First the general question: for whole house use how much benefit is there really for a tankless system vs a newer energy efficient tank hot water heater?
Now my specific.
We are a family of 6 and have installed two 40 gallon hot water heaters in parallel. One is crapping out and I am thinking of replacing it with a tankless.
Is that a good idea or not and is it worth the extra cost? If we do it does it make more sense to install in parallel? In that case we need fewer gallons per minute as it really only needs to provide enough for half of our maximum expected flow - so probably 3 gpm max - but it is heating from outside cold water temps in winter. Or in series? In that case it can be fed water from a tanked system that has prewarmed it and only has to heat it up a lesser amount most of the time - but we would need a greater flow per minute rating and it would need to be able to handle at least one shower running with cold water coming in to be useful after we’ve gone through the 40 gallons.
Also, is it worth in any case saving the old tank (it doesn’t leak) to use to accept the outside water and then feed the other systems - warming up a little in the furnace room in which the heaters reside.
My plumber is not crazy about tankless systems but it may just be what he knows.
From reading a previous thread on tankless water heaters, I got the impression that they work well if you use gas for heating the water. I thought someone said that the electric variety needed an insanely high amount of current (120 amps?).
One of several previous threads on the subject is here.
I hate tankless heaters with passion. A tank of hot water provides much more stable regulation of temperature. A tankless heater provides terrible regulation and pressure or flow variations will result in very uncomfortable temperature variations. Enough to scald or make you jump out of the way. If you are just filling the tub or basin that is one thing but if you are taking a shower I do not recommend it at all.
If you put them in series, then you could get the smaller one and put it first in line - use it to feed the 40 gal tank. This way, while that one is emptying, a trickle feed of hot water is still coming into it. I’m betting this way you would never use all the water, and the 40 gal would seldom have to turn on at all except perhaps to replace any lost heat.
Related question: do the current (inefficient) electric tankless heaters work via resistance heating? Have any tried to do it with induction heating or perhaps even a microwave-type setup?
When people refer to electric resistance heaters being “inefficient”, they’re talking about the entire process. For example, if the power plant for you electricity supply is natural gas, the energy transformation process is something like:
Gas->Heat->Work->Electricity(using a turbine)->power transmission lines to your house->Heated Water.
Each transformation process step has some losses along the way. The main loss is the Heat->Work step at the powerplant, which is only ~35% efficient. Also note that you started with Heat early on in the process, but transformed that energy several times in order to end up with heat again:smack:. Even though the last step is near 100% efficient, the entire process is still pretty wasteful when taken as a whole, compared to the simple:
Gas->Heated Water
Energy transformation process of a gas-fired water heater.
Maybe old ones don’t regulate the temp well, but the new ones are perfect. I just had a Rinnai tankless (propane) installed for whole-house hot water, and we couldn’t be happier. This one is absolutely rock solid at the whatever temp needed - no variation to speak of.
My research also indicated that the electric ones are a problem.
If it’s not leaking, how is it crapping out? Is it gas or electric?
I used to have a 10 gallon (got it for free) hooked in series to a 40 gallon. I used it to preheat the water to room temp. Or I would plug it in when I wanted to use the garden tub or had guests. That seemed to work quite well.
Fair 'nuff on the water heater vs hot water heater correction and I had not realized that this has been covered so much before. I should’ve done a search.
We have gas currently and would be looking at gas tankless systems … if the current one does turn out to to be irreparable. For quite a while what was happening was that the pilot would just go out - alright so long as I manually relight it oh half an hour before big shower times and honestly most of the time no one noticed it was out. Then it became unlightable. Plumber replaced the pilot unit (or whatever the whole ensemble is called) but it is still turning itself off, now after being on only for maybe ten minutes - not even enough to heat the tank. They’ve called the manufacturer and tried various fixes that they have suggested but it still isn’t working. Now a manufacturer’s representative is coming to see what they can come up with but I am preparing for the probability that the simpliest thing to do will just be to scrap it.
Right now there is a 30% (up to $1500) tax credit for tankless system installs so it makes it a bit more tempting.
By “superstore off the furnace” do you mean “indirect water heating”? How do those work during warm weather months? We currently have a hot water boiler for heat (under floor radiant in the family room and kitchen and hot water forced elsewhere) with a recirculation loop (again the contractor insisted) that is somehow supposed to make for more even heat. Could I tie into that recirculation piping to preheat the water going to the water heater(s)? Would it be worth the labor cost?
Any more thoughts of the in parallel vs in series question? That would go for two tank systems as well. When these went in out contractor insisted on in parallel and I deferred to him, but I am still unconvinced. While we are down to one functioning water heater we are not experiencing a deficient flow rate of hot water - it is not that we have three people taking showers at a time - the more common situation is that my wife takes prolonged shower and then the kids go and take their long showers and run out before they are done (especially the kid who uses the bathroom on the third floor, as it uses a lot of water to just fill the pipes to get it there). My thought, shot down by our contractor, was that a series set-up would’ve allowed the first tank to partially heat the water such that the second tank would be able to have a fast enough recovery time during high use periods that we would not “run out” … Same total flow of hot water, same ability to deliver hot water to the shower for three slowpokes in a row, but one tank storing less hot water and thus subject to less demands and less loss. And by the same token partially preheating water going to a tankless system would allow a higher gallon per minute rate than would running them in parallel.
Again I deferred to the contractor’s expertise but if we need to replace should I still defer?
I am refering to gas on-demand heaters. They are pretty much universal in Spain because there is not enough space for the tank types. I have not been to a single apartment in Spain where I could have a good shower without the temperature fluctuating. None. Even in the most expensive construction. I think they are crap.
Maybe some are worse than others but in my view a tank heater is always far superior. In fact, the people I know in Spain who have the means and the space will install a tank heater even though they are much more expensive here.
I think on-demand heaters could theoretically work in a situation where the water pressure is very stable but they are just very sensitive to pressure variations. Maybe water pressure variotions in apartment buildings are greater as neighbors use water but I just hate it when I am in the shower and I have to get the hell out because the neighbor flushed the toilet. Water pressure variations have a much smaller effect on tank heaters than on tankless heaters.
I have a visceral loathing of electrically heated water tanks. Regardless of lagging, one is heating a load of water that warms the environment - and runs out when you really need it.
A total waste of money, space and a source of aggravation.
Personally I prefer gas multipoint, but would be interested in an electrical solution - like an electric shower.
I follow the point about water pressure surges, that can be fixed with a header tank and/or a really good shower thermostatic control.
Well, I dislike using electricity for heating because it is quite a bit more expensive so that would be out of the question for me. Gas would always be my first choice.
But between tank and tankless I would always go with a tank if I could. I’ll tell you what would probably be the ideal sanitary hot water system I’d like:
One really large tank or, better still, two smaller tanks in series. First tank heated by solar panels adequately sized. Second tank heated by gas, with the thermostat set as low as possible (40~45ºC). This means most of the time no gas is needed and only is used to supplement periods of insufficient insolation. I had this system in my old house and I hardly used gas for heating water. Keeping the thermostat down makes solar much more efficient, less heat is lost in the pipes and it eliminates the risk of scalding. I had the thermostat set so that I just opened the hot water and did not need to mix. That is the very best because even huge pressure variations will not affect the temperature one bit.
If you need very hot water (say for the dishwasher) then you can add a much smaller heater after the second one.
On demand gas heaters are terrible in places where water pressure fluctuates a lot or is just low. The heater needs a minimum water pressure or it will just turn off. A header tank really is not a solution as we are talking higher pressure. A pump would be a better solution and I used to have one installed just for that reason. Now the building has a pressure group but, still, while I can have showers reasonably well, nothing beats a tank.
One other point. On demand heaters are much more complicated than tank heaters and therefore more prone to problems and more expensive. They have moving parts, rubber diaphragm, electronics, etc which a tank heater does not need. A tank heater is as simple as it gets and so they can be cheaper.
The only real advantage I see to a tankless system is saving space. And even on my boat I have a tank.
If the goal is well regulated temp with a nearly endless supply would installing them in series with the tankless heater feeding into the tank heater work? The tankless heater would heat as much water as you need. Then in the tank it would mix to prevent any heat spikes. The tank heater would then only need to fire enough to keep the water hot during periods of inactivity.
Well, it might work but would be a bad arrangement all around. Tankless heaters produce a pressure drop and require more water pressure. If the concern is “recovery” a gas tank heater correctly sized should have no problem. As a rule of thumb a 50 gallon tank heater will recover in a bit over an hour. Which means you could drain about 85 gallons in the first hour and about 0.7 gallons / minute indefinitely after that. A correctly sized gas tank heater should not have problems running out of hot water except exceptionally when demand is unusually high. An electric heater has longer recovery time and so needs to be sized larger. (All this is off the top of my head so please correct any errors.)
Rather than have the heater before the tank what can be done and is done is to have the gas heater separate from the tank but recirculating into the tank. You can just have a big tank and draw water from the outlet, sent to the heater using a pump and then into the tank again. This is not uncommon but it is not your typical on-demand heater but a special type specifically designed for this purpose.
I would also note that there are no universal hard and fast rules and that if you want to squeeze the last penny then every installation has to be studied in detail and certain assumptions about usage made.
For example, a place used only on weekends is not a good candidate for a tank heater because you have to heat all the water in the tank only to lose the heat when you leave. Many variables come into play and need to be taken into account.
In my HVAC system, I have a desuperheater, which is similar to the indirect heating that you show, except that it constantly recirculates in an electrically-unconnected water heater, which feeds my gas-fired water heater. Of course since the water going into the gas-fired heater is already hot (thanks to the HVAC), the gas-fired heater seldom fires. It’s actually more efficient to use hot water than not to use it, since the desuperheater only uses a small amount of electricity for the recirculating pump; the heat is free as a result of the heat pump.