Which unvented indirect cylinder to buy?

[virgilns]

not flawed, but work out the difference in cost to install a 300 now vs a 210 or a 250 and another in x number of years...price difference between sizes is not THAT much and the extra heat loss will be minimal...but you can work that out too...

 

[loizos]

The flow rate is somewhere between 18 and 20 litres.

I'm torn between a Unistor 210, 260 and 310. It's not about the price difference of getting the unit in - but I would prefer to do the installation once and be done with. So what is recommended out of these for the eventual use of likely 2 bathrooms + 1 en-suite for a family of 5. The house is 6 bedroom over 3 stories, but not sure if that matters.

I am erring on the 'safe' side to go for the 310 but can someone please help me to break down what the likely additional cost is of having a larger unit sitting there unused just incase we need more water storage in later years.

 

[indus]

The flow rate is somewhere between 18 and 20 litres.

I'm torn between a Unistor 210, 260 and 310. It's not about the price difference of getting the unit in - but I would prefer to do the installation once and be done with. So what is recommended out of these for the eventual use of likely 2 bathrooms + 1 en-suite for a family of 5. The house is 6 bedroom over 3 stories, but not sure if that matters.

I am erring on the 'safe' side to go for the 310 but can someone please help me to break down what the likely additional cost is of having a larger unit sitting there unused just incase we need more water storage in later years.

I think you are getting obsessed by the size of the cylinder and may not be sorting out the 'basics' first.

Your plumber has said that your flow rate is fine for your demands (on a 15mm pipe), you need to confirm what your demands are.

Many showers have a flow rate of approx 15l/minute. So given your existing flow rate of 18l/min you can see that you will not really be able to run two showers at the same time.

If you are happy with only being able to run one shower at a time then it's no problem. However should you want more flexibility then you need to increase your flow rate

 

[loizos]

Thanks. Today there is only one bathroom in the house so it's fine for now. He said I'll need to upgrade the pipe coming in in future but I want to make sure I'm not upgrading the cylinder again when I do that. We're talking 5 years down the road before I add another bathroom minimum.

So sizing for the future, I want to make sure that with a then decent flow rate (assumed), I want to buy a cylinder now which will run 2 bathrooms and one en-suite for 5 people. If 310l is the safe option, I'll go for that so long as its not going to cost me an extra £1 or more a day for the next 10 years to run it (£3650). That £1 is not something I calculated. If it really is more expensive by £1, it's clearly more cost effective to go with smaller now and upgrade / add to later...

 

[mfarrow]

The difference in heat loss if shown in the link I gave earlier for vaillant, consult your manufacturer for others. Multiply this value by the cost of energy to your home, then divide by the decimalised efficiency of your boiler to get the cost per day.

As you can see though, there's not a great deal of difference between the sizes, because most of the energy is lost at the connections. Once you connect your copper piping to it, that is only going to increase, but you will use the same or similar amount of copper pipework no matter which size your cylinder.

 

[virgilns]

These are obviously very rough figures, but you'll get the idea...and the 210l tank is your baseline heat loss, so we'll reference all the heat loss calcs from that...

Vaillant 210 loses 1.9kw/24hrs, costs £760
vaillant 310 loses 2.4kw/24hrs, costs £860

take gas at 10p/kw with 0.5kw extra heat loss of the bigger tank
0.5kw=5p/day
5years of 5p = £91.25

Both cost the same to fit - lets say £300

210l: £760+£300=£1060 over 5 years
300l: £860+£300+£92 = £1252 over 5 years

or 200quid more.

If you go option 1, you're then going to install another tank, lets assume the smallest they do 155l at £560 and 1.4kw heat loss at 14p/day

155l: £560+£300+£255 = £1115

so over a 10 year Total cost of ownership (TCO)
210l tank for the first 5 years, 155 l for the second 5:
£1060+£1115 = £2175

310l tank for 10 years (including 10 years of addition heat loss over the 210l)
£1252+£92 = £1344

or 800quid cheaper

as Mfarrow says most heat loss is at the fittings which you'll double when yo install two cylinders (a 155+155=2.8kw heat loss, a 310 is 2.4kw)

my Telford is surprisingly cool to the touch everywhere except the bits that stick out of it...hot water pipe comming out, immersion heater, safety valve etc...

Buy the biggets you can get in assuming the floor can take the weight...

 

[loizos]

virgilns - I can't thank you enough for breaking this down for me. I thought it would be hundreds of pounds a year difference in running costs, not 20 quid!

Thanks again to everyone on here who responded.