Water Heating

gary 

My partner and I often debate on the efficiency of turning off our hot water cylinder when our wetback is running. My theory is that with the fire ticking over pretty much 24hrs a day that the temperature of the water wont drop enough so that the element in the cylinder comes on. But on the occasions that the fire dies out, having the cylinder still on allows us to still get hot water without having to heat the cylinder from scratch. Anyone else out there have an opinion.

Re: gary 

Hello Gary,

I would maintain your cylinder element going to supplement your wetback as heating from cold not only has to heat the water but also heat the copper cylinder and pipes.

Consider the following – wrap your hot water cylinder with as much insulation as possible preferably with a wool based non irritant product. Next use the best and thickest foam/ foil pipe insulation from the wetback to the cylinder and up the vent pipe to under the roofing.

If you have any lagging on the hot feed pipes REMOVE it – seems illogical, but most hot water draw off runs for less than 8 minutes (even a decent shower), and it takes at least 8 minutes to heat a copper pipe to start to radiate to ambient. Any poor grade insulation also has to be heated and you will use more energy heating the insulation than you save with a bare pipe UNLESS the flow time is greater than 8 minutes.
In which case, you should have the same foam/foil pipe insulation but note that the capital cost of this grade insulation means it is rarely economical except for continuous operation.

Rex
(We have truth – just know that we do not have all the truth)

Re: gary 

Hi Gary

The lower the temperature of the water entering your wetback the higher the efficiency the wetback will run at transferring heat from the fire into the water. Thus keep the backup element off until such time the fire is turned off or someone wants to have a shower etc and the water isn't hot enough; then turn on the element.

Keep all your lagging on the pipes, preferably make sure its foamed rubber type not older carpet like backing. Copper pipes heat virtually instantly with water flowing through them, 8 minutes what a load of rubbish.

If you live in a very cold region, insulate the cold pipes also.

Re: gary 

Leaving the element on will mean using more electricity and less wood. Circulation through a wetback is very slow - when hot water is used the cylinder will have cold water in the bottom - with the element on it will be heating water until the thermostat cuts it off.
The wetback will then continue to heat the water. Turn the tap on again and cold water will again enter the cylinder - more element usage.
If you tend to keep the fire lit all day, turn the element off.
JK

Re: gary 

Forgot to add, the most important pipes to insulate are all hot water ones that go upwards from the top of the cylinder. These radiate heat all the time. Once a hot water pipe goes downwards it doesn't matter too much - these only lose heat when hot water is being used and savings with insulation are minimal.
JK

Re: gary 

Anon you have a point but it depends on if you are willing and available to turn your cylinder of at the critical times – most people do not.
I should have qualified; I would have the element set at 45oC and no hotter.

Solar Mike : You should have noted that I did not say that copper does not heat up for 8 minutes of hot water flow. We all know that copper conductivity is very high and heats almost immediately. I noted 'radiate to ambient' meaning the ‘emissivity’ of a bare copper pipe verses a hessian lagged copper pipe – I suggest some scientific reading of the relevant BRANZ Appraisal comment issued some 30 years ago to further your knowledge.

Rex
(We have truth – just know that we do not have all the truth)

Re: gary 

Re: gary 

Further information on why you could remove hessian pipe lagging to reduce energy loss on the under floor hot water pipes.

As a designer of high temperature pottery kilns, I observed the following.

The design for a continuous operating tunnel kiln, used 400mm of ceramic fibre insulation to reduce the 1300oC internal temperature down to 18oC on the exterior. It took about1200 minutes firing to heat the insulation to stability.

Conversely, a Raku kiln design using only 40mm of the same material as the tiles on the NASA space shuttle (yes, I had the same trouble with glue line shear that the shuttle tiles have), reduced the 1300oC internal down to 260oC external but took only 30minutes firing.

The thicker kiln was extremely efficient and radiated very little energy loss, but used, shall we say, 40 times the energy to initially heat the insulation, whereas the thinner kiln took very little energy into the insulation but leaked massive radiation.

Now the analogy is that the thin kiln represents the bare copper pipe and the thicker kiln represents the hessian lagged copper pipe which is conducting heat away from the copper pipe surface at a greater rate than the radiant energy loss from a bare pipe due to the low emissivity of copper.

Over 95% of domestic hot water draw off is for periods less than 8 minutes. It takes at least 8 minutes to heat the hessian, so this type of insulation is more of a liability than an asset.

Rex
(We have truth – just know that we do not have all the truth)

Re: gary 

Sceptic: What a load of twaddle. If radiant heating were your only concern from copper piping, you might have a small point, but it isn't. You've thrown in a load of pseudo scientific twaddle, and then covered up your fluff with talking about hessian lagging, and heating insulation, as if this was an issue.
By having lagging you do indeed force the insulation to be heated, that's the whole point - it resists it and reduces the flow of energy in that direction. Some of the energy is therefore retained in the pipe and the water, ready for next time you need it, reducing the energy needed to heat water because your first 20 seconds (say) are fulfilled from the water in the pipe, and not reducing your heat store.
You are talking nonsense about heating the insulation as if this was a problem. If you don't reuse the water in the pipe before the energy has flowed through the insulation, then it will simply be released more slowly into the area around the pipe. The same energy will be lost, it just takes longer.

Re: gary 

In reality there is little point in insulating home hot water pipes - regardless of the fact they may or may not waste energy any amounts will be miniscule in comparison with other measures. No matter how good the insulation hot water will likely have cooled to below 40 by the time the hot is next used.
The only hot pipe essential to insulate is the one that goes upwards from the hot water cylinder - this is a permanent source of loss. As soon as the pipe heads downwards insulation is probably not worth doing.
JK

Re: gary 

Hello anon,
As I said previously and I agree completely - pehaps CraigBayley should reread my posts 2nd June 3.48pm and 9.06pm and follow up on reading the BRANZ comment paper supporting my comments.

Rex
(We have truth – just know that we do not have all the truth)

Re: gary 

sceptic: I have reread your posts and I'm sorry to say I still disagree with you. Lagging may have minimal impact but it does have an impact. Your comments about having to heat the insulation are still nothing to do with your original post, and telling people to remove lagging is simply not good advice. Foam is obviously better than the old carpet style insulation, but removing it will not provide any benefit, and leaving it on might provide some benefit.
Yet again you have made reference to something without providing links, so it is a bit rich to suggest reading a paper without providing the supporting link.

Re: gary 

CraigBayley,

As I no longer have access to the BRANZ Bulletin number of about 30 years ago, I am sorry that I cannot help you further. I do not wish to pay $13 to obtain another copy but maybe someone with free access may help us?
Perhaps my description is at fault, but I did not wish to introduce physics and calculus into this forum.

Can I attempt another analogy to better explain?
Consider the flue temperature of a roaring pot belly stove – if the flue pipe is polished stainless steel and you touch it, skin is burned off as the emissivity is close to E=0.08. The energy is ‘held’ on the surface and liberated by conduction instantaneously.
Now paint the flue pipe matt black – the emissivity is close to E=0.95 and it will only be hot to touch although the inside flue temperature is the same.

As I understand the science, the emissivity of a bare copper pipe is low like polished stainless steel whereas hessian is high like the matt black surface. The surface energy on the bare copper pipe is ‘held’ in a still air environment and not radiated off as fast as the conduction rate into the hessian.

I originally made the comment only for interest as the difference in energy saving is so minor compared to the other methods noted in the post and agreed by others. Trust you also agree.

Rex

Re: gary 

Hi Gary
We run a 450 litre tank on solar and wetback. Last night after the fire was lit the tank went to 88 deg C from 55 deg C late afternoon. (One of my favourite features from the solar hot water set up is that the controller unit comes with enough sensors so you can see roof temp, tank temp and inlet water temp). My worry today, a fine sunny day, is that the cylinder is going to get even hotter - we once had to turn on all the hot taps to bring the system back under 105 deg C. (Cylinder is rated at over 110deg C).

The electric element has been turned off for months.

A few questions for you out of curiosity:
1. Do you know what temperature your cylinder gets to when the wetback is fired up?
2. DO you know if your cylinder is properly wrapped? (So that heat loss per day is about the 10 deg C drop level)

I also note in passing that we recently purchased some brilliant Thai manufactured neoprene lagging to put on the pipes from the wetback to the cylinder about 6 meters away - since doing this, instead of getting mid 60s deg C cylinder tank temperature, we are getting high 70s regularly, and even high 80s deg C like last evening.