Hot Water On Demand / Recirculation

Here's an interesting one that I'm wondering if anyone has tackled previously via C-Bus. Hot water recirculation pumps can be used to pump water around a ring main until hot water reaches the far most tap or outlet, this way hot water is instantly available at the tap (rather than waiting for cold water flow to become warm/hot) and the cold water can be recirculated to the hot water heat to minimise water waste.

Some of these systems use a timer (eg during typical occupancy hours), or a manual button to perform the recirculation until hot water has flowed through.

It seems to me that a great balance between convenience and economy could be achieved by linking occupancy detectors to such a switch or pump system. Eg when movement in bathroom is detected, trigger hot water recirculation system on. If it is retriggered soon after it wouldn't have any effect since these systems are microprocessor controller via the temperature (restrike delay could be a good idea however on a 12channel VFR).

Has anyone implimented such as system before in a domestic situation?

Save water, minimal electricity use, excellent convenience!

 

Interesting idea there, but I'm not sure how practical it would be. One of the advantages of instant hot water systems is that you don't have the water heater maintaining a few hundred litres of water at 55?C 24 hrs a day.

In my house, hot water is used primarily for showers, baths and hand washing dishes. Times when I walk into the bathroom and do not intend on using hot water include going to the toilet (wash hands with cold water), brushing teeth, getting first aid items, putting laundry in the basket. Then there's a very low percentage of time that I'm in the kitchen and need hot water at the ready.

Overall, at least in my household, having an occupancy sendor trigger my hot water would result in a lot of water being heated, not used and then cooled back down.

 

Yes, this is partly true - enter c-bus PAC and other logic devices and the following however.

These recirculation systems don't heat up water, they mearly fill the ring main with hot water so that it is only 1-2m away and ready to go (and recirculates the cold water back the the heater). On new installs the plumbers insulate the hot water lines. Most of the time these systems are used by being set on timers, or via a manual switch.

A PAC etc can be used to trigger the pump when hot water use is more likely (eg when you enter the bathroom in the morning, or near sink in the evening), and so it may be triggered a couple times in the morning, and 3-4 times in the evening (use restrike delays to minimise excessive triggering, altough these pumping systems are also thermalstatically controlled.

It will mean that heat loss will occur from the ring main a couple times a day, but less water will be wasted, or either smart logic in combination with manual triggers could be used to minimise that.

Has anyone used this? There are government rebates in some states (certainly Vic and Qld) for the use of water saving hot water recirculation systems and it seems to make sense to me that a good solution for many situations (domestic and commercial) can also minimise heat loss.

 

It seems like you are talking about using a recirculating pump with a traditional water heater, whereas I was assuming an instant on-demand type water heater like the Rinnai Infinity, where water is not heated until a hot water tap is opened.

The recirculation pump in those types of system is triggered by pushing the "pre-heat" button, at which point the heater fires up too. With this system your suggestion would not work very well, but with a traditional water heater I think there would be convenience advantages without a massive energy overhead.

 

Even with the rinnai unit (which i do have) this sort of thing could still be handy, if you had a flow switch and a thermostat (say in the kitchen) near one of the points that was commonly used, but traditionally not to close to the hot water unit, so the non-heated water in the hot water line could be re-circulated back into the cold water input input of the hot water unit, thus saving those precious few litres, until the water hit the thermostat limit.

This could be achieved via a simple 2 ch bus coupler (for the flow and thermostat inputs), and 1 relay output (to operate a solenoid). Don't know how much the flow switch and thermostat would cost, but the plumber would probably cost a fortune. So i am not sure if it would be economically viable.

Cheers

Grant

 

The Rinnai Infinity heaters already have this feature built-in. There is a pre-heat button on all their latest controller range, and the add-on recirculator unit comes in at ound $700-$800 last I looked.

What Guff is proposing is that a CBus PIR sensor used to try and predict hot water use, so it automatically starts the recirculation a minute before you want to use it, instead of you having to push a button.

The problem with this, when used with an instant hot water system is that if the automation system gets it wrong, you will be turning the gas on, heating for a couple of minutes, not use the water, and then it goes cold again. Massive energy waste.

If you have a something like a Rheem 160L hot water system, which keeps 160L of hot water at 55?C constantly, recirculating a couple of litres doesn't waste that much energy as a small amount of cold water is sent back into a massive tank of hot water, and if your pipes are well insulated, once the circulation is going, the only extra energy usage is in the pump.

 

when you say the latest range of infinniti have the pre heat function.. as from when? as my unit is about 3 years old... don't think i would have it.

 

You are correct that I was referring to storage type water heaters. The continuous hot water systems like the Rinnai Infinity have an "add-on" called SmartStart which does the same thing - except I'm not sure how easy it would be to cable that into c-bus.

With storage heaters, pumps vary in function - some with thermostats and some without. I spoke with a plumber yesterday and he suggests the best is usually the least complicated with respect to hot water recirc pumps - eliminate the thermostat ie just another thermostat to malfunction in a couple years.

Plumbers suggestion is to use a hot water pump designed for recirculation (two inlets, one outlet for non-return valve & pump) and simply use the smarts in cbus to turn it on and off with a suitably long restrike delay. PIR's via logic can then simply pulse a replay to the pump.

It is recommended that the hot water pipes (particularly ring main) be insulated to minimise heat loss. Either way water will be saved.

 

Not sure. Certainly the last 18 months, which was when I started looking.

 

The current model keypads (3 or 4 years old) have a pre-heat button, but without the module for the heater, which is an optional extra it does nothing. It isn't a simple case of adding it either, as to work properly you need a 'return' pipe from the most distant hot water tap.