Replacing a hot water cylinder with under sink water heater

Norfolk Broad

Norfolk Broad

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Nelson's County, Boudica's County
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Hello all

I'm returning to DIYNOT after a long hiatus and lapsed account. As ever, I only come on here if I want to know something or (as in this case) to reaffirm a decision I am about to make.


I'm in the process of buying a one bedroom first floor flat, built c.1990. It's on E7 supply with storage heaters, no mains gas. I work away a lot so, when I'm not using it, I hope to be able to rent it out via Airbnb. I'm guessing, on average, it will be occupied 4-5 days of the week.

It has been recently refurbished by the landlord:
- Kitchen units replaced, new sink, and space for a washing machine (but no plumbing yet).
- New toilet and basin, with bath removed and replaced by a shower cubicle with new 7 kW Mira unit.
- New carpets and vinyl throughout, walls repainted... you get the idea.

The original 1980s/1990s storage heaters are still in place, and the hot water cylinder is a Fortic type, also presumably original.

The hot water tank dominates a built-in cupboard in the bedroom, and seems awfully excessive given there is no longer a bath. This cupboard could be put to use as a wardrobe, or possibly a secure storage for when I'm not there.

[GALLERY=media, 106883]Plan by Norfolk Broad posted 10 Aug 2021 at 3:00 PM[/GALLERY]

I'm not going to open the debate about ditching the storage heaters and getting rid of the E7 tariff, but I am questioning the wisdom of having such a large hot water tank, especially as the flat may be unoccupied for approx. 35% of the time.

So, to the question(s):
1) Would a 15 litre under sink water heater be a "better" [more economic] solution?
2) Could one heater be used to supply the kitchen sink and bathroom basin?
3) Would fitting an instant hot water heater jeopardise the EPC rating?

I appreciate 1) is a "piece of string" type of question, but if there's a solid energy consumption reason for you to say "don't do it", then I'm all ears. But I'm also prepared for the initial outlay on this if it means I don't have to fork out for an additional wardrobe or other storage unit.

Any advice and opinions gratefully received.

Many thanks
NB
 
Thanks for the replies so far.

I'm surprised there's no detractors from this idea, though - surely this is too good to be true?

Looking at the hot water tank in the flat, being a Fortic type I understand this has its own built-in cold water cistern at the top. Is this a potential issue for water pressure?

I would like to think the water pressure is good enough to supply the cold taps in the kitchen and shower room (and the electric shower itself)?


Many thanks
NB
 
Norfolk Broad said:
Fortic type I understand this has its own built-in cold water cistern at the top. Is this a potential issue for water pressure?
Click to expand...
Pressure and flow from one of those will be pathetic at best, particularly with many modern taps which have small bore connecting pipes and tiny waterways inside.

An unvented undersink heater will have the same pressure as the cold water. Flow depends on the outlet size but will be far better than the Fortic junk.

The disadvantage is you will only have 15 litres of hot water before it runs cold, and then will have to wait for it to reheat. However with only a kitchen sink and bathroom basin hot water use will be minimal so it's not really a problem at all.
 
15 litres is about 3 gallons, sufficient to wash up in the sink and do a bit of rinsing.

If somebody wants to wash and shave in the bathroom at the same time I think they will need their own heater. If there are two people in the flat sharing one heater they will have ferocious rows when it runs out.

They heat at about 1 litre per minute from cold if 3kW
 
You asked for detractors, sadly I'm not one of them, I just have a few pearls of wisdom.
1. The heater will have a safety discharge valve. This needs to be run in copper to an outside wall, and have a constant fall to outside so water can't sit in it and freeze.
2. Some (Ariston too, I believe) heaters require a minimum length of inlet cold pipework between the heater and the mains stopcock, else an expansion vessel will be required. The final location of your heater may be influenced by this.
3. Closest to it's point of use will give best results...each litre that is run waiting for the hot to come through translates into a litre of cold water running into that heater, reducing its temperature.
4. Install a timer for it if you want to save power/money.
5. Don't expect your airBnB guests to save water or power. They may waste all the hot, then slag you on review that there was 'no hot water'.

(nice sketch BTW)

A friend has done very similar in a small 1 bed flat, 15 litre feeding both sink and basin. They have 2 occupants and they row constantly, but never over the hot water supply!! They don't waste their hot water, they use a plug in the basin, and they wash up each night. They have a bath installed, but only use it for catching the discharge from their 9kW electric shower. They live in a soft water area, if you don't then make sure you have good access to change or descale the heater. A spare descaled heater (they're only around £100) may prevent angst as the current one scales up.

MM
 
Ah, excellent answers everybody, thank you. And especially you, MeldrewsMate.

I was thinking instead maybe installing two smaller (10 litre?) units, one 3 kW for the kitchen, and maybe 2 kW for the basin.

But, if they need to have an overflow to an outside wall that might be an issue - the only outside walls are, as you can see, where the windows are!

I also hear what you say about guests not being particularly considerate when it comes to energy consumption - that's a risk/cost I'm prepared to burden.
Perhaps if I fitted a smart meter I could keep an eye on things?? (Probably a subject for a different forum thread!)

And yes, being in East Anglia, hard water is prevalent. I was thinking about fitting a water softener to protect the new shower unit, washing machine, and any new water heaters - more expense, though.

Just thinking about alternatives: What about instantaneous water heaters like the 9 kW Ariston Aures? Reviews on Screwfix aren't as good as the tanked Andris units.
https://www.screwfix.com/p/ariston-...r-9-5kw/305FJ?tc=MC5&ds_kid=92700055281954502

I'll need an uprated (40 A) supply, but at 3x the power consumption, am I asking for trouble?


Thanks again
NB
 
OK, so I've finally taken possession of the flat!

First things I did:
1) Turned on the water and the stop cock.
2) Switched on the E7 at the consumer unit and "night" switch in the cupboard.
3) Took electricity meter reading.
4) Unpacked a few things.
5) Returned to my current abode.

Observations on returning the next day:
- Overnight it used a massive 14 kWh on night rate to heat up the tank from ambient.
- Hot water pressure is.... pretty poor! It tool a long time to come through, too.
- Strangely, it still seemed like to tank was heating up during the day - 2 kWh recorded on the day rate in the past 24 hours, but no other lights or appliance connected yet.


Based on the water pressure and the amount of energy it uses to heat up from a standing start, I think I really want to go down the route of an Ariston Andris (or similar) undersink unit.


But this safety discharge business troubles me a bit:

MeldrewsMate said:
1. The heater will have a safety discharge valve. This needs to be run in copper to an outside wall...
Click to expand...

Isn't there already some form of overflow I can re-use for the Fortic tank? (See photos)

[GALLERY=media, 106948]Fortic2 by Norfolk Broad posted 21 Aug 2021 at 9:52 AM[/GALLERY] [GALLERY=media, 106947]Fortic1 by Norfolk Broad posted 21 Aug 2021 at 9:52 AM[/GALLERY]

Failing that, I guess I opt for a Hotun tundish? If I install two heaters (one for the kitchen, one for the basin), do I need separate tundishes, or do both heaters feed into the same one?

I think an expansion vessel is also required, given the proximity of the stop cock. Again, just one vessel for the two heaters, or one vessel per heater?


Thanks again
NB
 
The standing heat loss from a typical hot water cylinder at 60C is about 2.5 kWh per 24 hours.
100 litre of water needs about 4.2 x 100 x 45 (=18,900) kJ to be heated from 15C to 60C. There are 3600 kJ in a kWh, so this equates to 5.25kWh for initial heat-up. Add a figure to allow for standing heat losses along the way and you'll typically use 7kWh for the first heat-up. This is half the figure you quoted; I'm not sure why your figure is so high....Do you also have storage heaters?

1. The overflow pipe (the 'warning pipe') is not in copper at present, therefore it won't do as a safety discharge. I suggest you read Building Regs Approved Document G3 for a full answer to your question.
2. I've never considered using one tundish for two safety discharge outlets, and G3 doesn't either, though I would consider it essential to increase the final discharge pipe to 22mm if two 15mm pipes potentially discharged into it.
3. If the expansion vessel is on the cold feed then a single exp vessel of twice the capacity will do in my opinion; but it MUST allow expanded water from both vessels to enter without any valving (is that a real word?) between it and either vessel. Practically speaking this would mean that any maintenance on one heater would entail cutting off the supply to both heaters.

Tip: Make the heaters easily accessible and removable, especially in hard water areas.
 
Thanks for that, MeldrewsMate.


Yes, I thought the figure was rather high, even for the off-peak period. I guess if it is a 2 kW immersion heater, then running constantly for 7 hours would fit in with this.

It makes me wonder if the thermostat is not working properly, and the immersion heater didn't shut off when the water reached temperature. When the hot water eventually emerged from the tap, it was scalding.

The storage heaters were definitely turned off, and the fact that the Fortic tank still sounded like it was heating up the next day during peak hours was cause for concern. The red light on the meter was flashing, despite no other appliances or lights being switched on.

I thought the supply to the "night" switch feeding the immersion heater should cut off at 7:30 am, and if you wanted to boost it during the day, you flip the "day" switch??

I could spend time and money trying to troubleshoot and rectify the problem, or perhaps just cut my losses and go straight for the Ariston Andris (or similar) with expansion vessel (Kit A, and possibly Kit B) and a Hotun tundish (Option 3 on the online video).

In order to make it accessible, it will probably be fitted in the cupboard where the Fortic tank currently is. All the pipework feeding the taps is already there.
This cupboard is about 2 metres from the kitchen sink, and a little over a metre from the washbasin in the shower room, so I appreciate there might be some heat losses between the Andris and the taps.

If I put the Andris in a kitchen cabinet near the sink, I will lose valuable cupboard space, and access for regular descaling maintenance is is awkward. I'll also have to run new pipework back to the bathroom.


As the flat is primarily for my own use, I think I'll opt to fit just one Andris unit for now - I can always augment any hot water for washing up by boiling a kettle as and when I need it.
If I manage to rent it out on AirBnB, and there are complaints about the lack of hot water, then I might consider fitting a a second system.


Thanks again for all your help.

NB
 
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