Cylinder size

[pauldoc]

Had a conversation with my plumber about cylinder size. The house will be 5 beds and 4 bathrooms. 2 have a bath. We will be having gas and an un vented system like mega flo. He thinks I might need 2 cylinders of 200 litres each as he thinks it should be spec'd to the max usage. There are 4 of us (2 kids) at the mo our house has a 150 litre tank as seems to be plenty. I'm also worried about heating that much water every time. I was thinking a 300 litre tank would be plenty.  Any opinions welcome!  Thanks 

[Stones]

We are going with a 300L cylinder, heated via an ASHP.  Had we had the option of gas, with it's much faster recharge time, we would have gone for a 200L cylinder or do away with the cylinder completely and have a combi.  A 300L cylinder lets me store the quantity of DHW we need at a lower temperature (between 45C and 50C) to suit the ASHP.

[ProDave]

SWMBO and I were discussing this.

 

"I want a larger HW tank in the new house"

 

"Why, this one never runs out of hot water"

 

"You haven't seen the shower I want in the new house".......
 

[jack]

We have a 250L UVC with 5kW ASHP.  Seems to work fine.  As discussed elsewhere, we boost the top part with the immersion every morning but probably don't need to.  We've never run out of water in a year living here, although admittedly our water pressure is a bit low at the moment (temporary restrictor fitted by plumber to make absolutely sure we pass water calcs).

 

We have two young kids who tend to take longish showers unless you continually hound them out of it.  My wife and I both shower at least once a day, and what with sport, gym, etc, I'd say we average 5-6 showers a day between us, of anything between 4 and 10 mins each.

 

We do have shower waste water recovery, and I'm convinced that helps a lot.

 

One thing you could consider is plumbing in the basics for a second cylinder and seeing how you get on with just the one.  If you make the room now and put the main pipework in place, it won't cost much, and you'll avoid outlaying for something you may never use.

 

My inlaws have 2 x 250L in their 5 bed house with just the two of them(!)  They basically turn one off unless they have quite a few guests.  Personally I think they could do with just one by leaving the immersion running constantly on the odd weekend when they have guests.  You might consider something similar as well, especially if your peak needs are going to only be occasional (might depend on how often you actually use the baths) .  Bear in mind that although the occasional immersion usage seems wasteful, you have half the standing losses with a single tank.

 

1 hour ago, ProDave said:

"You haven't seen the shower I want in the new house".......

 

To be fair to her, she hasn't seen the restrictor you're going to hide in the plumbing to said shower! 

[pauldoc]

Thank you all for your replies. I agree with what you guys are all saying. My plumber has said a number of times that you 'should' have a system that can cope with a peak load, i.e lots of showers and baths or for future owners of the house. I really don't think having in excess of 300L is needed. And it's really just the occasional bath. Also as its gas the recovery is pretty quick. Our existing 150L takes about 20 mins so not the end of the world.

Jack I think I'll run your suggestion by the plumber. I really don't see how our usage as a family will rise that much. 

While on the plumbing topic he has also mentioned that we may need 2 boilers! 

Does anyone know how I can work out what size boiler I need? Again I can't see that being necessary but would rather be armed with the right info before I discuss it with him

[PeterW]

Boiler size is directly related to heat loss so if you want to calculate that then you need the heat losses for the house first. 

 

Hot water storage is just a big buffer - if you use an electric shower as an example, you need circa 9kw to get a decent flow on a shower at 12l/m - at a blended temperature you could say you have 9 l/m of hot water so a 300 litre tank will hold roughly 30 mins of water. A decent gas boiler will be pushing upwards of 20kw into the tank so you should be able to run 2 decent showers pretty much without a challenge. It's this reason why low input sources such as ASHP need larger tanks as the recovery is not as quick as with gas or oil.  

[pauldoc]

How do I calculate heat loss... I've gone for the best levels of insulation I could afford. Not the levels of some on here but 160mm celotex in walls and the same in the roof (vaulted ceilings rooms in roof) triple glazing. The downstairs is ufh and rads upstairs. It's approximately 425 m2.  Is there a website or app that does the calcs? Sorry for all the questions! 

[joe90]

Rather than run an immersion all day when guests arrive I plan an inline 9 or 12 Kw electric heater to cope with excess demand. It seems a shame to heat hot water than may not be used.

[tonyshouse]

But then in winter the heat should only escape into the house anyway, this applies to all cylinder so called heat losses in winter and the heat again worried about far to much that escapees from hot pipes too.

[joe90]

Perhaps inline heater in the summer and immersion in the winter?.

[SteamyTea]

The thing with heat losses from DHW cylinders is that they are uncontrolled.  So you may well end up with too much heat, and the associated temperature rise where you don't want it, or where it contributes nothing to the overall house temperature.  Te worse case is that you may have to pump some air out the house to cool it.

 

There is also the question 'What are you wanting to achieve with your DHW system?'

Do you want permanent hot water

Do you want lowest running costs

Do you want lowest capital expenditure

Do you want the simplest system

 

I am sure there are other things, but you get the idea.

[MikeSharp01]

22 hours ago, PeterW said:

Boiler size is directly related to heat loss so if you want to calculate that then you need the heat losses for the house first.

Not always that directly - the old DHW / space heating question again. We, most of us, have been around the dwellings on this, our passive house design model shows we will need around 2Kw of heat, worst case - peak in a typical winter and max of 3Kw once every 10 years. BUT following a conversation elsewhere with @Nickfromwales we are going, as things stand, for a 35Kw Combi because we want to run two simultaneous showers at 10+ L/min  and don't want the standing losses from a tank which @JSHarris has shown, and is discussed above, can be a problem in the well insulated home. The conversations above make the same point in that heating a well insulated home consumes a lot less energy than the DHW demand if we are all showering frequently as has become the norm or at the very least demands the ability to expend energy at a hi rate to recover hot water in standing volumes quickly in readiness for the next hose down! This is a classic 'no one right answer' problem - just an optimal one for each according to their circumstances....

[joe90]

ST may say I am misquoting him but I am sure he reduced his cylinder loss to nearly that of the sunamp unit! Tony says that heat loss from the cylinder in winter only helps heat the house and if that is well insulated it's not really a loss. I intend putting my tank in a well insulated cupboard so I have an "airing cupboard" ( as her indoors likes warm aired towels). I am tending to think 250-300 litres will be sufficient for our needs ( but will continue to read here.

[AliG]

Hi Paul,

 

JS Harris on here posted a heat loss calculator on eBuild, I don't know if it is here, but from what you say your house could well be approaching passive house levels of insulation, it depends on your air tightness, the insulation levels sound pretty high You'll barely need any heating at all.

 

The plumber is talking absolute nonsense on the two boilers. A lot of trades just aren't used to the amount of heating required in a modern well insulated house, you might only need 5kw of heat input if I had to guess. I am building a house twice the size of yours with a pool and will only have one 40kw boiler.

 

As Mike say, once your heat loss gets really low, actually how fast you want to heat your water starts to drive the size of your boiler. Your UVC will probably be ale to take 30kw of heat input and I would think this is the size of boiler you need. Anything else is overkill.

 

We have a 250l UVC at the moment and never run out of hot water, except when people stay. There are three of us. If you have people stay a lot and lots of people try to get up at 5am to catch a flight or something like that, running out of hot water is a pain. The real killer is if someone has lots of baths, a 150l bath fill plus two showers could drain a 200l tank.

 

I would say a 300l UVC is the right size for the house you describe. Is the suggestion of 2x200l so you can turn off one sometimes? 300l will have slightly higher standing losses than 200l, but the cost of an extra cylinder and extra installation costs would dwarf this. Indeed a 300l cylinder only costs around £100 more than a 200l cylinder and fitting costs are almost as high as the cost of a cylinder. Also two cylinders will take up a lot more room.

 

In fact, sorry to say this, but the more I think about it I wonder if your plumber is at it, he is suggesting a system with 2 boilers and 2 cylinders which is totally unnecessary and will cost around £4000 more than it should.

 

 

[CC45]

55 minutes ago, MikeSharp01 said:

Not always that directly - the old DHW / space heating question again. We, most of us, have been around the dwellings on this, our passive house design model shows we will need around 2Kw of heat, worst case - peak in a typical winter and max of 3Kw once every 10 years. BUT following a conversation elsewhere with @Nickfromwales we are going, as things stand, for a 35Kw Combi because we want to run two simultaneous showers at 10+ L/min  and don't want the standing losses from a tank which @JSHarris has shown, and is discussed above, can be a problem in the well insulated home. The conversations above make the same point in that heating a well insulated home consumes a lot less energy than the DHW demand if we are all showering frequently as has become the norm or at the very least demands the ability to expend energy at a hi rate to recover hot water in standing volumes quickly in readiness for the next hose down! This is a classic 'no one right answer' problem - just an optimal one for each according to their circumstances....

 

 

Hi Mike,

 

How much are you expecting that 35kw combi to be?  We are in a similar position to you.  Fitted 3 combi boilers and they are straightforward to do.

 

CC

[MikeSharp01]

Hopefully around £1200 + VAT (New build so will get that back) the unit is THIS one suggested by @Nickfromwales. It does have some standing losses because it has a very small heat buffer in it but I think, if I read the blurb correctly, you can configure it not to use the buffer tank if you are not going to experience high demand.

[Nickfromwales]

1 hour ago, MikeSharp01 said:

Hopefully around £1200 + VAT (New build so will get that back) the unit is THIS one suggested by @Nickfromwales. It does have some standing losses because it has a very small heat buffer in it but I think, if I read the blurb correctly, you can configure it not to use the buffer tank if you are not going to experience high demand.

When you switch off the preheat tanks you then revert to the DHW performance of a straight 38kw combi . The boiler is a Vaillant 938 ( 38kw ) so not 35kw as mentioned. 

You wont get premium DHW performance without the preheat so consider the annoyance of anticipating when youll have to activate / deactivate it according to demand. 

Also best to remember that all these devices ( combis / uvc's / TS's / instant inline heaters ) are ALL fully cold mains dependant. E.g. Crap water pressure / flow in = crap performance, regardless, so make sure you design a system based on that as well as the devices ability to provide X litres per min DHW. 

Regarding the plumbers suggestion of two boilers.....utter tosh. I would SERIOUSLY reconsider moving forward with a plumber that so poorly understands your remit. 

[Guest Alphonsox]

11 hours ago, AliG said:

JS Harris on here posted a heat loss calculator on eBuild, I don't know if it is here, but from what you say your house could well be approaching passive house levels of insulation, it depends on your air tightness, the insulation levels sound pretty high You'll barely need any heating at all.

 

The latest version can be found in this thread

 

[tonyshouse]

The bigger the cylinder is the larger the losses will be but proportionately the the larger the cylinder the lower the proportion of heat lost will be. 

 

The smallest cylinder that you can manage with should be the one togo for. Recharge time could reduce the volume of stored hot water that you need as some can be made while using it, the furthest end of this argument finishes uowith zero storage and a combi boiler.

[MikeSharp01]

9 hours ago, Nickfromwales said:

so not 35kw

Sorry Nick - Typo.

 

9 hours ago, Nickfromwales said:

When you switch off the preheat tanks you then revert to the DHW performance of a straight 38kw combi . The boiler is a Vaillant 938 ( 38kw ) so not 35kw as mentioned. 

You wont get premium DHW performance without the preheat so consider the annoyance of anticipating when youll have to activate / deactivate it according to demand. 

Yes this is a concern but I will do the sums on the heat loss, its not a huge mass but its hot - kept hot and not that brilliantly insulated. I think I was working on the basis that we would switch them off in the summer when showers can be a bit less sizzling and we don't want the heat loss warming the building. My only worry with the system as it stands is getting it to tick over at a low enough level to run the UFH and / or Air battery in the MVHR.

 

 

9 hours ago, Nickfromwales said:

Also best to remember that all these devices ( combis / uvc's / TS's / instant inline heaters ) are ALL fully cold mains dependant. E.g. Crap water pressure / flow in = crap performance, regardless, so make sure you design a system based on that as well as the devices ability to provide X litres per min DHW.

We have VG mains pressure and flow, I will do a full test with a direct connection to the meter, when we dig it up, later in the build but a single hose delivers 24L/min from an outside tap and throws it 12 meters so I suspect full bore will be fine for our needs.

 

[tonyshouse]

The bigger the cylinder is the larger the losses will be but proportionately the the larger the cylinder the lower the proportion of heat lost will be. 

 

The smallest cylinder that you can manage with should be the one togo for. Recharge time could reduce the volume of stored hot water that you need as some can be made while using it, the furthest end of this argument finishes uowith zero storage and a combi boiler.

[pauldoc]

On 10/12/2016 at 23:11, AliG said:

Hi Paul,

 

JS Harris on here posted a heat loss calculator on eBuild, I don't know if it is here, but from what you say your house could well be approaching passive house levels of insulation, it depends on your air tightness, the insulation levels sound pretty high You'll barely need any heating at all.

 

The plumber is talking absolute nonsense on the two boilers. A lot of trades just aren't used to the amount of heating required in a modern well insulated house, you might only need 5kw of heat input if I had to guess. I am building a house twice the size of yours with a pool and will only have one 40kw boiler.

 

As Mike say, once your heat loss gets really low, actually how fast you want to heat your water starts to drive the size of your boiler. Your UVC will probably be ale to take 30kw of heat input and I would think this is the size of boiler you need. Anything else is overkill.

 

We have a 250l UVC at the moment and never run out of hot water, except when people stay. There are three of us. If you have people stay a lot and lots of people try to get up at 5am to catch a flight or something like that, running out of hot water is a pain. The real killer is if someone has lots of baths, a 150l bath fill plus two showers could drain a 200l tank.

 

I would say a 300l UVC is the right size for the house you describe. Is the suggestion of 2x200l so you can turn off one sometimes? 300l will have slightly higher standing losses than 200l, but the cost of an extra cylinder and extra installation costs would dwarf this. Indeed a 300l cylinder only costs around £100 more than a 200l cylinder and fitting costs are almost as high as the cost of a cylinder. Also two cylinders will take up a lot more room.

 

In fact, sorry to say this, but the more I think about it I wonder if your plumber is at it, he is suggesting a system with 2 boilers and 2 cylinders which is totally unnecessary and will cost around £4000 more than it should.

 

 

Thanks all of you for the feedback and info. It's reassuring to know I'm not off in my thinking. In answer to a couple of the points made,  I do have fairly decent levels of insulation as I previously mentioned. However I haven't been overly zealous with air tightness (although done my best) and also don't have mvhr so not sure how much this will matter? 

I also think the plumber is working of old calculations as you all think . The online calculators that I have seen don't really take into account modern insulation levels and glazing. I guess the scary thing is over riding him and then finding the system isn't up to the job! 

I think I will tell him I want a second opinion if he comes back with the 2 boiler / cylinder scenario. Also in answer to your question about 2x200 cylinders,  he said they both run at the same time and can't just run one, is that also incorrect? 

Thanks again chaps

[PeterW]

55 minutes ago, pauldoc said:

I think I will tell him I want a second opinion if he comes back with the 2 boiler / cylinder scenario. Also in answer to your question about 2x200 cylinders,  he said they both run at the same time and can't just run one, is that also incorrect? 

 

Potentially..! If he is quoting a pair then he's either running them in series - so basically cold through the first into the second - or has a plan for parallel which will be a b!tch to balance properly ..! 

 

FWIW Telford do a 400 litre UVC and it's not expensive ... you just need a 40 litre expansion vessel which you would need with your current proposal anyway. 

[le-cerveau]

I think it has all been said but if you think of the basic physics.  Heat loss is via the external surface area and 2 x 200L cylinders will have a far larger surface area than 1 x 400L cylinder so your heat emitter is far greater, before you look at the insulation levels etc.  You are doubling up on penetrations into cylinders, more heat loss points and cost to install and maintain, it makes little sense to just put in 2 same size cylinders (unless you are running one as a low temp pre-heat fed from a low grade heat source like an ASHP).  Go with the single bigger cylinder and then go for the minimum you can work with (that is a long complex calculation).

 

As others have said I suspect your plumber has a ready supply of 200L cylinders and smaller boilers he is trying to use!  Just work out the cylinder size you require (not what he has) and the boiler power needed (reheat speed) and go from there, however cylinder size and boiler power are linked as a more powerful boiler will mean you can get away with a smaller cylinder for your needs, so you then have to look at the lowest boiler draw (Heating with no DHW) and can your selected boiler modulate down to that?

[pauldoc]

Just to be clear it is a gas boiler I am having. Also the boiler and cylinders will be in a plant room next to the garage so any heat loss won't really affect the rest of the house. It's a complicated business this housebuilding lark! Will I ever sleep through the night again?!! ?