Electric panel resistive element water heating

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Re: Electric panel resistive element water heating

Postby Gordon-Loomberah » Fri Nov 04, 2016 10:24 am

The instantaneous gas heater from Bosch I used to have just need a couple of D cells for the igniter, 230V not required.
I thought the requirement was to reach 60C once per week, my evacuated tube system has no booster, but I'm fairly sure it does reach 60C at least once a week even in mid winter. In any case, neither of the SHW systems I've used since 1991 have had boosting elements, and the legions are yet to attack me. I'd be more concerned about evaporative AC units for legionella.

At the moment, we only have to put up with a very few luke warm showers per year- I'm a bit more tolerant of the water temp than my wife. I have a heat exchanger to transfer heat from the 78 tube hydronics water heater after a couple of days of cloud, but it wont last 5 days. I do have a 1000l tank that I'll start to store water in one of these days, but that needs insulating and a housing built around/over it to keep the rain off and the rats out - anything costing money that isn't urgent is down the list of priorities of things to do ATM.


Instantaneous electric is another option, but they are typically 5-8kW elements, luckily they only need to run for short periods... well unless you have teenagers in the house :lol:
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Re: Electric panel resistive element water heating

Postby offgridQLD » Fri Nov 04, 2016 11:13 am

So what is actually heating your hot water Gordon for showers the same 78 tube hydronics water heater system or a separate smaller system?

I think I will just see how it works out with the 300lt tank and 30x 1800 long tubes. No point in removing the instantaneous gas from the wall as Im sure a 9kg bottle would sort out the odd cold shower in winter. If it not enough I could add a 2nd collector.

The standby power of the Instantaneous gas heater was flickering between 0 and 1w on the energy meter) So they are no big deal to have on 24/7

Though I'm counting on the vac tubes to be a lot more efficient than the flat plate at our other house and that's been good enough for us.

I just pulled the cover off and checked the temps (3 of us had a shower this morning (I think my wife had another one last night to) show head isn't that efficient and wife and daughter tend to use the shower as a meditation zone for long periods standing under the water :lol: along with two dishwasher loads. at 10.45 collector temp was 70C and tank was 54.4c at the very bottom. I'm 100% sure the system is only average at best due to how dirty the collector is and some shading in the morning + sub par insulation on the pipes. (I didnt install it) Mind you this is a good time of year to heat water from the sun.

Image


My new system will be very short pipe runs and very well insulated along with it being not very high off the ground I can clean it regularly.
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Re: Electric panel resistive element water heating

Postby Gordon-Loomberah » Fri Nov 04, 2016 11:39 am

The HW for the house supply is produced with the 30 evacuated tube system. Heat from the 78 tube system can be added to that water with the heat exchanger just before it comes into the tempering valve
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Re: Electric panel resistive element water heating

Postby offgridQLD » Fri Nov 04, 2016 11:46 am

Ok , I was thinking 78 tubes was a little over the top.

I was just looking at the SS tank and notice it had a M anode in one of the ports . Made me worry it wasn't SS at first though confirmed it is SS via the element cover access.

Trying to think what good it would do I guess if you had particularly acidic or harsh water into it you could get some pitting of the SS over a very long time.
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Re: Electric panel resistive element water heating

Postby australsolarier » Fri Nov 04, 2016 11:51 am

Kurt,
the efficiency between flat plate collectors and vacuum heat pipe collectors are not much different. in your climate the flat plate might actually be more efficient.
the reason for this is the flat plate uses the whole area to collect the sun. the vacuum heat pipe has gaps in between. so the aperture for a given area is larger for the flat plate.
the heat pipe collectors however are more efficient in windy and cold weather or at high temperatures. simply because they lose less heat. they are also easier to install. you install the lightweight frame, than stick in the tubes. also during hail damage they are easier to fix. the flat plate is recommended to dismantle and put the new glass in on the ground.
so if you are in a hail prone area, i suggest you get a few empty tubes for spares. the chinese usually send more tubes in their deliveries for breakages on transit. the retailers then keep them to sell them later. they have different lengths and diameters.
if you install them in sunny weather, keep the tubes in the shade until you insert them. the tip gets really really hot!!!! the easiest way is to pull out the copper head a bit so you can see where it goes into the hole of the manifold. then slide the glass tube into the large hole. the kit comes with a white heat conductor tube for the head. also lubricate the glass tube for easier insertion. when you touch the glass tubes and one is hot, it means you missed the hole in the manifold. if the inside of the tube gets white, it means the vacuum is lost.
when the temperature goes over 80 degrees or so, the circulation pump turns off and the system goes into stagnation. the water in the manifold evaporates and the temperature there goes up to 160degress or so. i personally don't like this to happen. in my case the hot water gets diverted into a cooling tank before hitting the manifold again. in kurt's case, well, a forced meditation session might be in order. or else maybe draining some hot water back into the rain water tank. the controller might have a relay for that function.

the faster you run the circulation pump the more energy you remove from the manifold. (this is not the same for temperature, thermodynamic laws) and vice versa. so the higher the turn on temperature and turn off points, the less energy collected though the water might be hotter at the top. some large systems for space heating use this to avoid stagnation. when there is a sunny day coming and no heating required they set the turn on point to 98 degrees. this delays stagnation or avoids it altogether. half the energy is harvested doing it that way. (having in mind in germany they usually have glycol in their systems and it degrades if you boil it. the boiling point is also up at 130degrees, depending how much glycol is in it and is less energy dense.
just food for thought
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Re: Electric panel resistive element water heating

Postby australsolarier » Fri Nov 04, 2016 11:56 am

Kurt,
the sacrificial anode in stainless steel tanks is because of the weld in the seam. also because of cheaper stainless steel i suppose. and yes rainwater is gentler on the corrosion than acidic bore water. i suppose that is one of the reasons the apricot or edward stainless steel tank systems are more expensive.
also keep in mind the hotter the water the more corrosion.
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Re: Electric panel resistive element water heating

Postby offgridQLD » Fri Nov 04, 2016 12:14 pm

Good points,
I was given a few extra tubes with my system. Just took them 105km on twisty bumpy roads to the offgrid house yesterday was a bit nerve wracking with them in the ute. No casualty's though :lol:

I think the weak point of the tubes would be that little nipple of glass at the very tip. One strike on that would be like hitting the corner of a pool fence glass sheet.

I put the collector frame together the last week and it looks ok though I will reduce the height of the back legs and cross bracing to give a lower inclination.

I was thinking its going to be to efficient in summer and a bit of shading of the collectors could help out in the very hot parts of the year.

As for the SS well yes it is imported but not to the point of being a 400 grade mildly ferrous SS. I think it would just be run of the mill 304.

That said in the past at work we rebuilt some 40 year old CIP systems that have steam injection to heat the water (hard town water) to very high temps and then all kinds of nasty caustic solutions get dosed into the water and no ill effects on 304. The worst I have seen is a mixer that would have a very strong vinegar solution that had some pitting of the SS from idiots who didn't wash the caustic cleaning solution out and let it sit for long periods with vinegar mixed. Though even that took 15+ year.

I would say the weld seems would be just the same as the tank and robotic tig seam welded with 316 wire. As long as correct passivization was done after I don't see it being a weak point.
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Re: Electric panel resistive element water heating

Postby Gordon-Loomberah » Fri Nov 04, 2016 12:25 pm

My 30 tube system has a Magnesium anode in it, I should have a look and see how much it has eroded after over 6 years of use.
We only have rainwater, and it is quite acidic due to dissolved CO2 (hey you people burning coal for your power and making my water more acidic, stop it now! ;) ), with a pH of under 6.
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Re: Electric panel resistive element water heating

Postby offgridQLD » Fri Nov 04, 2016 12:28 pm

I should check the PH of our rainwater. Perhaps all the dust on our roof from the dirt road would neutralize the acid a little :lol:
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Re: Electric panel resistive element water heating

Postby Gordon-Loomberah » Fri Nov 04, 2016 12:33 pm

australsolarier wrote:the efficiency between flat plate collectors and vacuum heat pipe collectors are not much different. in your climate the flat plate might actually be more efficient.
the reason for this is the flat plate uses the whole area to collect the sun. the vacuum heat pipe has gaps in between. so the aperture for a given area is larger for the flat plate.


I'm pretty sure area for area, the tubes are significantly more efficient, and when they have a reflector behind them, as my RunOnSun system has, the "gaps between the tubes" argument vanishes.
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