Gas continuous flow water heaters operate with an on demand principle; you open a hot tap and cold water flows into the unit which rotates a flow turbine, this sends a signal to the controller which then calculates a combination of information (from various sensors) and programmed temperature settings to initiate the ignition process.
Heater in standby mode
Once the electronic controller starts the combustion process the fan, spark ignition and gas follows a sequence to ensure smooth lighting. During the combustion/operation process electronic sensors are constantly monitoring the flame, over heat sensors, fan speed, water inlet and outlet temperatures and even air temperatures.
These constant checks ensure the correct operation of the appliance with an immediate (fail safe) shut down and self diagnosis facility should anything go wrong.
The appliance modulates both the gas solenoid valves and water flow valve to accurately maintain a constant pre determined outlet temperature at a given flow rate.
There are many options available for setting the desired operating temperatures by either using remote keypads with digital displays (some have voice facility) or altering the position of the temperature setting dip switches on the controller, however, all models are factory preset to 55 degree Celsius and can be used without any further adjustments.
Ignition sequence starts
Water heating commences
Once demand ceases the appliance turns off the burner, the fan runs for a short period of time to post purge the heat exchanger in preparation for re-ignition, the heater is then in standby mode.
Several advantages of continuous flow units are
- Energy savings due to water only being heated when required.
- Endless hot water
- Compact/lightweight unit makes installation easy
- Fewer control valves required
- Internal and external installation (model dependant)
All questions welcomed.
October 14, 2010 at 4:03 pm
can the continuous flow unit water supply be connected to a solar panel heated water supply with any real advantage
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David Waite:
October 15th, 2010 at 1:13 pm
Hi Roy,
A continuous flow unit can be installed in series with a solar system; this acts either as a booster for `warmed` water from the collectors during low irradiance days and would most likely not operate during hot summer days, the hot water would simply pass through the appliance without it operating.
The advantage is similar to a standard solar low line system which can achieve a 70 % reduction in energy consumption.
Thanks for the question.
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October 23, 2010 at 8:42 pm
How long would these typically take to use a 9kg bottle of gas, assuming “typical” usage for a couple of family of four? Or would a large cylinder installation be needed?
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David Waite:
October 28th, 2010 at 10:55 am
Hi Martin,
Using a 9KG cylinder to supply a continuous flow water heater is not practical as the appliance demand (up to 4.1KG/HR max on continuous high) would not only drain the cylinder in approximately 2.2 HRS, it would also freeze up, the cause being the rate of vaporisation (changing the liquid into gas) for that size cylinder (surface area heat transfer from atmosphere) would be excessive and continual draw off exacerbates the problem.
A family of four could use an average of between 40 – 60 litres of hot water per person per day, this (at 60 p/p/day) would equate to roughly 1+ KG of LPG per day.
Another concern would be likelihood of the distribution pipe system having no standard compliance in regards to supply and operating pressure parameters, this would render the installation un-certifiable (illegal).
Generally speaking a twin cylinder installation usually meets most domestic installation requirements and would be suitable for all of our range of continuous flow units.
Thanks for the question.
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March 7, 2011 at 10:32 am
Hi, We are a family of five and looking to renovate and change our hot water. We have gas to the house. Should we look at electric storage, gas storage or gas continuous? We have three bathrooms, a laundry and kitchen all at opposite sides of the house (up to 17m apart) so I am worried about the delay for hot water when you turn the tap on, especially to the kitchen.
Cheers.
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David Waite:
March 7th, 2011 at 12:04 pm
Considering all the parameters mentioned (especially the distance between draw off points) I might suggest that you consider a Rheem Stellar 160L exterior gas storage cylinder connected to a pumped ring main (to negate supply delays) which should meet a family of 5`s demands.
Some benefits with this hot water system are very short delays to the hot taps with considerably less water wasted during run off, the Stellar range of cylinders are 90% + efficient (see previous listing) improving benefits to an extremely high degree, some being with cost savings (especially when gas prices are usually cheaper than electricity) and benefitting the environment with less resources squandered, two other benefits are the internal space saved installing an exterior cylinder and the fact that no electrical supply is needed for this appliance to operate, great during power cuts.
In comparison an electric cylinder (domestic) would not be able to recover quickly enough due to its lower `power` input rate and continuous flow appliances attached to a circulating ring main would wear proportionally and are not as efficient as the stellar (soon to change with the introduction of the continuous flow condensing range).
I hope this helps but if you have any further questions please call.
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April 6, 2011 at 6:21 am
We are reviewing the way we design our hot water systems for coastal beach houses. Would you recommend this type of system for holiday homes used infrequently throughout the year?
[Reply]
David Waite:
April 8th, 2011 at 1:50 pm
The continuous flow units are certainly recommended for any home and the only specific consideration would be installing the unit on the leeward side of the property and/or fitting it with a recess box to reduce corrosion from sea spray.
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May 5, 2011 at 5:16 pm
Tell us more about the “pumped ring main” – this appears to be what I need too. Presumably it would need to be a sizeable diameter pipe and heavily insulated over its many metres of length. Even then the heat losses would be greater than than those from storage tanks? The pump would need to operate 24/7. Sounds costly. What standard offerings does Rheem make/supply? Any info on the economics of their use? Thanks.
[Reply]
David Waite:
May 6th, 2011 at 3:54 pm
A pumped ring main is effectively a continuation of the supply pipe work returning back to the cylinder thereby providing hot water without the losses associated with `running off` lukewarm water.
The pipe size does not necessarily have to be the equivalent of the supply pipe as its purpose is to circulate only, not act as the supply pipe.
The pipe work is usually only insulated when installed outside the building envelope, and pipe work installed within the building has relatively low heat losses due to the insulating properties of the building itself.
For a rough comparison an electric 180 litre cylinder has around 1.8 kW standing heat losses per day and a 10 metre return leg of 20 mm insulated pipe containing 2.25 litres loses about 0.02 kW per day roughly 1.25% of the cylinders losses.
At rates of 21 cents per kW you have $0.37 cents and $0.004 cents heat loss cost respectively.
A circulating pump using 100 watts per hour will cost 50 cents per day to run.
40 litres of water heated through a 50 degree Celsius rise will cost $0.50 cents.
It would only take an average kitchen tap 3.25 minutes to pass 40 litres.
If an average family of four all ran the sink, shower and wash hand basin for say 30 seconds several times each day before useable hot water was available this could equate to 11 minutes of running off and at an average flow rate of 9 litres per minute would equate to 99 litres of water run to waste with a similar volume needing to be heated as it enters the cylinder.
The savings in these circumstances are easy to quantify.
Another benefit to a ring main is the satisfaction of not having to wait for hot water.
Rheem do not offer plumbing systems as such.
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Martin Kannegieter:
May 7th, 2011 at 9:30 am
Hi David,
I’ve done a little research on this a while ago, and found some figures on heat loss of copper tubes – see link.
It may be a little more than what you say, but I guess it depends on the lagging of the pipes and the heat difference of the water in them.
http://www.engineeringtoolbox.com/copper-pipe-heat-loss-d_19.html
The circulation pumps are pretty small and since there’s almost no head loss, the power requirement is tiny. There is a Grundfoss unit available that has different pumping (& therefore power consumption) rates, up to 45W. You would hardly notice this on your bill.
[Reply]
David Waite:
May 9th, 2011 at 9:47 am
Thanks for the link, I recalculated the heat loss cost per day using the information you mentioned and came up with an additional $0.52 over 24 hours using the 13mm minimum insulation thickness allowed.
This could be reduced using thicker insulation and replacing the 20mm return line with 15 mm.
Another consideration is the $ value of water lost down the drain!
May 9, 2011 at 7:46 pm
Hi David, Not a whole really is it?
Could a system that had both pipes as say, 10mm [3/8"] work, as the recirculating flow is not very high. Then both pipes could be wrapped in the same lagging, reducing heat loss even further.
To achieve the required flow at the tap, shower, whatever, there would be a low pressure check valve to allow the flow in the return line to go the other way, in the same direction as the supply line, so that both pipes contribute to the supply.
When the tap is shut, the check valve shuts and recirculation continues.
Would this work?
Cheers,
Martin.
[Reply]
Martin Kannegieter:
May 9th, 2011 at 7:47 pm
Not a whole lot! sp…
[Reply]
David Waite:
May 10th, 2011 at 10:39 am
10 mm supply pipe work is only suitable for supplying wash hand basins with strict limitations on distances (G12) preventing flow rate problems thus limiting the application.
Reversing the flow on the return line and flowing against the circulating pump would be detrimental to the pump as the pressure required would have to exceed the ‘metre head’ rating.
Unfortunately either of these two reasons would make the idea unworkable!
Appreciate the question.
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May 10, 2011 at 9:07 pm
I don’t think I explained this properly…
The pump pushes the water around the circuit, so the return line with no large restriction would allow the water to flow back to the tank with the tap closed and away from the tank with it open.
The pump would have a swing check valve in parallel with it, so when there is recirculating water it is closed, but when there is a demand for higher flow – when the tap is opened – the bulk of the water bypasses the pump and goes through the check valve – no high pressures involved…
If there are more hot taps in the circuit, the pipes would need to be a little larger, but that would be the case with any system anyway.
I’ve drawn a diagram of this, but no way to upload to this site.
Cheers,
Martin.
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May 11, 2011 at 10:42 am
The return line tees into the main cold water supply pipe between the pressure limiting set and the cylinder, both flow through the same connection into the cylinder and without a non return valve fitted to the return pipe prior to the tee connection the MCW would take the least resistive path and flow directly up the return pipe when the hot tap is opened. Having the NRV at this point therefore makes it the most practical place to fit the circulating pump.
I have not seen a cylinder with an additional dedicated return connection available.
This layout method would prevent a practical application of your theory.
You can forward the diagram by email to david.waite@rheem.co.nz
Refreshing idea though!
Thanks,
David.
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