Hot water cylinders - help me pick

R.C.Anderson

R.C.Anderson

R.C.Anderson

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Currently undergoing a bathroom renovation, and as part of this, I'm installing a vented HWC, that will be pumped for the shower (expecting ~20-25L/min combined flow out of the shower).

An unvented cyclinder was considered, but we've only got a 15mm incoming supply, so have limited flow available, so am going with a pumped vented, so the flow to the shower will be uninterrupted.

The thing I'm having trouble deciding on, is the size/type of vented cylinder. I currently have a gas boiler to heat it, but am likely to switch to a heatpump at somepoint in the future & don't want to have to replace the cylinder at the same time. This has pointed me towards getting a cylinder with a heatpump coil (from what I can find, this essentially means there's a much larger surface area in the coil, so the heat is transferred more effectively to the water in the tank than with a standard coil). However as this cylinder is for the shower only, does this matter? If it takes 4+ hours to fully reheat from a heatpump (rather than say 40mins), that doesn't really matter, as long as it's large enough to begin with to provide enough hot water for 2 showers in quick succession. This makes me think I should just aim for a ~300L tank, but with a standard coil, which would be significantly cheaper than a HP version. But is this a bad idea?
I've also seen cylinders that are twin & even triple coil and/or solar cylinder, which could be a middle ground.

In summary, I think I have the following options:
  1. Get a ~300L standard cylinder & live with slower heatup times if/when I get a heatpump
  2. Get a ~300L HP cylinder
  3. Get a smaller (~200L?) HP cylinder (I'm not sure this is worth it, as the price difference is fairly minimal from option 2)
  4. Get a ~300L twin/triple coil/solar cylinder (which I expect would operate somewhere in-between option 1 & 2 for heatup times)
Has anyone got experience with this & got any pointers for me?
 
Not personal experience, but one thing I have come across is that if you're going to get a heat pump fitted at some point and take advantage of the BUS, the installer is supposed to make sure that the whole plan is realistic.

So, you can't ignore all of the unsuitable radiators and just tell them you'll complement with electric radiators or whatever. They need to be comfortable that the whole install is fit-for-purpose or it isn't grant eligible. That could mean that if you went for option 1, you might find that some installers won't go ahead without replacing the cylinder.

The other option is a cylinder with an integrated heat pump, but that would be more expensive and the location is critical to make sure you've got airflow that you want to be cooled. So probably not a solution.
 
I hadn't considered if a future HP install could be hindered by it, because they decide it's not suitable, although I'd probably have the option of just not telling them about it (the install location is pretty hidden away & unless you knew about it, you wouldn't know it's there).

I didn't realise you could get tanks with built in HPs, but they seem to be 2-3x the price of the other cylinders I'm looking at, so not sure it's something I want to consider (and I think from the quick look, they're all unvented, so I'm back to the same flowrate problems).
 
I wouldn't bother with a twin coil if solar is your intention. The previous owners of our house installed one without actually installing the solar thermal panels, I suppose it was going to be a future consideration. I costed in solar thermal panels and their installation and the calculated ROI was about 27 years, that excludes the cylinder :cry:. So I didn't bother.

Anyway, I want to replace the hot water cylinder due to it's silly location, size and inefficiency. I was looking in to smart water cylinders so that I know how much hot water I have and balance that with how much I need at any given time.

I use a brute force approach right now - since I'm on an EV tariff I installed a wifi controlled immersion heater switch so that it's switched on during the cheap period (currently 3ppkWh, soon to be 9ppkWh). It works well enough and is cheaper than using heating oil especially at recent prices. However, it's only controlled by two things - the set time period and the water temperature that the cylinder's single thermostat at the bottom of the tank detects. So it heats the entire tank whether I need that much or not. There's no real indication of the temperature and amount of hot water stored, only the maximums.

With a smart water cylinder I could have a much larger capacity stored but only heat up 10-100% of it, varying it as the needs demand. Some have heat pump integration already. The cost of smart water cylinders is an issue and calculating ROI for them can be difficult to calculate.
 
I didn't even realise smart cylinders were a thing, but after a quick google, I don't think I'd gain anything with them. I'm already planning on attaching a couple of zigbee thermostats to the cylinder (one high & one low), so I can determine how much HW I have & if it needs heating, all via home assistant. I also plan on attaching a CT clamp to my shower pump, so it knows when the shower is turned on & can begin heating then (don't know 100% if I'll implement this, but it's an option).
 
The downside of slow heat up times is that a heat pump can’t do heating and hot water at the same time.

It’s one or the other. So if it takes 2-3 hours to reheat, that’s 2-3 hours it can’t heat the house.

300l is a big unit for just a shower, even at say 50C max.do you need such a big cylinder?

Either way, if you can afford the heat pump one, id suggest getting it. It’s not just heat up times, it’s also heat pump efficiency. The bigger coil is also more efficient.
 
Mysterae_ said:
That's a good solution @R.C.Anderson. If you have the know-how and home assistant then go for it! I need to get in to home assistant but I'm too deep in to other projects at the moment :D.
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I know that feeling, and it can take over a bit, but worth it IMO.
b0rn2sk8 said:
The downside of slow heat up times is that a heat pump can’t do heating and hot water at the same time.

It’s one or the other. So if it takes 2-3 hours to reheat, that’s 2-3 hours it can’t heat the house.

300l is a big unit for just a shower, even at say 50C max.do you need such a big cylinder?

Either way, if you can afford the heat pump one, id suggest getting it. It’s not just heat up times, it’s also heat pump efficiency. The bigger coil is also more efficient.
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The lack of house heating was a consideration & probably the biggest consideration I had (although I could set it to heat for an hour, then run the house heating for the next hour & switch back etc until up to temp (or just wait & heat overnight)).

I was trying to work out quite how big I need. The shower has a large rainfall head, & the valve is rated for 20-25l/min at the pressure I'm expecting. If I assume 2xshowers in quick succession @ 10mins each, that's 400-500L total water & a 2:1 hot:cold ratio means I will need in the region of 300L to avoid it starting to go cold.

I hadn't thought about the efficiency aspect of the HP. I guess that's probably the biggest difference. It's ~50% more for the HP cylinder, but TBH it sounds like the right decision & will hopefully last for several decades.
 
The water requirement seems high 20-25l/min for a shower equally so. 300L is a big unit.

I think the rule of thumb is usually 50 litre/person.

I’ve got a rainfall head but on an unvented (3 bar max) and we’ve only got a 180L. You can get 3 pretty long or 4 regular showers out of it heated to 50C. We’ve never ran out but have come close to.

You can’t really partially heat a cylinder, it doesn’t really work. You need to heat the whole thing in a single cycle in practice.
 
b0rn2sk8 said:
You can’t really partially heat a cylinder, it doesn’t really work. You need to heat the whole thing in a single cycle in practice.
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That's true of most hot water cylinders including mine, but the new breed of water cylinders use stratification so that they can be partially heated. The top of the tank is heated rather than the bottom with as traditional tanks, and the layering of hot and cold water moves down as the water is heated. The 3,000 litre hot water buffer tank of my biomass system uses the exact same method of stratification.

Some of the new systems even learn what you use on a daily basis and will only heat the volume you that you need. It would need an "in-laws" override button for when they come to visit :D.
 
Yes, cylinders like Mixergy exist and have done for a long time but they cost ££££ and are not worth the cost.

They are also unnecessarily complex as they need a pump to pull cold water from the bottom of the tank to be heated, all the associated electronics to control it all and the ‘smart’ functionality they offer.

Compared to a coil in the bottom of a tank, it’s quite complex and just not needed.

I love a gadget like the next nerd but I’m yet to see the need for WiFi to my water cylinder. It costs peanuts to fill the thing daily so why wouldn’t you just fill it?

Perhaps I should have included ‘standard’ in that sentence :p
 
b0rn2sk8 said:
The water requirement seems high 20-25l/min for a shower equally so. 300L is a big unit.

I think the rule of thumb is usually 50 litre/person.

I’ve got a rainfall head but on an unvented (3 bar max) and we’ve only got a 180L. You can get 3 pretty long or 4 regular showers out of it heated to 50C. We’ve never ran out but have come close to.

You can’t really partially heat a cylinder, it doesn’t really work. You need to heat the whole thing in a single cycle in practice.
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What size rainfall out of interest?

I'm aware 300L is a larger than average tank, but I'd much rather spend a bit more now, than have to replace it at a later date when it's not big enough (I've got 2 young girls, who I imagine will start wanting long showers in the next 10 years or so :cry: )

I'm fitting a 400mm rainfall head. In my last property I had a 300mm, which ran off the combi & worked well at ~15L/m, but it was the only shower in the property. Where I am now, we will have a smaller shower over the bath (off the combi) in a neighbouring bathroom, then the larger rainfall shower in a walk in shower (I wanted bigger than we had previously, which also means more flow required). I wanted both to be usable at the same time, hence the tank & pump. I expect there to be ~3.5bar pressure at the shower from the pump. With the larger head being ~33% larger surface area, for a similar performance, I would need ~33% more flow, which is how I set the aim of ~20L/m minimum for the shower.
 
I'm not sure it will make much difference. You'll be limited by the 15mm supply pipe to the shower head which is 15l/min at 3.5 bar in any case. The monobloc valves tend to be pretty restrictive as well as having mesh screens in the inlets to catch debris so the real flow will probably be lower anyway.

If you want more pressure out of the rainfall head, I'd recommend the smaller 300mm, if you want more spread but less pressure, the larger one is the one to go for.

The default setup is to also use a flow restrictor so they don't use a silly amount of water. That said, I removed the one on mine (300mm) :p

Edit: just be careful on the thermostatic valve you use, they tend to expect hotter water than 50C so make sure you look at the spec sheet carefully.

Get the big cylinder if you have a big family ;)
 
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Yea, there'll be a limit at the showerhead itself, as the mounting arms & head ocnnections are 1/2" threads, which have a ~15mm bore. The rest will be 22mm pipe though to reduce the flow restrictions as much as possible.

I'm not overly fussed about pressure from the rainfall, it's more the flow I'm after.

Yea, I've seen most have a flow restrictor included (I'm assuming it's some water saving regulation, but they can get round it by not building it in permanently).

The valve I've gone for wants 55-65C water, and pumps max out at 60-65C too, but 55C doesn't look to be a problem for modern heat pumps, and it'll have a bottom mounted immersion anyway, so I can always heat the majority with the HP, then turn that on overnight during a cheap rate, or when I have excess Solar to increase by the last few degress if need be.

Think I'm just going to go for the 300L HP tank. It's only ~£150 or so more than a smaller HP tank, so may as well to make sure we don't run out of water in the future!
 
If the heatpump would be doing house heating too and other dhw(another tank?), will this shower tank be close enough to that,
or will you install another hp close enough.
 
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