Scott3:

 

Given you already have a boiling water unit, and (hopefully) have space for a separate under-bench hot water cylinder, the latter is likely to be a better bet than a combined unit.

 

Without knowing how much water you use in the kitchen, no way of answering how big you need, but they seem to be available in 15, 25L and 40L.

 

https://rheem.co.nz/products/home/electric-water-heating/low-pressure-copper/19901513

 

https://rheem.co.nz/products/home/electric-water-heating/mains-pressure-vitreous-enamel/31202519v

 

https://rheem.co.nz/products/home/electric-water-heating/mains-pressure-vitreous-enamel/31204515

 

Recovery rates on a 50 degree rise are

 

• 2.0kW: 34L/hr
• 2.4kW: 40L/hr
• 3.0kW: 51L/hr

So with the 40L, if you completly run it out, it will be fully heated again in 47mins... Obviously you need to confirm if you can get a 16A feed to your kitchen for this.

 

No need for a vent on these, but they do need a drain (assume you could share your skin drain pipes?).

 

Instant electric hot water heaters are sweet, but need some fat wiring i.e. the below has 30amp and 40 amp versions, and even then the flow rate isn't going to be great. Few NZ homes have the electrical capacity for this. My house for example has a cira 60A feed for everything... but could solution for something like a batch, with few electrical demands, only wanting hot water for the shower, not caring about low flow rate, but wanting infinite run time, and no standing losses. Could also be good in commercial /industrial buildings with ample electrical capacity. 

 

https://www.stiebel-eltron.co.nz/dhce-50-instant-single-phase-water-heater

 

Short of ripping out the hot water feed pipe, and replacing it with a skinnier one,  increasing the flow rate with a high flow tap or similar, or changing the entire house to a storage based hot water system with reciruclation, I don't know if you have many other soultions.

 

Funny, I had just looked at that 25L mains pressure Rheem you linked. I got thoroughly confused trying to decipher the install instructions/drainage requirements etc. But if it can be drained into existing kitchen sink drain it could be a good option. I guess I'll need a plumber to advise on that. The dimensions don't sound overly big, but I'll have to measure the space - as with waster disposal and existing multi-tap boiler I might be out of luck.

I ruled out the electric instant heaters after I looked up the specs and saw the power requirements!

I just saw this thing. Not a huge capacity but looks like a very easy install. Says it's for caravans etc, but also says suitable for cabins and sleep outs etc so I guess this could work for a kitchen... but I'm a bit wary if that's not really what it's designed for.

Small instant heater. It doesn‘t waste electrical power to keep a reservoir at a specific temperature when not in use and you don‘t have the risk to raise germs below 60 DegC.

Paul1977:

Inphinity:

...and were suggested having a 15 - 20L underbench hot water cylinder, fed by the instant gas hot rather than my the cold feed.

I'd thought of that as an idea, but don't cylinders need venting? If so, how could this be done in a retrofit into an existing kitchen island?

nickb800:

 

A modern mains pressure cylinder can vent down through your sink drain.

There's a cold water expansion valve and temperature/pressure relief valve, both can run into a normal wastewater pipe through a tundish.

Thanks

billgates: We put in a Steibel 15L unit during home build via plumber in our scullery for this very same reason and IIRC it was around $1200 incl install. Very happy with it. Hot water comes out instantly and it fit fine under the sink bench area. https://www.stiebel-eltron.co.nz/shc-compact-storage-water-heaters

@billgates That doesn't look too bad, and price seems reasonable including install.

Did they plumb the input from a cold feed, or from a hot feed from the gas califont (so after if running for longer than a short period it refills with hot water)?

Paul1977: Our is direct, it's the distance that's the problem for us (potentially combined with the pipe diameter).

Paul1977:

 

Funny, I had just looked at that 25L mains pressure Rheem you linked. I got thoroughly confused trying to decipher the install instructions/drainage requirements etc. But if it can be drained into existing kitchen sink drain it could be a good option. I guess I'll need a plumber to advise on that. The dimensions don't sound overly big, but I'll have to measure the space - as with waster disposal and existing multi-tap boiler I might be out of luck.

 

I ruled out the electric instant heaters after I looked up the specs and saw the power requirements!

 

I just saw this thing. Not a huge capacity but looks like a very easy install. Says it's for caravans etc, but also says suitable for cabins and sleep outs etc so I guess this could work for a kitchen... but I'm a bit wary if that's not really what it's designed for.

 

Plumbing is basically this:

For low pressure you also have a pressure reducing valve on the cold water feed.

Only time you "must" have a vent pipe is if you have an uncontrolled heat source (such as a wet back), and there is a risk of boiling the cylinder. Otherwise, you can have a relief valve plumbed to a drain. In short, if taps are closed everywhere, and the cylinder heats up, due to the thermal expansion of water it needs to dump a little water to avoid over-pressure.

Would recommend mains pressure.

If you are happy with 60 deg+ water from your kitchen sink you can omit the tempering setup too.

Paul1977:

 

billgates: We put in a Steibel 15L unit during home build via plumber in our scullery for this very same reason and IIRC it was around $1200 incl install. Very happy with it. Hot water comes out instantly and it fit fine under the sink bench area. https://www.stiebel-eltron.co.nz/shc-compact-storage-water-heaters

 

@billgates That doesn't look too bad, and price seems reasonable including install.

 

Did they plumb the input from a cold feed, or from a hot feed from the gas califont (so after if running for longer than a short period it refills with hot water)?

 

Having a water cylinder fed from another (non solar) hot water source would be fairly rare. One of the selling points of an under-bench cylinder is that you don't waste all the energy from heating the volume of water to make up the pipe run.

As a side note, the time to get hot water to our kitchen tap when from really quick to slightly annoying when we upgraded our kitchen. I think this is solely the fault of the tap itself. I seems to have a substantially lower flow rate on hot compared to cold (mains pressure system), and the tap itself (fairly fat arched tube) seems to hold quite a volume of water.

Not quite bad enough to replace the tap, but I would get a lower volume tap if it broke.

Recovery time on a small-ish tank with a 2-3kW element is going to be pretty good. Half an hour or less.

There are under-bench boilers that deliver both ~95C and 50C water, but they're ludicrously expensive and hard to find. I've seen them somewhere, though.

@gzt total distance is (counting vertical inwall etc) would be around the 18m mark. 11.5m of this is in the roof cavity, the rest is in walls and under floor (conduit in concrete slab). I'm working on the assumtion it would only be feasible to replace the pipe in the ceiling cavity.

I tried to have a look last night at the pipe size, but I was a bit big to contort myself enough to get to that particular run of pipe. I could see that the other (non-kitchen) trunks are 20mm branching off to individual fixtures with 15mm. If the trunk feeding the kitchen and scullery is the same it could be up to nearly 5.5L of water between the califont and kitchen sink. That sounds like a lot. But I think the kitchen and scullery are the only things that branch feeds, so I guess that entire run could possibly be 15mm, which would make it about 2.8L.

If the 11.5m length in the roof space was replace with 12mm it would take it down to about 2.1L.

So I think if the kitchen trunk is 20mm it would be well worth replacing it, but if it's 15mm maybe not worth it.

In the kitchen it takes about 40 seconds for the water to start warm up, and about a minute or so to actually get hot. But I don't think the Insinkerator Multitap has an overly great flow rate, so that might be contributing.

I need to do the bucket test, and try to access the kitchen trunk in the roof again (after some stretching!).

As @Scott3 said, an instantaneous electric heater is beyond what most domestic electrical supplies can do, especially for a kitchen sink where you want enough grunt to get the water really hot.  It's like another cooker circuit.

You can't do a hot water recirculation system with an on-demand gas heater.

Under-bench storage heaters are usually limited to 25 litres (by the dimensions).

In the "old days" we piped up under-bench heaters as "push-through" heaters, which eliminates the expansion valves, relief valves, vents, etc. There are still lots of these around. Unfortunately it requires a 3-hole tap set, so it doesn't work with modern mixers.

I'd say your solution is a 25 litre mains pressure under-bench storage heater. It's enough for 2 full sinks of 100% hot, so not perfect. And you still need a new power circuit.

Also remember that although you are running off 40-60 seconds of apparently cold water, it's actually hot water that you are wasting.

Paul1977:

I think we'd have room for an underbench water cylinder, but have a few questions about this idea: