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Topic: Wetback thoughts

by Donald Christchurch 18 May 10, 10 replies : Last Post Sort by:
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6 posts
This forum thread has been marked as a question for other Ecobob users to answer. Wetback thoughts 
Posted 18 May 10 1:35 PM
Gidday Folks ,
I'm new here today and been having a read , good site.
I'm keen to hear your thoughts regarding my ideas.

I have a small home 85 sq m with seperate header tanks in my ceiling space for both hot and cold supply with a hot vent pipe on my roof.

I'm getting a new Ethos Aquos fire 1.8 kw wetback to replace my existing non compliant log burner.

I have 2 water panel heaters on standby I would like to use, I have experimented for several months with one in my bathroom via a small 12v dc hot water pump. The hot water supply from the hot water tap and returned via the cylinder drain. This works very well and it's operation is close to silent so suitable to put in my hall beside my bedrooms also.

I would like to make a smaller tank below my existing HWC. The wetback will be connected across this. I plan to have my hot water panel heaters via a room thermostat controlling the hot water pump across this lower tank so as not to use up my existing hot water.

I'm considering also fitting a 5.2 cop 3kw output room heat pump with thoughts to modifying this to include a heat exchanger into this lower water tank in parallel with my wetback.
When panel heaters are not in use then excess heat being absorbed by the existing HWC.

I see 2 advantages of this lower cylinder,
The wetback and heatpump will be working with the coldest water, therfore absorbing more heat energy from the fire and or heatpump.
The panels will deliver more heat when the fire is going, therefore less of a big deal if I forget to turn the panels off.
I will use 25mm copper for the wetback.
The wetback pipework will be approx 3m horrizontally from furthest fire pipe to cylinder.

I agree it would be more simple to have 1 tall cylinder with multiple tappings or take offs down the side, my existing HWC is fine.

I did wonder if with one taller HWC the circulation due to the panels operating would tend to mix the water a little therefore reducing the temperature of the hottest water at the top of the HWC.

my questions so far.

1 What is the minimum slope I can have in the wetback to cylinder pipes to maintain best flow rate so allowing maximum heat absorbtion from fire ?
2 Do I have to make any special considerations because the fire wetback is a horizontal pipe with in and out pipes at the same exit height above the floor ?
3 Should the cold water supply be into the bottom of the lower tank or remain into the bottom of the upper existing tank ?

4 What size should this lower tank be ?


Regards
Donald
Christchurch


126 posts
Re: Wetback thoughts 
Posted 18 May 10 9:14 PM
Slope for thermosyphon > 1 in 10, if you are going 3 meters then use 25mm pipe, put your 2nd tank in the ceiling above the fire for best flow or right next to the fire in a cupboard etc, the bottom of the tank should ideally be higher than the wetback.

Doesn't matter too much about your wetback connection as long as the entry/exit pipes flow constantly upwards with no horizontal bits, dips, or too many sharp bends etc.

A secondary tank makes sense in that you don't disturb the stratification in the main HWC tank, the action of the pump and its return flow will totally mix up the water in the secondary tank no matter what size it is. The cold water inlet should be moved from the primary tank to the secondary, the secondary hot outlet then feeds the primary inlet. Thus as you use HWC water, preheated water is drawn in to be heated to your thermostat setting.

If you want the fire or whatever you intend to heat the secondary tank to warm up quickly so you can circulate warm water through the radiators then it has to be small say 45 L, 1.8kw wetback will only put out this much when the fire is on full blast, that isnt much heat to move to the radiators, If its going then why do you need radiators or doesnt the warm air travel to some areas.

Cheers
Mike

6 posts
Re: Wetback thoughts 
Posted 23 May 10 11:49 AM
Gidday Mike

Just discovered your reply today, thankyou.

Regarding gradient
I contacted Chriistchurch city council's plumbing compliance inspection and was advised the hot pipe from wetback to cylinder to be a average of 1:7 with no portion less than 1:20. The cold pipe only needs to have a gradient to eliminate the posibilty of trapping air or that any air is able to travel through the hot pipe.
I set my cold supply pipe with a slight upward gradient toward the cylinder to eliminate the posibility of air being trapped in the wetback.
My hot pipe setup as above using 25mm copper and it works well, making a few clanking noises for a start whenever I light the fire or put more wood on when I've let it die down for a while which I'm guessing is the water expanding with heating until the water velocity accellerates up to a constant state of equalibriam reducing the wetback water temperature so not allowing the water to boil. I read somewhere steam is approx 1000x less dense. My guess this velocity being proportional to temperature differential between the hot and cold ends of the wetback and friction in the pipes from the cylinder.

Regarding why bother with the water radiator, this means I can heat my bathroom quickly ( say 10+ x more powerful than average heated towel rail) from my water cylinder without needing to light my fire and wait the hour, I can also keep the doors closed and only heat my living area with the fire therefore less wood and faster room warmup. My bathroom is vented to outside.

My new burner a Ethos Aquos is a new beast.
For me I ignore the instruction to close the door immeadiatly after lighting, otherwise 3x normal kindling is required to accellerate the cold air down the flue outer before the fire is out. I leave the door ajar for 5-10 minutes to allow the kindling to get properly established. I'm used to being able to shut the fire down a bit when the room is hot, If your a little cold and put too much wood on your going to be cooking as you cannot close it down, the air control seems to regulate from 100% to around 50 % so only way to regulate the temperature down more is by limiting the wood you put on the fire. Duration from loaded up to sufficient residual heat to ignite a new log without excessive smoke is approx 4 hours so if you want it warm in the morning you need to get up once or twice in the night to feed it.
Wood consumption is similar to my woodsman ebony freestanding. Heat output is around 2-3x so I'm happy with it's performance. I have the cooktop option and it's about equivilent to electric ring being on 1/3 so alot faster than a slow cooker, I've cooked 3 meals sofar, spuds slow boiling in 20 min from cold. Pork chops in a light stew cooked in large pot from cold in 90 minutes.

Donald

3840 posts
Re: Wetback thoughts 
Posted 23 May 10 5:19 PM
With the newer clean air regulations, modern wood burners cannot be closed off like they used to, so they will not burn all night. Rather than get up early in the morning to feed the fire, you could store some excess heat in a large mass, eg big tank of water, then circulate it through your radiators early in the morning before everyone gets up, this is really DIY stuff as otherwise would be too expensive to implement.

Mike

19 posts
Re: Wetback thoughts 
Posted 23 May 10 5:51 PM
They may not burn all night but it would be convenient if there were enough hot embers to start a new lot of kindling otherwise its a bit of a chore to start from scratch every morning.

3840 posts
Re: Wetback thoughts 
Posted 23 May 10 6:02 PM
So much for modern technology, wood burners seem to have progressed backwards.

3840 posts
Re: Wetback thoughts 
Posted 23 May 10 6:53 PM
going backwards in this country for sure.

Have a look what they are doing with things like outdoor wood boilers in the US, or the wood gasifier boilers in Europe, some very cool stuff.

We seem to have lost or way with wood heating here.



3840 posts
Re: Wetback thoughts 
Posted 23 May 10 8:49 PM
Yes as usual NZ is behind the times, currently unless you have a large property you cannot use them, even though they put out less pollution than your standard wood burner.

quote: Whilst modern wood gasification boilers such as the ETA SH are significantly more efficient and cleaner burning compared to locally manufactured heating appliances, New Zealand environmental standards currently do not provide for wood burning boilers (e.g. with output of less than 40kW) on properties smaller than 2 hectares.

3840 posts
Re: Wetback thoughts 
Posted 4 Oct 10 8:11 AM
Hi Donald
Just reading your thoughts on your Ethos Aquos fire and would be interested to get your feedback.We are in Nelson and also have to change our wood fire to meet the new regs.A few questions -
1.With this fire does the pre heating of the combustion air work and drawing the air in from outside have you noticed any improvment in air inside ie.less stuffy etc?
2.Wetback-I was told 1.2kw size,as there are only 2 of us in this house is this large enough.We only use the current fire for 5-6hrs/day.We are also looking to install solar so I wanted to avoid possible boiling of HW cylinder.
3.Are you happy with the fire-any negatives?

Thanks Ian.

14 posts
Re: Wetback thoughts 
Posted 20 Jun 13 1:08 PM
Hi Ian
We are also looking for a clean air fire (we're in Tasman), with a wetback and one that has an external air source. Did you go with the Ethos Aquos in the end? Or something else? How are you finding the size of wetback etc. We are a family of four, and our house is going to be superinsulated (probably made from SIPS) and pretty airtight so my other worry is that we are going to overheat!! There's not many choices available from the approved list that tick all the boxes. Thanks, Sarah

334 posts
Re: Wetback thoughts 
Posted 21 Jun 13 12:30 AM
Sarah

Have you had a HERS (home energy rating) done for the house you are designing/building? This will give you an idea of the rate of heat loss from the house, and hence rate of heating required, and this may indicate how likely you will need the fire going.

A wetback needs a fairly full-on fire to drive any decent water heating volumes, and you may not want the fire that full-on, that often, in your well insulated home as you noted in your post above.
 

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