Help me choose a domestic hot water tank

[Sideways]

This is an appeal to anyone who has worked in the heating and plumbing trade or has any specialist knowledge to help me select a new domestic hot water cylinder.

I want to swap out our 40 year old copper cylinder for a larger modern one.

1. Because new tanks are far better insulated
2. To use surplus solar electricity in the summertime. To do this most effectively means two immersion heaters. One at the top for a quick hit. Another at the bottom for bulk heating of the whole tank. I already have the smart controls to do this.
3. To increase the tank size - the new one needs to be 250 litres.
4. To get two indirect heat coils. One to hook up to the existing gas boiler. The second to be ready to hook up to an air source heat pump in future.
5. I'll be installing this myself and retaining the vented config and the header tank in the loft. I'm inclined to buy an unvented tank and install it vented because this gives me the option to have a qualified plumber change it to unvented in future, but I have nothing to gain from doing that now. The shower is pumped and water flows are just fine as it is.

What I don't know is:

• which brands use good duplex stainless steel ?
• weld everything properly ?
• stand by their warranties instead of looking for excuses to duck and dive ?
• have really efficient insulation ?
• have any genuine design / features / quality advantages that are worth paying for ?
• offer quality at a competitive price

And, are there any brands to avoid ?

Thanks !

 

[Spectric]

For unvented tanks OSO are good, neat and tidy without a mass of pipe work on the sides as the expansion vessels and connections are all on the top at the rear. The pressure / temperature relief valve is factory fitted which is a requirement of the regs for a sealed vessel so may not apply in your case. I know you cannot use a solid fuel backboiler with a sealed cylinder because you do not have direct control of the heat source and do not know if this could be the case if your solar panels are heating the water, if electric no problems.

https://osohotwater.co.uk/

 

[sirocosm]

With multiple heat sources you might be better off with a thermal store. I am not in the trade, just a DIY guy. Here is what I did:
https://www.plumbersforums.net/threads/t50-on-new-install.121196/post-1128880
The plan has unvented because I only have a gas boiler, vented is simpler if you foresee a solid fuel heat source. I don't like kit with all the bits "built in". When something goes wrong, and it always does, then it is more expensive to replace. I prefer to build it myself from off the shelf bits, then I can troubleshoot and fix it myself, and specialized parts won't become obsolete.

 

[robotmannick]

I have an Akvaterm tank as part of a biomass setup. It's a 3000l thermal store but they do make them in the size you're looking for.

I can't speak for the welding etc as the insulation comes pre-installed, but the build quality appears good and it's been in place for 10 years now with not a single issue. It loses maybe 1 degree C per day in winter when it's at 80+ degrees, installed in a drafty garage.

 

[flying haggis]

we have had a gledhill copper cylinder for thirty plus years without problem and I know they do s/s as well
https://www.gledhill.net/i think they will also make one to your spec if you so wish

 

[Agent_zed]

I've done similar to what you are doing. I have a 250 (might be 280?)litre stainless tank. It is an unvented tank but I run it vented.

I have a gas boiler feeding into it directly as well as 2 immersions although I only use the 1 which i swapped out for a 1kw as I regularly make more than 1kw on solar but not always more than 3kw. I fitted a 5 hour boost timer so I can turn it on for 1-5 hours when i leave the house for the day and know it will turn off incase I am back after I'm not making enough elec. Although I have this setup it is not that efficient to use PV to heat water. I want to install water panels soon as that is far more efficient.

Boiler also runs my CH and is set up in 's' plan.

As the boiler is direct I then have to run an external heat exchanger for the hot water. This has a thermostatic mixing valve so the output will never exceed the set value and I can heat the tank as high as i want. This setup means I have full mains pressure hot water. Only disadvantage is it require a pump, flow switch and relay to circulate hot water through the exchanger when the tap is turned on. It only requires a small header tank as it is not using the water in the tank directly.

 

[Spectric]

I am not in the trade, just a DIY guy.

Do you realise it is illegal to fit unvented hot water cylinders without the required what used to be G3 certification because of the risk of explosion.

 

[sirocosm]

Do you realise it is illegal to fit unvented hot water cylinders without the required what used to be G3 certification because of the risk of explosion.

It is not an unvented hot water cylinder, it is a thermal store with an external heat exchanger, what some people here call a heat bank. My house came with a vented system which will be sealed when I replace my old boiler, otherwise I would have gone for a mild steel tank with a large internal stainless coil. It would be one less pump and no exchanger to clog.

 

[Sideways]

Thanks for the replies so far.
Sadly I don't have space for anything bigger than 250L and I'm not sure that a thermal store is worthwhile at that small size.
With a simple hot water tank I can heat it to 70C in the daytime, draw a full 250 L of water before it runs cold and get three good baths courtesy of my PV for a useful part of the year. I couldn't do that with a 250L thermal store because once the tank temp down much below 50C it's little use.
PV isn't as efficient as solar thermal but it's much more versatile as the energy is high grade.
250L x 65C increase in temperature (5C upto 70C) needs 19kWh of solar PV energy. Sometimes we have that to burn. Other times, I imagine we'd be better off using much less to run a heat pump and preheat the 250L so it only needs topping off by immersion or gas to reach a useable temperature.

I wish I could buy a domestic size sand battery. A 500 Celsius heat source could really drive the heating and hot water but no one is offering those yet.

@Agent_zed, if you haven't yet, look at myenergi's Eddi. This takes surplus electricity from a PV system and drives resistive heaters like immersions and electric underfloor with it. It will monitor your electricity meter rails and feed only surplus energy from zero upto a max of 3.6kW into an immersion heater. Eddi does all the smart stuff. No need to size down the immersion heater. If there's 600W surplus, Eddi puts exactly 600W into the immersion whatever rating that is. When the water is at full temp, the bimetal switch in the immersion stops it from heating any more.

2 immersions are important because they can be used in turn. An upper immersion will give us a modest amount of hot water quickly for dishes or a quick morning shower, and replenish that as a priority. The lower one will soak up energy heating the bulk of the tank.

 

[dgs2001]

I've done similar to what you are doing. I have a 250 (might be 280?)litre stainless tank. It is an unvented tank but I run it vented.

I have a gas boiler feeding into it directly as well as 2 immersions although I only use the 1 which i swapped out for a 1kw as I regularly make more than 1kw on solar but not always more than 3kw. I fitted a 5 hour boost timer so I can turn it on for 1-5 hours when i leave the house for the day and know it will turn off incase I am back after I'm not making enough elec. Although I have this setup it is not that efficient to use PV to heat water. I want to install water panels soon as that is far more efficient.

Boiler also runs my CH and is set up in 's' plan.

As the boiler is direct I then have to run an external heat exchanger for the hot water. This has a thermostatic mixing valve so the output will never exceed the set value and I can heat the tank as high as i want. This setup means I have full mains pressure hot water. Only disadvantage is it require a pump, flow switch and relay to circulate hot water through the exchanger when the tap is turned on. It only requires a small header tank as it is not using the water in the tank directly.

Agent_zed

Why do you think heating water direct by solar is more efficient than using Solar PV?

My understanding was that £ for £ invested, Solar PV is the most efficient and by some margin.

 

[Agent_zed]

Agent_zed

Why do you think heating water direct by solar is more efficient than using Solar PV?

My understanding was that £ for £ invested, Solar PV is the most efficient and by some margin.

My understanding is completely the opposite. Water solar heating is pretty cheap in comparison to PV for the actual kit, and will be cheaper to replace parts etc.

Solar Water has a COP higher than 1, so you are getting more energy back into the system than you are using to run it. Normally by a good many times more. PV has to convert the suns rays into electric and you can only get a COP of max 1. E.g. You generate 1kw and roughly 1kw goes into your immersion heater, whereas for every 1kw you put into solar thermal you get 2kw or more back.


A 4KW PV system takes up pretty much all of a roof as they are only about 20% efficient at converting sunlight to electric. Most water systems I've seen only require a couple of m2 as they are 60%+ efficient at capturing the heat.

In the UK you can get around 8months of water heating from solar thermal. My in-laws have a system which gives this return.

In general though a hybrid approach is best if you can, as no one system is perfect.

if you haven't yet, look at myenergi's Eddi.

This is something i have considered in the past but a £450 outlay would take a while to pay back as we already manage our spare electric quite well as my other half is home most of the time or we program things to turn on when we are likely to have excess. We have a fair few appliances which need electric such as fridge, freezer, dishwasher, drier and washing machine so it wouldn't help converting to heated water. It is something I may look at in the future though.

In theory we could add a thermostatic mixing valve and add hot water direct to the washing machine and dishwasher so they don't have to do the heating (both only have cold fill). But they still have a background amount to run the motors.

I'm planning on investing in water panels to compliment my PV panels and a heat pump as I would like to move away from gas. The solar pv should then provide most of the elec to run the heatpump for CH and the water panels should hopefully provide most of the hot water we need for washing.

One of the most annoying things though is that when we come to cook dinner at around 5pm the sun is around the back of our house and the amount of elec drops to a few hundred watts. If I had panels on the back of the house as well I'm sure we'd be making 1kw or more (depending on the time of year). But its a trade off with making the most you can at the sunniest part of the day vs spreading it out further over the day but making less overall.

 

[Spectric]

With a simple hot water tank I can heat it to 70C in the daytime, draw a full 250 L of water before it runs cold and get three good baths courtesy of my PV for a useful part of the year.

Just three baths a year, you must have very low water bills !

 

[dgs2001]

My understanding is completely the opposite. Water solar heating is pretty cheap in comparison to PV for the actual kit, and will be cheaper to replace parts etc.

Solar Water has a COP higher than 1, so you are getting more energy back into the system than you are using to run it. Normally by a good many times more. PV has to convert the suns rays into electric and you can only get a COP of max 1. E.g. You generate 1kw and roughly 1kw goes into your immersion heater, whereas for every 1kw you put into solar thermal you get 2kw or more back.


A 4KW PV system takes up pretty much all of a roof as they are only about 20% efficient at converting sunlight to electric. Most water systems I've seen only require a couple of m2 as they are 60%+ efficient at capturing the heat.

In the UK you can get around 8months of water heating from solar thermal. My in-laws have a system which gives this return.

In general though a hybrid approach is best if you can, as no one system is perfect.

Hmmm I'm not sure.

Solar PV is a much simpler system to design and install, I'm really not sure why Thermal would be cheaper unless your going DIY.

Generally speaking it is much simpler to site an inverter and run some cable within a property than accommodate a buffer vessel, pump set, heat exchanger and safety discharge pipework

Solar PV also has a much longer expected lifespan than Thermal

 

[Agent_zed]

Hmmm I'm not sure.

Solar PV is a much simpler system to design and install, I'm really not sure why Thermal would be cheaper unless your going DIY.

Generally speaking it is much simpler to site an inverter and run some cable within a property than accommodate a buffer vessel, pump set, heat exchanger and safety discharge pipework

Solar PV also has a much longer expected lifespan than Thermal

Yes I generally do things myself.

Last time I looked the cheapest 4kw PV system was about £3000. Solar thermal was about £1000. Just for the self install kits. I don't know the relative costs to get someone to install.

I can't connect a PV system to the grid as I'm not qualified so I think it's about £250 to get someone to certify the system. I assume I'd need a plumber to check a pressurised system but not sure of this cost.

And as above you will only ever get a 1-1 return from PV.

If you can afford both I think it would create a really great system.

 

[Jones]

I'd go for a UK manufacture for the cylinder though they probably all offer similar guarantees. Ours is a Telford tempest solar 250l which sounds like it would suit you. The insulation is effective but not that thick so it's a bit smaller on the outside than the 250 L would suggest. I think they'll all come as unvented with the expansion vessel nowadays and even if you think you don't need it it's probably less effort for the plumber to fit it as unvented.

 

[Sideways]

I'd go for a UK manufacture for the cylinder though they probably all offer similar guarantees. Ours is a Telford tempest solar 250l which sounds like it would suit you. The insulation is effective but not that thick so it's a bit smaller on the outside than the 250 L would suggest. I think they'll all come as unvented with the expansion vessel nowadays and even if you think you don't need it it's probably less effort for the plumber to fit it as unvented.

I haven't priced it yet but I did speak to Telford technical yesterday about exactly this tank.
There's no problem with me DIY installing the unvented tank as a vented one.
They will fit a second immersion ("just mark on the drawing where you want it") and the 250L and bigger tanks have a hot water return connection on the side of the tank near the top. That would be perfect for my power shower which is fed from a york flange on the present copper tank.

 

[aet1]

Just three baths a year, you must have very low water bills !

He really does like being clean - surely one a year it quite adequate.