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?

 

[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.

 

[Dragoon]

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.

Thanks Ray, so would you suggest that in my case the shower and the sinks etc be fed by the unvented cylinder, while the large bath and the bath basin be fed by the instantaneously heated side? That sounds pretty good to me, as a small drop in flow rate at the bath shouldn't make too much of a difference.

 

[Dragoon]

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.

Hi Cardiff Plumber. That sounds good actually. But would that be the same for an unvented cylinder i.e.. that it would just be topping up temp each day?
If I lived anywhere near you (Cardiff) I might seriously consider it, but as I've said before it's hard to find people who have a good working knowledge of Thermal Stores.

 

[chris watkins]

Sounds like a good idea don't it, however your bath would be cold by the time it is full & you are ready to get in.
Your bath 400+ Litres I believe, combi flow rate lets say it's a 28kW @ 12 L/M @35deg C rise = 33mins to fill it i.e. cold & a long wait.

 

[Dragoon]

Sounds like a good idea don't it, however your bath would be cold by the time it is full & you are ready to get in.
Your bath 400+ Litres I believe, combi flow rate lets say it's a 28kW @ 12 L/M @35deg C rise = 33mins to fill it i.e. cold & a long wait.

Hi Chris
Yeah, that's true. Do you think that what Cardiff Plumber says is true for unvented i.e. that I would just be topping up temperature each day so it wouldn't be too expensive?

 

[Dragoon]

Sounds like a good idea don't it, however your bath would be cold by the time it is full & you are ready to get in.
Your bath 400+ Litres I believe, combi flow rate lets say it's a 28kW @ 12 L/M @35deg C rise = 33mins to fill it i.e. cold & a long wait.

However, as you said before, filling a bath would be 60(hot):40(cold) so for a 400ltr bath I would need 240 ltrs hot water which, at 12 ltrs/min would take 20 mins.
Is 20 mins okay to fill a bath? I don't really have baths often so I wouldn't know. This is really just for the girlfriend!

 

[Ray Stafford]

Thanks Ray, so would you suggest that in my case the shower and the sinks etc be fed by the unvented cylinder, while the large bath and the bath basin be fed by the instantaneously heated side? That sounds pretty good to me, as a small drop in flow rate at the bath shouldn't make too much of a difference.

Nope. Other way round.

Infrequent use but lots of water = bath. Take that off the cylinder.

Frequent use but middling amounts of water = shower/basin/sink. Take them from the combi.

Maybe also think about the most likely clashes, and make sure they are on separate supplies

 

[Nostrum]

How about a smaller bath? :hurray:

I'd stick with a 250-300 litre cylinder. Wire the system to prioritise hot water so when your running a bath the boiler is diverted to hot water only.

Also remember to subtract the water displaced by people being in the bath. (Depending what you were going to use a big bath for :rolleyes2

The heat losses between the 300 and the 180 is less than a kW a day. Gas is approx 4.5p per kW so (ignoring boiler efficiency) and assuming it all goes to waste (ie not airing clothes etc) your looking at less than £20 a year. The additional cost in complex installation would far outweigh the saving.

 

[Dragoon]

Nope. Other way round.

Infrequent use but lots of water = bath. Take that off the cylinder.

Frequent use but middling amounts of water = shower/basin/sink. Take them from the combi.

Maybe also think about the most likely clashes, and make sure they are on separate supplies

So the cylinder would be heated up every day, but not used until Bath day while the Combi would be in use everyday for the rest of the outlets?

Wouldn’t it be better to have it the other way around where the cylinder can be used like a ‘normal’ unvented system (where the cylinder is heated up everyday), and then on Bath day we can have the extra hot water “straight from the tap” without any storage?

Or am I missing the point? (which wouldn’t be the first time I can assure you!)

 

[Dragoon]

How about a smaller bath? :hurray:

I'd stick with a 250-300 litre cylinder. Wire the system to prioritise hot water so when your running a bath the boiler is diverted to hot water only.

Also remember to subtract the water displaced by people being in the bath. (Depending what you were going to use a big bath for :rolleyes2

The heat losses between the 300 and the 180 is less than a kW a day. Gas is approx 4.5p per kW so (ignoring boiler efficiency) and assuming it all goes to waste (ie not airing clothes etc) your looking at less than £20 a year. The additional cost in complex installation would far outweigh the saving.

Hi Nostrum.
You’re right! I completely forgot about the displaced water! Also, it would actually be worth it if the rolling eyes were applicable! But she likes lighting candles and reading her books in the bath.

And it has been bugging me if I’m being overly concerned about the extra expense. When you say that the heat losses between the 300 and the 180 is less than a kW a day what does that mean?

 

[Nostrum]

The energy losses of a good unvented cylinder are 1.5 - 2.5 kWh's per day. Ie naff all. You are worrying over nothing, or pennies per day.

:welcome: