where we are now), but not so great in the Pacific NW (where we plan to be). I will look into that drainwater heat recovery system, sounds interesting, I can't even begin to visualize how that would work. How large is the tempering tank? ie. would fit into our wall cavity... We thought about a solar pre-heat (we have this currently) but we want to keep our envelope tight and don't want big tanks or water heaters eating into our living space, plus then we'd have to run 2x as much pipe everywhere... We were thinking of having a recirculating inline heater for the tub to keep the water hot for longer. We take family baths (in turns or pairs) & often by the time my husband gets in it can be lukewarmm. Just curious, hypothetically, would any of these potential elements be mutually exclusive: recirculating heater, tempering tank, heat exchanger? THANKS :)

sorry for the block text, for some reason I can't get the formatting to stick??....

Drainwater heat recovery? Visualize this! :-) :



Natural Resources Canada has a standard for measuring performance between vendors & models, to sort out the mutts from the fast dogs in apples-to-apples comparison:

http://oee.nrcan.gc.ca/residential/personal/retrofit-homes/questions-answers.cfm#q45

(Hopefully they'll update it from time to time- there are other vendors/models out there.)

A cost/benefit analysis based on 7.5cent/kwh hot water heating was done for the state of MN here:

http://www.renewability.com/uploads/documents/en/analysis_dwhr_minnesota.pdf

The take-away message from that analysis is, that if you're gonna do it at all go with the biggest/longest/fattest that actually fits, because the net-present-value of the increased performance is far greater than the increase in cost from the larger units.  In your situation it makes even more sense to go big, since it's a capacity issue: How much  shower flow can you get out of a marginally rated electric tankless.  6-foot long 4-inchers like the 4x72" PowerPipe can deliver over 60% of the heat back at 2.5gpm, but it takes an optimally deep basement clearance to install one of those. It's easier to accomodate at the planning stage than retrofitting one that size.  They can only return the heat when both the drain & potable water is flowing simultaneously though (showers, not tub fills.)

The size of a tempering tank need be no bigger than your daily use.  If you'r'e on a well, your storage tank buffer & tempers a few 10s of gallons, but a tub fill is probably less than 1/4 of the total.  Water use patterns are highly variable, but 70gallons/person/day is typical, 45-50 gallons/day if you're conservative.  If it's just in series with the shower/tub, it's a matter of timing how long your showers are and measuring the flow, and figuring how often you fill the tub.  For "typical" families showering is 40% of all hot-water use, not sure how that breaks down to total water use.

A tempering tank is heated to roughly room temp by the home's heating system (wood stove, in your case), which is roughly half the finish temp in cold water areas.  In humid zones it's likely to "sweat" quite a bit of condensation when under high flow- so a tolerant floor finish (concrete pad?) or drip pan is usually a good idea.

An alternative to a tempering tank can sometimes be a 100-300' length of PEX embedded in a concrete slab-floor/wall that's completely inside conditioned space.  The thermal mass of the room-temperature concrete is effectively your storage & heat transefer medium to the water in the PEX.

There are many climates & regions in the PNW- active solar radiant can work great in some (the Columbia River Basin in WA, or much of eastern OR), but for much of the western slopes of the Cascades you can fuggedaboudit.

Forgot to mention- the recirculating heater for the tub can be much lower power than the rest (500W is probably enough 1KW is likely overkill.) Keeping your service drop to the house under 200A is probably more reasonable than 500A service, from an up-front cost POV, eh? ;-) You can probably make it work with a tiny point-of-use mini tank (eg. Bosch Ariston 2.5 gallon), but there may be better options.

And none of it is mutually exclusive.

we installed 2 tankless and 1 point of use  units in a 3000 sq. ft. house , bath/bath/kitchen...80amp/80amp/20amp and used a 300 amp electrical service

80 amp units kick on at 80 for the initial surge then stge down to 60 then 40

Dana1~ thanks for the diagram, this is what my husband and I figured, I knew the waste pipe could not wind around the water pipe for obvious reasons. I'm presuming that this is recommended to do on a horizontal (with grade) section rather than vertical...

Yep as I thought the Tempering tank will not work for us, with a slab on grade (no crawlspace)

I do LOVE the idea of the PEX in concrete! THANKS!

Yes we are planning 200 AMP service, and we also need to make sure my husband can use his welder!
My husband is British and I lived there long enough to learn to not run the kitchen tap when someone's in the shower ;o)

Chris~ thanks for the feedback! I think we should be able to get away with a 60 amp point of use upstairs and a 40 amp point of use downstairs

The drainwater heat exchanger needs to be vertical to work (and as plumb as possible for maximum performance.)  Hopefully the bath is on the second floor to be able to use drainwater heat recovery(?).  There is at least one vendor with a horizontal version, but little real-world feedback (and no 3rd party NRCan testing on it) yet:  

http://www.ecodrain.ca/



(I expect you may become all-too-intimate with the use of a plunger with horizontal plate-type heat exchangers though.  TBD.)

With any of 'em they're more effective if the output of the HX feeds both the hot water heater AND the cold feed to the shower. Sometimes it's more convenient to plumb it to feed the post-tempering distribution point to the whole house, which means the cold water from other taps might feel a little tepid (but not hot) while someone's in the shower. (That's the way it is at my house and it's barely room temp,  but I'm starting with ~48F+/- water, not tempered water. It'll be a bit warmer in a tempered water flow, but not body-temp.)

Whether you use PEX-in-concrete tempering on the cold water feed or not, it's usually worth putting at least R5 worth of XPS or high-density EPS rigid foam under that slab in heating dominated climates (R10 min if it's used for radiant heating). The most critical portion is the first couple feet around the slab edge however (put R10 min. there, even if it's nuthin' in the middle.)

Dana1~ Yup the slab has gotta be insulated! Our slab and foundation on our current house are and boy does it make a world of difference!!!

Thanks for all the great advise :)