Vented system, cold water tank questions

[daven1986]

Hi all,

This is my first post here so please be gentle! I have been reading lots of threads about central heating and hot water solutions, however I still have a few questions!

Scenario:
I have 2 flats that I'm converting into a 4 bedroom house, the flats currently have a combi boiler each to drive the central heating and hot water. However they do have shared mains supply which makes it easier to combine the heating systems. I don't want to use an accumulator because the difference in my mains dynamic vs static pressure is fairly high, and the flow is not always brilliant (I notice drops in flow when having a shower even though the other flat is currently empty - so it must be from the mains). I also want to ensure that I can have 2 showers running at the same time, without any drop in flow even if someone uses the washing machine / taps - this is an issue with my current combi boilers and it is really annoying - both my wife and I love showers.

My proposed solution:

• Vented hot water cylinder (250-300l)
• Cold water tank (400l)
• 2x Pumped showers
• 35kw system boiler
• Mains cold water feeding all basins, toilets and bath

Problems / Questions:

• I could put a cold water tank in the loft, however due to the way the building is structured I would need some pipes running up the wall of one of the bedrooms to reach the loft (the bathrooms are in an extension under a flat roof, with no access to the loft). This is not the end of the world, but it irritates me. To resolve this problem I thought that maybe I could have all my cold water running from the mains, except for the showers which would run from the cold tank and the hot tank via a pump. In this way, I could have the cold tank on the ground floor or the 1st floor, instead of in the loft.
  • Would this work?
  • How could I balance the hot/cold flow for basins / bath / etc.?

• When sizing a cold water tank, do I need to ensure it isn't "too big" so that water doesn't stagnate, or is it just a case of ensuring it doesn't fall through the floor?!

• If I wanted the toilets and bath to be fed from the cold water tank, can I place it anywhere but add a pump?
  • Again, how would I go about balancing the hot/cold flow?

I appreciate these are probably questions to ask my plumber, however I am trying to plan the layout of the house for now, and I don't yet have a plumber contracted to do the work.

Thanks in advance,

Daven

 

[stardanny]

Problems / Questions:

• I could put a cold water tank in the loft, however due to the way the building is structured I would need some pipes running up the wall of one of the bedrooms to reach the loft (the bathrooms are in an extension under a flat roof, with no access to the loft). This is not the end of the world, but it irritates me. To resolve this problem I thought that maybe I could have all my cold water running from the mains, except for the showers which would run from the cold tank and the hot tank via a pump. In this way, I could have the cold tank on the ground floor or the 1st floor, instead of in the loft.
  • Would this work?
  • How could I balance the hot/cold flow for basins / bath / etc.?

• When sizing a cold water tank, do I need to ensure it isn't "too big" so that water doesn't stagnate, or is it just a case of ensuring it doesn't fall through the floor?!

• If I wanted the toilets and bath to be fed from the cold water tank, can I place it anywhere but add a pump?
  • Again, how would I go about balancing the hot/cold flow?

Cold water tank need to be at highest point, a loft is the best place, to ensure a good water pressure.
Normally a 50 gallons tank is enough.
Cold feed for bathroom is better from cold water tank. No need to balancing hot and cold flow as they are the same flow from one tank.
Can't you run pipework from loft to first floor via airing cupboard?

Daniel.

 

[Madrab]

I could put a cold water tank in the loft, however due to the way the building is structured I would need some pipes running up the wall..........
Would this work?
How could I balance the hot/cold flow for basins / bath / etc.?

Yes....
Balancing mains to pumped depends on taps used. If they're single flow mixers then either PRV on the cold or match the pump to the mains pressure. Individual or dual flow mixers wouldn't be affected.

When sizing a cold water tank, do I need to ensure it isn't "too big" so that water doesn't stagnate, or is it just a case of ensuring it doesn't fall through the floor?!

Cold cistern size should be fine. As long as the hot and cold are used regularly and the cold cistern is piped correctly then stagnation should not be an issue.

If I wanted the toilets and bath to be fed from the cold water tank, can I place it anywhere but add a pump?
Again, how would I go about balancing the hot/cold flow?

Yes if you spend more on a universal pump but using a standard pump with a cold water cistern and hot cylinder you really need the cold tank above the level of the shower head, the higher the better, to avoid any issues with pump activation or negative head.

 

[daven1986]

I could put a cold water tank in the loft, however due to the way the building is structured I would need some pipes running up the wall..........
Would this work?
How could I balance the hot/cold flow for basins / bath / etc.?

Yes....
Balancing mains to pumped depends on taps used. If they're single flow mixers then either PRV on the cold or match the pump to the mains pressure. Individual or dual flow mixers wouldn't be affected.

When sizing a cold water tank, do I need to ensure it isn't "too big" so that water doesn't stagnate, or is it just a case of ensuring it doesn't fall through the floor?!

Cold cistern size should be fine. As long as the hot and cold are used regularly and the cold cistern is piped correctly then stagnation should not be an issue.

If I wanted the toilets and bath to be fed from the cold water tank, can I place it anywhere but add a pump?
Again, how would I go about balancing the hot/cold flow?

Yes if you spend more on a universal pump but using a standard pump with a cold water cistern and hot cylinder you really need the cold tank above the level of the shower head, the higher the better, to avoid any issues with pump activation or negative head.

Thanks, good to know re the dual mixers. Just to confirm if I used a universal pump on my showers, then I could place the cold tank wherever I wanted? That seems like it might be a good solution, and then the rest of the cold feeds would be mains water (I'd definitely prefer that in the basins).

Problems / Questions:

• I could put a cold water tank in the loft, however due to the way the building is structured I would need some pipes running up the wall of one of the bedrooms to reach the loft (the bathrooms are in an extension under a flat roof, with no access to the loft). This is not the end of the world, but it irritates me. To resolve this problem I thought that maybe I could have all my cold water running from the mains, except for the showers which would run from the cold tank and the hot tank via a pump. In this way, I could have the cold tank on the ground floor or the 1st floor, instead of in the loft.
  • Would this work?
  • How could I balance the hot/cold flow for basins / bath / etc.?

• When sizing a cold water tank, do I need to ensure it isn't "too big" so that water doesn't stagnate, or is it just a case of ensuring it doesn't fall through the floor?!

• If I wanted the toilets and bath to be fed from the cold water tank, can I place it anywhere but add a pump?
  • Again, how would I go about balancing the hot/cold flow?

Cold water tank need to be at highest point, a loft is the best place, to ensure a good water pressure.
Normally a 50 gallons tank is enough.
Cold feed for bathroom is better from cold water tank. No need to balancing hot and cold flow as they are the same flow from one tank.
Can't you run pipework from loft to first floor via airing cupboard?

Daniel.

Thanks, unfortunately the two rooms next to the bathroom are bedrooms and the bathroom only has a flat roof above it, therefore to get to the main loft space the pipes need to go into the bedroom then into the loft. It isn't the end of the world but something I'd love to avoid if it isn't too much work.

Appreciate the responses,

Daven

 

[Madrab]

Well within reason of course, there is an optimum cistern/cylinder position even with a universal pump, you wouldn't really be placing the tank and cylinder on the floor below. The cold tank really needs to be above the hot cylinder with the pump at the base level of the cylinder. Ideally the cold cistern would be as high as possible within your ceiling height range, do remember though, it's a big heavy cistern, and HW cylinder too and you will have the fill valve noise especially if at room ceiling level.

Just a few considerations. As D mentions, if at all possible, it may be worth considering the attic space, pipes can always be hidden quite creatively. I do believe you'll need that larger cistern size primarily if there is the potential of 2 long power showers at once, you will drain that cylinder quite quickly, pump pressure/flow dependent. Adequate supply pipe size to the cylinder will be important too.

I'm sure your plumber will cover all this with you anyway, some good design work and thought and you should be able to set up what you're looking for, if constant flow over 2 showers is key and the mains cannot handle it.

Can I assume (maybe wrongly) that you had the mains checked to ensure that there isn't an option available there?

 

[daven1986]

I haven't had the mains checked although I do know it is a 15mm pipe rather than a 22mm. Personally I prefer a vented solution to an unvented due to reliability of mains flow, complexity and maintenance.

Thanks for the comprehensive response, I think that given what you've both said, the best thing is to have the cold tank in the loft and hide the pipes.

Thanks,

Daven

 

[Madrab]

That's ideal, yes. Do remember as well, you will have a bit of a drop in supply to the shower that is already on when the 2nd shower is run until the pump catches up, only way to avoid that is to have separate feeds and pumps for each shower but that brings in another set of considerations.

 

[daven1986]

Thanks, I will be having two separate pumps I think (at least that is the plan at the moment!). What considerations are there?

Thanks

Daven

 

[Madrab]

Separate feeds from both cylinder and cistern to each shower should remove the drop off of supply when the other shower starts up. Noise and location will need to be considered. 2 pumps will likely sympathetically resonate quite significantly therefore separate concrete bases (sit on concrete slabs that sit on high density rubber mat). A thought should be given to how quickly that will empty your hot water cylinder and cold cistern therefore a suitable supply to the cistern and a fast recovery cylinder would be recommended, could use anywhere from 30-50L every 5 min HW depending on pump/temp setting/HW temp/shower head type of course.
To get this just right it will need fine tuning with pipe layout, outlet specs, delivery capacities etc to map the potential usage limits, again your plumber/designer should work through all of that with/for you.
Would be worthwhile engaging a resource that can do the design work for you and get the figures and layout correct so you know what is achievable, as you seem to be looking for quite a specific setup

 

[daven1986]

Wow thanks for all the detail. I plan on aiming for around 12l per minute flow so that is where I got the tank sizes in my original post from.

Your replies have been really helpful, I feel much more prepared to go into a discussion with a plumber.

Many thanks

Daven