Unvented Cyclinders

[franky]

Alright guys just a question for you on the coils on the cylinders. The recovery on these can be as little as 7mins but at a massive 20kw how do you allow for it when sizing a boiler. I'm talking to my old boss about it at the mo as he seem to be putting in bigger boilers to factor in this, but I was thinking that it would be a waste to go down this route? I'm not sure what the best way is but wondered what you guys did?

 

[AWheating]

if the demand for hot water is very high then it would be best to increase the size of the cylinder to cope with demand, there no need for such a high recovery rate if cylinder is sized to suite.

 

[Kev the gas]

Which cylinder manufacturer is quoting 7 min recovery rates?

Kev

 

[That Extra Mile]

When using the 'Whole house' method to size a boiler you would allow 2kw for the hot water. As has been mentioned if you size the cylinder correctly you won't have any issues. Jonathan, That Extra Mile Heating and Plumbing, London

 

[tamz]

Use a divertor valve if it turns you on. Full power to the cylinder for X minutes to top it back up.
Same principle as a combi. Nobody notices the heat off for a few minutes.

 

[franky]

oso doing 7min recovery if you use 26 kw to heat the coil. Thats what they say I don't think I believe it. Thought I'd ask because I have been leaving 6kw on the boiler for hot water, but went with my old boss for a few days and noticed he was using masssive boilers so he could get the recovery time, (even though the cyclinders were correctly sized for the property) like putting in 37kw boilers for a 3 bed semi two bathrooms , he's a very good plumber and has a right statto working for him.
Bit unnecessary to overdo, it its been working fine for me so far

 

[josh]

Sounds well over the top,
normally you allow a provisonal or the rule of thumb is 1kw for every 50 litres of stored water you have
the coil rating doesn't mean you allow an extra 26kw of boiler for the coil !!
as tamz said though if you did use a diverter you wouldn't need to allow for it but i do not see that as neccisary specially if it was a larger house with more than one heating zone or one bigger zone where you require 28mm for the first section of pipe work as a diverter may restrict the flow of this down a lot more than a 2 port valve.
not only using a massive boiler is more expensive to buy it will never be at full rate and not as efficient, yes i know they do modulate down to nearly nothing but that's not the point. hope this helps atall.

 

[Gray0689]

That's not a good plumber that's just silly
Where is the energy efficiency there 37kw boiler in 3 bed house lol

 

[Deleted member 2053]

Why not fit a matched boiler and store like the Vaillant? The ebus system turns the boiler on to 85C to reheat the hot water when required (hence higher transfer rate across the heat exchanger) and then adjusts back down to the central heating setting once demand for DHW is satisfied. That way you seem to get the best of both worlds.

 

[franky]

Grahamep have you installed the 937 is it any good compared to an unvented set up? Have one to put in next month.
Why is the coil rating there at all? I never even looked at it until my old boss pointed it out.
Also I will stand by that he's a good plumber he's just got this wrong

 

[Gray0689]

Grahamep have you installed the 937 is it any good compared to an unvented set up? Have one to put in next month.
Why is the coil rating there at all? I never even looked at it until my old boss pointed it out.
Also I will stand by that he's a good plumber he's just got this wrong

Ok mate but it's very wrong and if building control new it would fail as waste of energy

 

[tamz]

The coil rating is there as the manu's have to make the cylinder to a certain std for reheat times.

 

[doitmyself]

i know they do modulate down to nearly nothing but that's not the point. hope this helps atall.

The problem is they do not modulate down to nearly nothing. Most boilers have just a 3:1 or 4:1 modulation ratio. So if you have a 30kW boiler, the lowest it will go is 7.5kw and more likely 10kW. That's not much help if your house only needs 8kW.

The only boilers with a wide modulation ratio are the Geminox (e.g 1kw to 10kW and 2.5kw to 18kW).

 

[Deleted member 2053]

Grahamep have you installed the 937 is it any good compared to an unvented set up? Have one to put in next month.
Why is the coil rating there at all? I never even looked at it until my old boss pointed it out.
Also I will stand by that he's a good plumber he's just got this wrong

Sorry about the delay in replying - had a busy week and only just seen this. Yes I have installed a 937 and it wasn't bad but I wasn't that impressed with it really. I have a bit of an aversion to everything in one place and it is quite a large unit. This really is only a combi with storage so you overcome some of the limitations of the combi but without the need for an unvented ticket as the storage is only (as I recall) about 10 litres. I much prefer vaillant's own unvented cylinder with ebus controls that link back to the boiler and allows the rapid reheat I mentioned above. Actually the reality is that you can probably use any unvented cylinde with their Vaillant ebus and have the same effect. But their cylinder is also a nice piece of kit. Vaillant do lots of ebus stuff and my experiences with it are very good.

As has been pointed out above there are lower limits on modulation and you only really want to be putting a 937 in a fairly large house and probably not one with new-build levels of insulation . But a large house will almost certainly have multiple showers etc and that is where the whole combi based philosophy starts to break down.

 

[Deleted member 2053]

oso doing 7min recovery if you use 26 kw to heat the coil. Thats what they say I don't think I believe it.

7 min recovery from 26kw would imply around 70 litres capacity (compare 24kw combi give 35c rise at around 9.5 litres/min). Those good old boys at Newark Copper cylinders reckon they fit high recovery coils to match the boiler.