Linking Two Unvented Cylinders

[Dragoon]

Hi looking for some help please.

Need a plumber to install heating and hot water from scratch in a new-build. I want to go a bit flash and install a round bath. The bath holds 405 litres of water. I also have a steam shower cabin to install which has about a zillion shower jets. The rest of the house is pretty normal.

The problem is that my unpredictable girlfriend may want to run a bath while I’m having a shower (also a friend who may be staying may want to do some washing up at the same time). Point is that I would like to avoid any sort of loss in hot water flow if possible.

So realistically Combis are not really viable. So a system boiler with an unvented cylinder would be the choice to go for.

However as the large bath would only be used very occasionally, it would be a waste of energy having to constantly heat up a large cylinder to provide for the occasional use of a large bath.

So my question is, is there any way of linking up two smaller unvented cylinders so that only one is heated and the other can be heated when the large bath needs to be run?

 

[Ch4 plumbing]

having 2 unvented cylinders won't improve hot water flow rate as this is governed by the incoming cold water main flow and pressure, It will however give you more stored hot water.

 

[Ch4 plumbing]

Also the only practical way I can think of doing it would be to have the bath run off the 2nd cylinder only, otherwise the unseated cylinder would cool the hot at the point where they join.

Then if only used occasionally you would enter into the realms of a high legionella risk.

 

[Dragoon]

Hi Ch4, I'm not looking to improve water flow rate by using two cylinders, I'm looking to save on bills by not heating up one very large cylinder everyday.

 

[Ch4 plumbing]

Oh okay I read your post wrong.

" Point is that I would like to avoid any sort of loss in hot water flow if possible "

 

[Dragoon]

Oh sorry! Yes, I made that point to validate why I was ruling out using Combis. Because if Combis didn't lose flow rate when another outlet is being used, a Combi would be ideal.
Also yes, I didn't think about the legionella risk.
Any other way to not constantly heat up a large cylinder for the occasional use of a large bath?

 

[Ch4 plumbing]

If you were strict with your hot water times that would save you money, also most unvented cylinders don't lose a lot of heat so as long as you and your family were careful you should be okay, what size cylinder have you been advised to have fitted?

 

[Dragoon]

I have approached a couple of plumbers, and they haven't got back to me as yet so I don't know what cylinder size, or even what system they will suggest. It's just that talking with them we couldn't really pin down which system to go with, a powerful Combi or an unvented system.

I just measured the incoming flow rate this morning and it's pretty impressive. It's about a litre per second or 60 litres/min.

 

[Ch4 plumbing]

I wouldn't be fitting a combi! A Worcester high flow can only deliver 25ltr a min at 30 degree rise, and that's not going to be enough for you at a guess.

 

[Dragoon]

Yeah I'm not really keen on a Combi, but they would be cheaper. But as you say unvented cylinders don't lose a lot of heat anyway, and also the cylinder will be housed in a dedicated small boiler room along with the boiler so heat loss would be even less I suppose.

I'm probably being needlessly concerned, but I thought I should look into a cheaper way of doing it because it's best to get it right the first time.

 

[Ch4 plumbing]

No your right I agree completely, there's no point in heating 500ltr of water if you don't ever use it! Sit down with the installer and go through the exact demands and they should be able to size the cylinder correctly for you and your needs, good luck.

 

[Dragoon]

Cheers Ch4! Will do!

 

[Sharp Point]

I

I have approached a couple of plumbers, and they haven't got back to me as yet so I don't know what cylinder size, or even what system they will suggest. It's just that talking with them we couldn't really pin down which system to go with, a powerful Combi or an unvented system.

I just measured the incoming flow rate this morning and it's pretty impressive. It's about a litre per second or 60 litres/min.

Are you sure it's 60l/m, how are you measuring?

whats the static and dynamic pressure?

 

[Ray Stafford]

I just measured the incoming flow rate this morning and it's pretty impressive. It's about a litre per second or 60 litres/min.

Thats very high for a domestic supply. What size is the incoming main? (The blue pipe coming into the house from the road)

 

[Best]

To avoid having a second, but seldom heated unvented cylinder which would risk stagnant water & take up space & cost you more, I would suggest you use one large unvented unit that has different height heating coils inside it. That way you can use the lower coil only when you want more storage. Ask the manufacturer directly about this.
Unvented cylinders won't cost much extra to heat if larger anyhow, and they don't lose their heat quickly. They are the best job & will not cost much if you avoid the overpriced Megaflows

 

[AWheating]

Hi looking for some help please.

Need a plumber to install heating and hot water from scratch in a new-build. I want to go a bit flash and install a round bath. The bath holds 405 litres of water. I also have a steam shower cabin to install which has about a zillion shower jets. The rest of the house is pretty normal.

The problem is that my unpredictable girlfriend may want to run a bath while I’m having a shower (also a friend who may be staying may want to do some washing up at the same time). Point is that I would like to avoid any sort of loss in hot water flow if possible.

So realistically Combis are not really viable. So a system boiler with an unvented cylinder would be the choice to go for.

However as the large bath would only be used very occasionally, it would be a waste of energy having to constantly heat up a large cylinder to provide for the occasional use of a large bath.

So my question is, is there any way of linking up two smaller unvented cylinders so that only one is heated and the other can be heated when the large bath needs to be run?

sounds like an interesting job to work in. What area are your from? maybe a plumber from this forum is from your neck of the woods?

welcome to the forum.

 

[SimonG]

How about a 300 litre unvented for all the house bar the bath and a gas fired instantaneous water heater for the bath?

or bigger cylinder and multiple coils as best recommended above.

 

[chris watkins]

The multi coil cylinder is a good shout but still need to take care with leaving the bottom un-heated for more than couple of weeks.

You can have a go at working out your own hot water storage requirements.
Bath is easy, what was it 400L (BTW notifiable under Water Regs) so approx 60/40 hot to cold mix to give bathing temp water you will need around 240L hot @ around 65deg C depending on the time of year.

Shower is a bit more awkward, does the manufacturer give any flow rates? they normally do against different pressures. Decide how long she will be in the shower for & multiple by the Litres per minute give and do the 60/40 bit again but remember in both cases the recovery rate of these new cylinders is such that given the boiler power they can re-heat (recover) in little over 20-30 mins, so don't do it all on the storage of hot water.
Don't worry about the washing up bit.

Either way it will be a large cylinder, hope that helps.

 

[Gray0689]

A 300 to 400ltr ACV cylinder will do your job as the reheat times are amazing
Stick in solar thermal if your hot water demand is going to be that high

 

[Ray Stafford]

The multi coil cylinder is a good shout but still need to take care with leaving the bottom un-heated for more than couple of weeks.

You can have a go at working out your own hot water storage requirements.
Bath is easy, what was it 400L (BTW notifiable under Water Regs) so approx 60/40 hot to cold mix to give bathing temp water you will need around 240L hot @ around 65deg C depending on the time of year.

Shower is a bit more awkward, does the manufacturer give any flow rates? they normally do against different pressures. Decide how long she will be in the shower for & multiple by the Litres per minute give and do the 60/40 bit again but remember in both cases the recovery rate of these new cylinders is such that given the boiler power they can re-heat (recover) in little over 20-30 mins, so don't do it all on the storage of hot water.
Don't worry about the washing up bit.

Either way it will be a large cylinder, hope that helps.

Sorry Chris, there is an error in your maths.

The calculation for stored water is:

W = 210 + (x^3 x 100) + (y^2 x 100)
--------------------------------------------
n

Where W is the amount of stored water in litres, x is the number of teenage girls living in the house, y is the number of women of any other age in the house, and n is the number of wild horses you have to hand in order to get said females out of the bathroom.

 

[SimonG]

If x and y are greater than 2, then install an iso valve outside the bathroom to prevent over long showers.

 

[rpm]

If x and y are greater than 2, then install an iso valve outside the bathroom to prevent over long showers.

One with a timer to reduce the chance of getting caught turning it off?

 

[chris watkins]

Sorry Chris, there is an error in your maths.

The calculation for stored water is:

W = 210 + (x^3 x 100) + (y^2 x 100)
--------------------------------------------
n

Where W is the amount of stored water in litres, x is the number of teenage girls living in the house, y is the number of women of any other age in the house, and n is the number of wild horses you have to hand in order to get said females out of the bathroom.

Ah well, 4 boys & 1 wife only ever exposed me to basic arithmetic's Ray, I will stand corrected by your high math skills, as ever.

All that algebra, just like the fairer sex, was something I just could never get my head around!

Being a more practical man I use have applied something someone once told me & ended up with this when applied to yours N = X+YS<W were S is the showers required W should always be less than this.
Field testing has always indicated a substantial drop in the number of horse power required.

 

[Dragoon]

Hi All.

Still waiting for a response from the plumbers, but in the meantime, what do people think of an average sized unvented cylinder for shower and the rest of the house, and a separate small Combi to heat the occasional bath water?

The incoming flow rate seems pretty good enough to cope with this. However if it wasn't good enough, the resulting drop in water flow to filling the bath, wouldn't really matter as much as if there was a drop in flow rate to the shower.

It would obviously work out more expensive, but may be cheaper in the long run.

 

[Dragoon]

Hi I don't really know why but until I just posted my above question, I wasn't able to see a reply from anyone else other than Ch4. Now suddenly I can see all the other replies. I thought no one was reading this thread!

I'm new to this posting thing and must be doing something wrong.

Anyway I'll read all the other replies and get back to you soon.

 

[Cardiff Plumber]

Hi Dragoon,

Use a thermal store with a pumped return, if cws supply is an issue use a break tank with a pump and pressurized vessel.

I have a lot of experience with hot water systems. I fitted similar system and all the underfloor heating including the heat extraction system on the telecom's room on top of the tower. This was on this project in Lymm, Manchester, see here: Lymm Water Tower - the art of housebuilding

 

[Dragoon]

Hi Ray.

The size of the incoming blue main is 25mm (I just measured it with a tape measure so may be a millimetre or two out.)

 

[Dragoon]

Hi Best. I think that's a really good idea about the different levels of heating coils.

 

[Dragoon]

Hi AWheating. Thanks for the welcome!

I would really like to say where I am located, however I think I should wait for the replies from the plumbers I have already asked, otherwise they may get a bit offended that I didn't wait for their answers. I may do so at a later stage though.

 

[Dragoon]

Hi Simon G.
Yes as posted earlier, I was thinking along those lines as well. However I don't know if that would be better than what Best suggested about the different coil heights.

 

[Dragoon]

Hi Gray0689

Could you explain what a solar thermal is please.

 

[Dragoon]

Hi Chris Watkins, I’ll do the calcs and get back to you later.

Hi Ray Stafford, the calculation should be quite simple as Y (No of women) =1. However is there a there a variable I should add to the eqn. for the Predictability of Y?

 

[Dragoon]

By the way, I think I realise why I couldn't originally see all the other replies. I didn't realise that there were different pages so was only looking at page one:blush5:

However, I didn't receive any email alerts either so I still don't really know.

 

[Ray Stafford]

Hi Chris Watkins, I’ll do the calcs and get back to you later.

Hi Ray Stafford, the calculation should be quite simple as Y (No of women) =1. However is there a there a variable I should add to the eqn. for the Predictability of Y?

You still need at least 1 wild horse (or reasonable fraction thereof) otherwise the calculation comes out as div by 0.

The predictability of women is such a vanishingly small number that it eludes description even by advanced quantum theoreticians. However, the square root of FA is a good approximation.

 

[SimonG]

Hi Dragoon,

Use a thermal store with a pumped return, if cws supply is an issue use a break tank with a pump and pressurized vessel.

I have a lot of experience with hot water systems. I fitted similar system and all the underfloor heating including the heat extraction system on the telecom's room on top of the tower. This was on this project in Lymm, Manchester, see here: Lymm Water Tower - the art of housebuilding

Definitely not a thermal store, unless you are connecting in multiple fuel sources.

 

[Dragoon]

Hi Cardiff Plumber.
Wow what place that Lymm Water Tower is!!! I wish mine was anywhere near as good! Impressive work!!

I have considered Thermal Stores before and they sound a good idea. The only problem I have with them is that not many plumbers are familiar with them, so if and when things go wrong with them, I wouldn't be able to just phone the first available plumber to sort the problem.

Also in my particular case, wouldn't it again involve heating a large cylinder of water for the occasional use of the bath?

 

[Dragoon]

Lol!! Yeah, wasn't it this Predictability factor that first lead scientists to discover quantum theory?!

 

[Dragoon]

Hi Bogrodder.

There's a temporary curved 25mm copper pipe, shaped like a tap with a lever ball valve as a tap connected to the blue incoming mains. (there is also stopcock on it but I've left that open).

I filled a bucket to 25 litres in 25 seconds. I don't know the pressure as I don't have a gauge.

Bye the way I'm sorry if this is the second time you've received this reply as I'm sure I've already sent it, but I cannot see it on the page. It may have something to do with me using Safari as a browser. Anyway apologies again.

 

[snowhead]

However is there a there a variable I should add to the eqn. for the Predictability of Y?

This is the missing constant ∞

 

[Dragoon]

Hi Snowhead.
Lol! I think that symbol (which I can’t do on my keyboard) = Unpredictability.
So Predictability would be 1/ (that symbol) which would make it pretty close to zero as Ray stated.

It’s a good thing that there aren’t many female plumbers out there or else we’d get deluged with lots admonishing replies from irate members.

In fact I’m going to have to have the Hide button ready when writing in this forum, just incase she walks in! . . . . . . Which will be okay cos she’ll think I’m just looking at naughty stuff again, which is usually considered a lesser crime than any form of direct criticism!

 

[AWheating]

Unvented cylinder and combi boiler used as a system boiler but the hot water from combi supplying the bath and basin in the bathroom.

 

[Ray Stafford]

Unvented cylinder and combi boiler used as a system boiler but the hot water from combi supplying the bath and basin in the bathroom.

If there is no shortage of incoming pressure and flow (and it sounds like there isn't) then this would be an excellent solution, and very cost effective both to install and to run.

 

[Gray0689]

Hi Gray0689

Could you explain what a solar thermal is please.

Solar thermal
Basically tubes or panels on roof (South facing preferably)
That when sun out will heat hot water through a coil at the bottom of your cylinder

And stay away from thermal stores there not good

 

[Ray Stafford]

And stay away from thermal stores there not good

Thermal stores are excellent if you need to integrate multiple heat sources, particularly hard-to-control or unpredictable sources like solid fuel or renewables.

If there is no need to manage such challenging inputs, then its difficult to make the case for a thermal store in a domestic installation.

 

[BLOD]

You could fit an unvented sized for just the shower & hot taps and fit a combi boiler zoned for heating & cylinder but using the hot water side just for the bath.

 

[SimonG]

I wouldnt be keen using a combi to fill such a large bath. It takes about 25 minutes just to fill a standard bath with our gas combi.

 

[Dragoon]

Unvented cylinder and combi boiler used as a system boiler but the hot water from combi supplying the bath and basin in the bathroom.

Hi AWheating

Could you please explain the above system a little more. So would that be the one boiler (a Combi boiler) and one unvented cylinder? If so, could you explain a little more how it would work?

 

[Ray Stafford]

Hi AWheating

Could you please explain the above system a little more. So would that be the one boiler (a Combi boiler) and one unvented cylinder? If so, could you explain a little more how it would work?

Hi Dragoon. I'm not sure whether AW is online, but in case he isn't, this is what he is suggesting.

On the heating side, a combi is just like any other boiler - it has a flow and return which can be used as an S plan (preferred) or Y plan (if you must) to heat a cylinder as well as heating the radiators.

In addition to that, it has the instanteous hot water system, albeit with a limited flow rate, as you have identified.

So if you mate a combi with an unvented cylinder, you can have stored water available for some outlets and instantaneously heated water for others. Exactly how you divide them up is up to you, but normally outlets that use a lot of water, relatively rarely (say a bath) should be fed by stored hot water, whereas outlets that frequently use just a few litres (say kitchen sink) would be better on the instant.

The limiting factor (assuming that the cylinder is unvented) is the capacity of the incoming cold main, but yours sounds like it has plenty of flow at least.

 

[Cardiff Plumber]

Hi Cardiff Plumber.
Wow what place that Lymm Water Tower is!!! I wish mine was anywhere near as good! Impressive work!!

I have considered Thermal Stores before and they sound a good idea. The only problem I have with them is that not many plumbers are familiar with them, so if and when things go wrong with them, I wouldn't be able to just phone the first available plumber to sort the problem.

Also in my particular case, wouldn't it again involve heating a large cylinder of water for the occasional use of the bath?

Well you would have superb insulation with a decent thermal store, so you would be just topping up temp each day. It works very well and have never had any problems in the past.

 

[Dragoon]

Thermal stores are excellent if you need to integrate multiple heat sources, particularly hard-to-control or unpredictable sources like solid fuel or renewables.

If there is no need to manage such challenging inputs, then its difficult to make the case for a thermal store in a domestic installation.

Hi Ray.
Thermal stores sound like a good idea, however I don't have multiple heat sources so it's not really needed in my case. But also I heard that they can be problematic (as with all systems really as they age), and because not many plumbers are familiar with them, that would be my biggest cause of worry with them.