Renovation - UFH + 2 showers

[VXTfour]

Hi Guys,

I am renovating a house and the old boiler, hot water tank, everything has been removed. I am starting from scratch. I require two showers to be run at the same time. The heating will be underfloor.

I have spoken to a few plumbers and have received pointless answers such as "why would you want to run two showers at once?" "leave it 2 years and you'll be glad to get away from your wife" etc.... No one has actually said how to achieve this.

I have ruled out a combi boiler (not enough flow), an unvented system (same problem), and by default have been left with storing a lot of water via a vented system. So... would this work:

Regular boiler https://www.plumbnation.co.uk/site/vaillant-ecotec-plus-430-regular-natural-gas-boiler-erp/
Large hot water tank (vented in the loft) https://www.plumbnation.co.uk/site/discount-copper-vented-indirect-cylinders/
Large cold water tank (stored in the loft) http://www.screwfix.com/p/polytank-cold-water-tank-50gallon-uk-1190-x-610-x-500mm/10134
3 bar (40l/min) Pump to suck the water from these tanks http://www.plumbworld.co.uk/stuart-turner-monsoon-30-bar-2388-18611

The cold water tank will just feed the showers, the taps will be mains pressure.

Am I along the right lines or totally wrong about this?

Thanks,

 

[Sunshine12]

I'd like to know the answer to this too. We are having 3 showers installed and UFH throughout.

Plumber has advised combi boiler is a no no. He recommended a large tank in the loft, he never mentioned what type of tank though sorry.

Haha why would you want 2 or 3 showers running at the same time - these people have clearly never tried to get in the bathroom at 8am with teenagers!!

 

[Madrab]

The key aspect of any system design is -

Adequate dynamic pressure and flow from your mains - this is key to understanding what is achievable and needs to be known up front
A pipe sizing exercise to understand total requirements and the system designed accordingly
A large, fast recovery HW cylinder - Unvented could suit if the supply pressure and flow is suitable.
Alternatively a large, fast recovery vented HW cylinder and a large supply cistern, high enough above the head of the shower(s) to supply adequate head or pumped
Suitably sized pipework to minimise any drop in supply flow to and from the cylinder - applicable to both designs (part of the sizing exercise).
Outlet's suitable for the system design.

 

[Sunshine12]

From what I read here, unvented is better than vented: http://www.mcdonald-engineers.com/Vented-vs-Unvented-Hot-Water-Cylinder

 

[JohnD]

(not enough flow),

Have you considered improving it, by upgrading the supply pipe into your house from the pavement?

How old is the house?

 

[Madrab]

From what I read here, unvented is better than vented: http://www.mcdonald-engineers.com/Vented-vs-Unvented-Hot-Water-Cylinder

It really depends on a number of factors. They both have highs and lows, some of them are -

Vented plusses - They are usually cheaper to install and require little maintenance. They are also safer as the hot water is not held under pressure. Do not really require servicing unless issues are encountered. Minuses - They do need a higher height cold cistern and larger supply pipework to the outlets to provide suitable gravity pressure or need to be pumped.
Unvented plusses - They use less space (no cold cistern), they deliver mains pressure hot water which is the main driver for these IME. Minuses - They are more expensive to buy and install and require a yearly safety service.

 

[VXTfour]

Hey guys,

Thanks for the quick replies!

From what I read here, unvented is better than vented: http://www.mcdonald-engineers.com/Vented-vs-Unvented-Hot-Water-Cylinder

Because the Hot water and Cold water would be supplied at the pressure and flow of the incoming main, there would be a noticeable drop in flow rate when the second shower is turned on. We are trying to avoid this, hence the pumped cold water and not relying in mains pressure for anything.

Have you considered improving it, by upgrading the supply pipe into your house from the pavement?

How old is the house?

The house is 1980s, I would rather not dig up my driveway though, that's the one part of the house that is new!

It really depends on a number of factors. They both have highs and lows, some of them are -

Vented plusses - They are usually cheaper to install and require little maintenance. They are also safer as the hot water is not held under pressure. Do not really require servicing unless issues are encountered. Minuses - They do need a higher height cold cistern and larger supply pipework to the outlets to provide suitable gravity pressure or need to be pumped.
Unvented plusses - They use less space (no cold cistern), they deliver mains pressure hot water which is the main driver for these IME. Minuses - They are more expensive to buy and install and require a yearly safety service.

One of your Unvented plusses is a minus in this situation, correct me if I am wrong: It will supply hot water at mains pressure, but because both hot and cold need to be run by this single mains inlet and because this type of system cannot be pumped, the pressure and flow rate will drop substantially and not be sufficient to run two showers?

 

[JohnD]

The house is 1980s

What's the colour and diameter of your incoming water pipe? I presume it is plastic, probably blue, might be black. Might be 20mm OD.

It probably changes to 15mm copper after your indoor stopcock.

 

[VXTfour]

What's the colour and diameter of your incoming water pipe? I presume it is plastic, probably blue, might be black. Might be 20mm OD.

It probably changes to 15mm copper after your indoor stopcock.

Black, plastic and yes about 20mm. (I'll have to check exactly when I get home). Correct, it changes to 15mm after.

 

[Sunshine12]

Just been doing some further research. I'm not a plumber or qualified but have you considered an unvented cylinder and a megaflo (or similar) pump to boost pressure? http://www.megaflo-unvented.co.uk/megaflo-accumulators-and-pump-sets.php

That way you only need one tank in your loft

 

[VXTfour]

Just been doing some further research. I'm not a plumber or qualified but have you considered an unvented cylinder and a megaflo (or similar) pump to boost pressure? http://www.megaflo-unvented.co.uk/megaflo-accumulators-and-pump-sets.php

That way you only need one tank in your loft

I have checked out the Accumulator/Pump combo and figured that as we don't need the drinking water boosted, a cheaper option would be to just store the cold water in the loft and pull it out when we need it for the shower. I don't mind having the tanks in the loft as long as we can get a sufficient flow rate for the two showers. An added benefit is that when someone washes their hands downstairs, the showers shouldn't be affected, only the top-up rate of the cold/hot water storage (which you won't notice).

 

[Madrab]

One of your Unvented plusses is a minus in this situation, correct me if I am wrong: It will supply hot water at mains pressure, but because both hot and cold need to be run by this single mains inlet and because this type of system cannot be pumped, the pressure and flow rate will drop substantially and not be sufficient to run two showers?

Which leads back to my original response to you, where a dynamic pressure and flow exercise would need to be performed to ensure suitability. The second comment was in response to Sunshine's comment about the site saying that un-vented was better, IMO each system has it's benefits and drawbacks.

 

[VXTfour]

Which leads back to my original response to you, where a dynamic pressure and flow exercise would need to be performed to ensure suitability. The second comment was in response to Sunshine's comment about the site saying that un-vented was better, IMO each system has it's benefits and drawbacks.

Surely in my proposed system, this is redundant as there is stored water to provide a consistent flow?

 

[Madrab]

Not really redundant, no, do you know what your current supply is capable of delivering?

You do need to consider the capacity of the cold cistern (50+ gallons), how quickly that can be emptied when feeding 2 pumped showers running full pelt with both hot and cold water and if the current supply can make up that level of draw off quickly enough. Key here to get this working correctly for you is proper design & the starting point is knowing what the supply can deliver.

If you are not looking to upgrade the supply and it's not currently suitable to feed an unvented properly then your original suggestion of a cold water cistern and vented HW cylinder with a pump combination will be your easiest option. Do note though that a supply upgrade may be needed if your pipework drops to 15mm inside. 15mm pipework may not be able to cope with the cistern/cylinder demand.

 

[VXTfour]

I understand what you mean, though I'm still not sure it matters.

Using the links above, the largest hot water tank is 210l. The cold water tank is 50gal (227l).
Picking a 20l/min shower x2 will be 40l/min combined hot and cold flow rate.
Dividing that into 60% hot and 40% cold will be 24l/min hot and 16l/min of cold.

The 210l tank of hot water would last for 8min 45sec before the hot water tank is "empty" (replaced with the cold water from the hot water storage header tank (also assuming that the header tank is 50gal so a complete refil would leave 17l of cold water))
The 227l cold water tank would last for just over 16 min.

So the max shower would be 8 min 45 seconds at max flow rate. In this time, neither header tank would empty before the showers got cold and my wife complains that I have messed up our hot water system.....

The current mains supply only comes into account when refilling the storage tanks in the loft (after the showers and time is not a factor), and as they would also be refilling during the showers (even at a slow rate) they would never run dry.

Correct?