Water heater to choose with solar preheat

[elcerritotom]

I'm hoping to get some sage counsel from you all about the following situation. I live in the San Francisco Bay Area and have a 15-year old integral collector storage system (40 gallon Copperheart) on my roof. It works via pressure from the municipal water system to serve as a pre-heat to a 15-year old natural gas 40-gallon tank conventional water heater. That conventional tank heater should be replaced given its age and condition, but using a conventional tank with solar preheat doesn't seem to take advantage of the solar preheat very well. But given my climate, I need some sort of a backup.

So what should I use as the backup for my solar preheat? I'd been planning on using a natural gas tankless unit. Installation of a new larger gas line would be relatively straightforward (given locations of gas meter and the water heater closet location). But it appears that the required 'delta T' (temperature differential between incoming and set-point/outgoing water) has to be fairly large to activate the units. While some need as little as 0.4GPM to activate the tankless burners/heating element, and 0.26GPM to sustain flow, that is at a fairly high delta T. And the point of solar pre-heat is to eliminate or reduce that delta T. I have learned that at least RInnai and Rheem are going to be releasing new models that are designed to work with lower delta T's sometime this year. (Their motivation is not the tiny solar thermal market, but warmer groundwater parts of California, where low-flow fixtures are the mandated normal, and homeowners want their units to fire/heat water at very low flow rates.) But a disadvantage of natural gas tankless units are methane losses from the unit, which NRDC and others are increasingly pointing to as reducing the climate change benefits of these units.

I know very little about heat pump water heaters, but my water heater plumbing is all in a small closet in a basement. There is an efficient condensing natural gas furnace in that space, but in general that room has a temperature often in the 50's or low 60's, and it doesn't get much harder in the summer. The furnace is turned on very infrequently to heat the house given the yearround moderate climate. The closet is adjacent to a large basement room, and we could keep the door open between the closet and basement, but the basement room has similar temps. I don't want to move the water heater backup location from the closet, given the existing plumbing.

What advice would folks have for me on what to use for a backup? If you've got an ICS system with a natural gas tankless backup, how does it do? I assume you would put a thermal mixing valve between the solar preheat incoming and the tankless unit. What temp do you set that at, given a typical 120F setpoint for domestic hot water out of the tankless? Do you have problems with the unit not always firing because of too small a delta T? Do people use electric heat pump water heaters with solar preheat?

I'd welcome any thoughts or wisdom you're able to share with me. My goals are trying to reduce my carbon footprint from water heating and to some extent reducing water heating costs also.
Cheers

[J.P.M.]

That conventional tank heater should be replaced given its age and condition, but using a conventional tank with solar preheat doesn't seem to take advantage of the solar preheat very well. But given my climate, I need some sort of a backup.

Why do you think that ? I've been doing that for a long time and it works just fine.

[jflorey2]

That conventional tank heater should be replaced given its age and condition, but using a conventional tank with solar preheat doesn't seem to take advantage of the solar preheat very well.

I think it would work just fine.

A tank heater is a simple beast. It has a thermostat that turns on the element (or burner) when the temperature is below a threshold and turns it off when it is above the threshold. If you feed it 55F water from city mains it will run a long time to heat the tank again if you drain it. If you feed it 80F water from your preheater it will run for a shorter time. If you feed it 110F water (and your thermostat is set for 110F) it will not run at all.

The only gotcha is that a tank heater will not cool water, so if your preheater can get your water to dangerous temperatures you need a tempering valve to reduce scald risks.

[elcerritotom]

Thanks for your replies, guys. It's quite kind of you to share your knowledge.

I guess I thought the conventional water heater tank backup wasn't ideal, since it stores a large quantity of water at the desired/set output temperature. In the winter in my climate on cloudy days,or on our frequent foggy days throughout the year, I assume I am getting no increase in temperature in my solar preheat into the conventional tank. So the tank will turn itself on periodically to keep the water therein at the output temperature. That seems wasteful and inefficient compared to a tankless unit that would only heat water when it is needed. Basically, I'm parroting back the standard language as to why tankless units are more efficient than tank units. (I've heard people say, "Would you leave your car idling in the garage overnight in case you wanted to go for a drive?"). So it would seem that when I'm getting no heating in my solar preheat, a tankless unit would be a more efficient backup than a conventional tank. It also seems to me that when my solar preheat is getting the water warmer than incoming from the city mains but below the desired output temperature the tankless would also be more efficient. But I am sure that you both know more about this than I do! Basically, I think of myself as well-intentioned here but probably ignorant.

Clearly my solar preheat is beneficial (when it's sunny enough to do some preheating) compared to not having a solar preheat. But wouldn't the tankless as the "backup" be a more efficient alternative than a tank? There's probably something I don't understand or see. Or maybe a solar preheat with a conventional tank backup is good and works well in comparison to no solar preheat. But maybe a tankless backup is better? Although maybe it wouldn't work as well because of the minimum flow rate issues? Certainly the current setup with the conventional tank has done a great job of providing hot water when needed for household use. I just wonder if there is a better alternative from an environmental perspective that would also work well.

And jflorey2, thanks for the tempering valve advice. I do have one on the output from the backup to the household fixtures to prevent overheating/scalding. And it has always seemed to do its job. Cheers

[Mike90250]

I have a well planned system, that has been working for 10 years, no troubles. I have a rooftop flat plate collector with glycol loop, heating it's 40 gal tank on the roof. That preheated water feeds my propane fired tankless which does the final boost to 115F
Often my summer water from the roof is 140F and I have a mixer valve before the tankless to prevent it's thermal safety from tripping off. (and prevent scalds).
The better tankless have multi stage burners and flow sensors to keep the outlet20160406_153916.jpg temps steady. The cheap ones, work like a yo-yo: hot cold hot cold hot cold
Additionally, I have a SS loop in my masonry heater that thermosiphons into an 80 gall storage tank on the 2nd floor. Winter time, I get 90F water out of that tank. The colder it is, the more I fire the masonry heater and the hotter the water from it.
The heat pump water heaters are at least into the 2nd & 3rd generation now, and the major bugs have been worked out, but they do need some warmth to work, or their resistance heater kicks on

( sorry, you'll have to dl the sketch to see it all, or pan back and forth)

1711_HotWaterSchematic_h.jpg

[LucMan]

Most of todays HP water heaters are ductable, so if your mechanical room is on an outside wall, or a short run it would be easy to vent to the outside with no worries of over cooling the room.

[J.P.M.]

Tom: I've always been in favor of not making things more complicated than necessary. If I was in your shoes (and I'm pretty close, with a flat plate solar water heater circulating potable H2O to/from an 80 gal. storage tank in the house), I'd skip the tankless. But before I scrapped what I had and went looking for something new, I'd make sure it's reached the end of its service life.

I'm still wondering what your system is doing that makes you think it needs replacement.

To your question on what to use for backup: If it was me, early/immediately I would:
1.) Find out /educate yourself about how solar thermal works.
2.) Get the system inspected by someone who knows what they're doing.
3.) Get components replaced in kind if the remaining service life is estimated at 2+ yrs. or so.

With that, and longer term, if the solar water heater is deemed to be approaching the end of it's service life, but not there yet, I'd probably keep it until it fails. Same for the nat. gas heater. But if the nat. gas heater failed first, given your climate, I'd probably keep the solar and replace the gas fired tank heater with a heat pump water heater (HPWH) and hang on to the solar water heater until it failed/popped a leak. Then, when the solar water heater fails, it could be replaced, with a portion of the HPWH load possibly added to that of the rest of the electrical load of the home for a larger PV system as appropriate. Even w/no PV, given your climate, the cost of fuel (electricity) for the HPWH may well be about the same, +/- a bit, as the cost of nat. gas you're burning now.

As Mike writes, things with tankless may have improved, but I'd suggest, partly because simplicity of design is one of my mantras and also because they're more expensive with respect to initial and installation costs than the conventional types of DHW tanks, and also partly because they are not yet as simple as a dumb gas fired tank heater - and so more prone to problems - I'd go with a HPWH for your situation. As for the running car analogy, I don't think it's a good one as it seems to overstate the losses and do in a somewhat scare tactic fashion. For example, the (measured) annual standby losses to my tank was the equivalent of ~ 40 therms of nat. gas/yr. burned at 70 % efficiency. I then put 6" of fiberglass around the sides and cut that loss by ~ 50 %. If you're paying about $1.50 or so per therm for nat. gas, given the cost, including mat. & labor of tankless vs conventional tank type, it may take you some time to recoup the cost of standby losses by going tankless. I'd check the cost of both as well as the cost of a HPWH.

BTW, the running car analogy does work in the case of continuously circulating systems that continuously pump hot water through (and back to the storage). Those things are a major waste any way you look at it, but that's off topic.

Another thing to think about with tankless is the flowrate needed for proper operation. This is somewhat ironic, but if you practice good conservation techniques, one thing you'll probably do is use low flow shower heads. Mine use (as measured) ~ 1.2 - 1.3 gal./minute. Some/Many gas fired tankless units require more than that (maybe 2 gal./minute) before they'll fire up, or if they do, they'll "chatter" if the flow rate is too low. So, save nat. gas with tankless and cause it to run more by using shower heads that use 2X as much energy heating 2X as much water so they don't chatter - and most likely fail sooner for the chattering.

Tankless units have, at their heart, what are called heat exchangers. I spent a good part of an engineering career designing heat exchangers and boilers. One of the things that causes problems and shortens service life of heat exchangers, particularly the fired type is high firing rates and short times of intermittent firing, which is the way instantaneous water heaters operate. Then, there's also the not talked about (partly because most sales folks and even many designers are clueless about it) but ever present problem of heat exchanger fouling where the heat transfer surfaces collect deposits from the water being heated and cake up/coat with stuff that causes real performance problems over time - sooner with harder water. While always present, that performance killer is made worse by the same intermittent operation. So, for my money, other and probably more reliable is better for the residential domestic water heating application, even if it doesn't wring the very last BTU out of the energy source. Intermittent heaters do have their place. Such units are designed for larger applications like hotels, hospitals, large apt. buildings and and industrial/process industry applications. I've designed such equipment. From what I've seen of the residential units, they are, IMO, less than fit for purpose for that application, and, given the current state of engineering heat transfer, I'd doubt they can be fit for purpose in a way that will make them as safe and reliable, and as cost effective as other means of meeting the duty of domestic water heating any time soon.

Water heating is a "dumb" task. It doesn't need bells/whistles and more stuff to go wrong. For my money and what I think I might know about the subject, if my HOA allowed it, I'd have put a nat. circulation batch heater on my roof as a preheater and piped it to/from a conventional heater in the house. I got as close to that as I could with what I have.

[Ampster]

I built my first solar thermal water heating system in 1979. It worked great but eventually it wore out. Since then the cost of PV has come down and efficient heat pumps have changed the economics. Solar thermal is still economical for swimming pools. For domestic hot water natural gas is the most cost effective but my preference is to store my excess PV generation via a heat pump water heater.

[elcerritotom]

Thank you all for your thoughtful replies. This forum is a real community service. There's a lot of good information for me to digest.

Ampster, I don't have much extra PV generation at his point, and before long will have none when we get a plug-in vehicle. But I really like your concept of heating water via HPWH with excess locally-generated electricity via PV.

Mike, you have quite an impressive system. Well done. What temperature do you set your mixing valve before your propane tankless unit to? Looks like your domestic water out from the tankless is 115, so I'm wondering what the input temp is (how big is that delta T)? It seems that if the delta between water into and out of the tankless is too high, there will be a range there where the tankless won't fire at low demand flow rates from fixtures. Have you had any issues with that? Did you have to play around with the water temp coming in to find the right spot? Do you have low-flow fixtures?

JPM, super helpful thoughts. The solar part of my system is in great shape and should last I would think for many years to come. It's the conventional natural gas 40-gallon tank that has at 15 years old come close to the end of it's life. Our water quality here is great (ie, soft), so problems of scale do make things like tank water heaters or tankless natural gas heaters last longer than other places. But my tank is corroded and needs replacement (just normal lifespan, exacerbated by a complete lack of maintenance, anode replacement, regular draining etc). And it's in a location where a catastrophic leak would be a big soggy mess. So the solar part of the system is not broken but I need a new "backup" of some sort (and in our climate, which is foggier than many other parts of California) that backup is the sole heat source a significant part of the time. Your point about needing more water to trigger the tankless units, and thus pitting water conservation in conflict with fuel conservation makes a lot of sense. The highest efficiency condensing tankless units I've been considering have a flow activation rate of 0.4GPM, and 0.26GPM to continue heating once activated. But those rates are only true for delta T's of over 35F or more. At delta T of 20F, you need a flow rate of 2GPM to activate, which becomes tricky with our low-flow fixtures. Now Rinnai and Rheem are coming out with units specifically better at tripping at lower flow rates than today's models, given a certain delta T. But you make pretty compelling arguments against tankless generally. I haven't done nearly enough investigation of heatpump water heaters, but given your advice, and others above as well, that is what I will explore next.

Our location would be against an exterior wall, so venting outside is definitely an option. Thanks Lucman for making me aware of that. There's a furnace in the small closet the water heater is in now, so that raises the temperatures slightly when the furnace is on. But it is rarely on. The closet is next to a big room that is largely below grade, and seems to therefore maintain fairly constant temperatures throughout the year. I think it rarely gets over 68F, and rarely below 50F, and often is at a temp between the low 50's and the low 60's. It'd be easy to keep the door between the closet and that room open. Anyway, it looks like I need to explore the HPWH option. It wouldn't be impossible for me to pull a new 220V line from my electrical room to that closet, although it certainly wouldn't be straightforward. Again, I just want to reiterate how kind it is of all of you to volunteer your wisdom to people like me, helping us make better choices.

[Mike90250]

We have had zero low flow hot water issues. The mixer valve is set for about 120F, so in the summer, the water gets a bit hotter the longer you run it !!
in winter, the tankless inlet is about 70F. so the heater just loafs along at idle.

And in Kalifornia, low flow everything is mandated, even your lawn sprinklers, so you let them go for 4 hours instead of 30 min. 4th time this month i forgot to turn it off !!

[LucMan]

I would like to point out a few facts on high efficiency mechanical equipement such as instantaneous water heaters, 90%+ furnaces and HVAC equipement. These are sophisticated pieces of equipement with additional parts, that can and do fail. If you can repair these yourself that is excellent but if you can't and you need a tech to trouble shoot and repair your equipement the repair cost will be equal too or be above the energy savings you have realized. Parts for units over 10 years old may be difficult if not impossible to source as models change yearly. The newer models are easier to trouble shoot as they have a self diagnostic feature that at least will point the tech to the general area of the problem reducing trouble shooting time. OEM parts are extremely costly as they are usually manufacturer specific.
If you can KISS even if the EPA is limiting your choices

[J.P.M.]

We have had zero low flow hot water issues. The mixer valve is set for about 120F, so in the summer, the water gets a bit hotter the longer you run it !!
in winter, the tankless inlet is about 70F. so the heater just loafs along at idle.

And in Kalifornia, low flow everything is mandated, even your lawn sprinklers, so you let them go for 4 hours instead of 30 min. 4th time this month i forgot to turn it off !!

Q : Have you measured the flowrate of your faucets and shower heads ?

[Mike90250]

Q : Have you measured the flowrate of your faucets and shower heads ?

They are Calif Low flow . Miserable

[J.P.M.]

They are Calif Low flow . Miserable

Thank you. I'll take that as a no.

FWIW, I found a shower head something like 40 yrs. ago that was called "The Incredible Head". The name was changed from a hard PG-13 to something else, but it's still available. I've used then ever since and while opinions vary, I find them adequate and fit for purpose. As mentioned in prior posts, I've measured the flowrate at 1.2 - 1.3 or so GPM at 40 or 50 PSI line pressure. I think they cost ~ $10 or so at the present time.