Our kitchen is just about at the furthest point possible from the gas hot water cylinder. We have to draw off a lot of water before we get the hot coming through. Is there a simple (instant?) electric heater that can be plumbed into the hot water supply (under the sink?) and if so, will it save enough in both water and gas to make it worthwhile?
Or, should we divert the cold water into the garden instead of wasting it, as we are paying the Auckland waste water charges anyway!
lythande1,
Mar 23, 10:25pm
Yes you can get under bench cylinders.
Have you not got mains pressure though? That's all you really need.
tintop,
Mar 23, 10:28pm
You are paying to heat the water - and then using it on the garden ?
From a point where you have not used any hot water for a number of hours. Run the hot tap and measure how much is drawn off until the water is hot enough for what you want to use it for.
Measure the temperature of the heated water as close to the cylinder as you can ( a bathroom tap or a tap close to the cylinder) , measure the temperature of the cold water feed to the cylinder ( at any cold tap will do) From those numbers it is possible to calculate the amount of heat wasted, and further - be able to calculate the cost of the waste heat. Also calculate the cost of the wasted water itself.
Will also need the per unit cost of power/gas. Electric heating is close to 100% efficient, gas less so - the efficiency of a gas hot water system should be able to be found somewhere - given the make and model of the cylinder, and if natural gas or lpg.
socram,
Mar 24, 5:03pm
The first few litres from the hot water system are cold, which is why I posted. You can't use them if you are wanting hot water, so better they are diverted to the garden until the water starts to come through heated, rather than down the waste.
All water is mains pressure and mains gas.
nzjay,
Mar 24, 5:17pm
You are best to install a small under bench electric cylinder with it's feed being off the cold supply (will need a pressure reducing valve etc). Do not use the existing gas heated water supply to feed the new cylinder.
nzjay,
Mar 24, 5:21pm
Use an empty measuring container (plastic 1 or 2 litre milk bottle) to see exactly how much water you are wasting from cold before the hot comes through and make you decision based on what you think of that.
tintop,
Mar 24, 6:23pm
I agree - if the $ stack up in favour. It's not too difficult to work out if the cost of the waste was to be worked out as I outlined.
It may well be better to just swallow the cost of the wasted heat, although there is the added convenience of not having to wait too long with a local heater.
tintop,
Mar 24, 6:24pm
My reference was to the waste of heat rather than the water itself.
aredwood,
Mar 25, 5:05am
Are you using natural gas or LPG? As Natural gas is far cheaper Than LPG. (on a per unit basis). Also an electric cylinder is NOT 100% efficient. Sure an electric element is 100% efficient. But remember a cylinder has heat losses. So you will be using electricity just to have the hot water sitting there waiting.
But overall you are unlikely to save any money by installing a separate cylinder. But other things to consider - Does the cylinder have a tempering valve on it? If so then re route the water to the sink so it gets the hotter water direct from the cylinder. And leave the tempering valve to just feed the bathroom. As depending how hot you need to fill the sink you might then be able to put plug in sink, turn on hot tap, And then hot water mixing with initial cold that was in the pipes might still be hot enough for what you need. Also replace the pipe to the kitchen with a smaller diameter one. Less water sitting in the pipe = less cold water wasted.
tintop,
Mar 25, 5:31am
Agree on the heat loss from the cylinder itself. I had overlooked that :)
socram,
Mar 25, 3:32pm
Why not? That is what I was hoping for!
I don't want or need a new cylinder in the kitchen and my main concern is the waste of water, as the heating cost on gas in minimal anyway. I would have thought that by feeding the hot water supply through an electric heater, that the heat would be virtually instant and therefore once the water arriving from the hot water was indeed hot, it wouldn't need to be heated and any inbuilt thermostat would then switch off the heat.
That seemed logical to me, but obviously there is no such device. All water for laundry and bathroom use is at the rear of the house and close to the gas heated water. The kitchen is at the farthest point possible.
Maybe I should just fit an electric under bench heater in the kitchen and put a stop cock on the hot water supply, that would only need to open if there was a power cut?
Thanks for the suggestions.
socram,
Mar 25, 4:28pm
Thanks for that. Looks interesting.
nzjay,
Mar 26, 3:44am
Because you have an amount of cold water in the pipe that has already cost you to heat up (and then cool). Lets say there is 5litres of cold water before the hot comes through from the gas cylinder. That wasted 5litres has cost you, say 25cents. You are then going to put it through the electric cylinder and heat it again. another 25cents. Bear in mind, there are many times the hot tap is used just to rinse something, a cup or two, a plate. and the hot from the gas won't even get there.
The oval cylinder linked is border line in capacity in my experience for a family kitchen. You may have times where it will run cold, rinsing plates etc before a wash and then filling a sink. I always looked for 25litres minimum. The pressure rating is 500Kpa (70psi). sometimes mains pressure well exceeds that, so I suggest this is meant for a reduced pressure system, ie. a pressure reducing valve is required.
socram,
Mar 26, 4:02pm
I hope not!
Thanks anyway.
johotech,
Mar 26, 11:52pm
I can't seem to pinpoint what your main concern & priority is. Is it the wasted water, the wasted energy (from the hot water wasted when it cools in the long pipe), or the amount of time it takes to get the hot water at the tap?
Also, how much money are you prepared to throw at the problem?
There are instantaneous under bench electric water heaters. They aren't cheap to buy, and require a high current circuit to your switchboard, so there will be at least a few hundred dollars in electrical work on top of the plumbing work required.
An under bench cylinder would be cheaper to buy and install - even so, probably no change out of $1000 or so.
Or, as Aredwood said, change the pipe to the sink to a smaller size. If you change from 20mm to 15mm, you shouldn't have any problem with flow as you are mains pressure, but there will be only about 52% of the volume of water wasted in the pipe each time (1.3L / 10m of 15mm pipe compared to 2.5L / 10m for 20mm).
Another option is to change to a recirculated system for that tap, so there is hot water instantly available at the tap, with no water loss. That would require a return pipe be installed back to the cylinder, and a small pump to circulate the water (uses very little power).
The disadvantage with recirculation is that there will be a continuous heat loss from the circulated water - that can be reduced with good insulation, and the pump could be on a timer so it only runs during "working" hours.
Maybe Aredwood has done it before and could comment further?
socram,
Mar 27, 5:19am
Thanks for that. It is mainly the waste of water as Auckland is all metered and there is a 78% charge for the waste water, so it seems a bit pointless just putting more decent clean water down the drain.
My thought was that if it is too expensive or difficult to install a booster of some sort (which it looks as though it is) then I may as well use that initial 5 litres or so of cold water from the hot tap, to water the tomatoes or the citrus trees.
aredwood,
Feb 20, 2:59pm
Thanks for the recommendation johotech. Yes have done everything as you have described on previous jobs. Yes you can connect a little electric cylinder to the output of a gas instant cylinder. House I did that on was owned by an elderly customer who hated waiting for hot water to get to the kitchen. He didn't have a dishwasher and didn't want one. So would be using a reasonable amount of hot water in kitchen. Gas heater used natural gas so far cheaper than electricity. Having the gas heater feeding the electric cylinder helped it to maintain temperature. I also ran 10mm copper from the cylinder outlet to the tap. To speed up the hot water some more. Customer now has instant hot water.
So yes Socram can have a little electric cylinder feeding off a gas heater. Note that all of the usual rules for installing mains pressure hot water cylinders still apply. So you will still need at least 3 pressure control valves to go with it. So installing such a cylinder is well into "get a plumber" territory.
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