Unvented how good? Combi how bad? H E L P !

[batsran]

Hello,

We live in a 3 bed detached house - soon to be a 4 bed. The existing ten rads will probably increase to 15 rads. There are two baths and three showers. Our existing gravity fed system with a roof tank and cylinder in the airing cupboard need to go due to the airing cupboard being removed (and the fact the 30yr old glowworm boiler died about a month ago). As far as I can see there are two main options. One is install a combi, the other to install a mega flow (or similar system in the disused garage).

We have a 15mm water main coming in with a flow rate of 14 ltr/min and standing pressure of 4 bar. We've also got mains gas.

Now my concern is that a combi would vary and struggle to maintain supply to have a decent shower if someone else flushed the loo or started the dishwasher/washing machine. What has thrown me a bit is that one plumber has said that an unvented system would also struggle since it relies on mains pressure to provide the hot water. This is obviously disturbing since it seems to negate the whole point of the system. Assuming that a decent combi can provide 15-19ltr/min water - then why have a mega flow!

One other plumber has suggested that we should increase the size of our copper 15mm pipe that runs to the street (23 or 27mm I think). When I asked how this would work, i.e. wouldn't there only be a 15mm hole on the stop cock for a pipe leading to the property he seemed a little vague and said that even if that was the case, the extra storage capacity of the large pipe would still benefit us despite a possible 15mm restriction at the street.

Any and all help much appreciated. Currently we have been looking at Worcester range.

 

[TicklyT]

Increasing the bore of the rising main may help, even retaining the original stopcock. The water velocities, and consequently the friction between the water and pipe walls is reduced. The storage capacity within the pipe is immaterial, as water doesn't compress.

Some will suggest an accumulator to supplement poor mains flow rates, but they will require some recovery time once depleted.

A megaflow can work well given good mains pressure and flow rates. They can deliver hot water faster than a combi could heat it, but would eventually need some recovery time to re-heat.

Combis are usually limited by the maximum rate they can heat the water at. The colder the mains water, the lower the flow rate for a given output temperature.

 

[gas4you]

If you go to the bother of replacing your incoming main, then you may as well go for 35mm (I think thats the next size up )

4 bar is good and as the pressure reducing valve would limit this to 3 bar, should give a good balanced supply to the balanced taps in the house.

Increasing pipe size increases flow rate only, it will not help with the pressure.

You really need to be looking at 20 l/min flow rate incoming as a minimum for maximum performance from a Megaflo.

With your sized property I would always recommend an un-vented cylinder as opposed to a large combi, even a storage type.

 

[batsran]

If you go to the bother of replacing your incoming main, then you may as well go for 35mm (I think thats the next size up )

4 bar is good and as the pressure reducing valve would limit this to 3 bar, should give a good balanced supply to the balanced taps in the house.

Increasing pipe size increases flow rate only, it will not help with the pressure.

You really need to be looking at 20 l/min flow rate incoming as a minimum for maximum performance from a Megaflo.

With your sized property I would always recommend an un-vented cylinder as opposed to a large combi, even a storage type.

I'd rather if possible avoid upgrading the water main into the property. I read somewhere else on the forum that a plumber was aware of someone with a similar 15mm feed and an unvented solution that appeared to be working well (interpreted that as meaning the customer was happy) for several years.

If I can be perhaps a little more specific - assuming that a plumber runs 22mm from the mega flow to the bathrooms, would there be any noticeable change in hot water delivery if someone else in the house opened a tap - and is it true that even using the cold (mains) would affect the hot flow?. In simple terms if the tank has the pressure reduced to 3 bar, then hopefully this would mean there would be close to 1 bar that could be used to cold fill the dishwasher/washing machine without having any major difference on the hot feed?

From our current cylinder I measured the hot and cold (attic tank fed) water has 20 l/min into the bath. Could a mega flow match this? or is this where the limiting factor of 14 l/min mains flow comes into play. Apologies if I am being dim here .

Finally - if it transpires that replacing the feed from the street is needed, how is the 35mm pipe fitted to the stop cock?; would the water board need to come out and change it (and presumably charge) or is there some sort of reducing rings fitted as standard to attach the current 15mm copper? The estate was built about 35yrs ago.

 

[Agile]

I must try to make you understand that an open pipe flow rate of 14 li/min is ALL that can be used anywhere in your house!

Boilers dont suddenly dream up their own water flow! It all comes from the 10 li/min dynamic flow rate that you have. Thats for toilet flushing, showers, washing machine, dish washer, garden hose etc.

Unless you just have only one, or just about two, people living there then a combi or unvented is just not possible without upgrading the water supply.

There is nothing wrong with upgrading the water supply, thousands of people do it. With an up market house (?) a proper water supply is expected! Usually costs about £1000 though and can be done by moling so the whole route does not have to be dug up.

Otherwise stick with your loft cistern system perhaps increasing the storage capacity if required. They work fine and if more pressure/flow is required a simple shower pump can be fitted.

Leave combis for one bed flats where they belong!

Tony

 

[mickyg]

From what you've said I would see your options as thus:

1 Best option, upgrade your mains supply pipe to the road, I might be wrong but I think the water board are obliged to upgrade their pipework if its in 15 and you get your bit done up to the boundary line. and then have an unvented cylinder or if desired a highflow combi.
2 Fit unvented cylinder and accumulator(the 4 bar standing pressure should be sufficient to make the most of the accumulator).
3 Stick with what you have, maybe upgrade to larger cylinder and storage tank, and utilise shower pumps if required.
4 Look into the option of fitting a grundfos home booster, sounds like a decent bit of kit but I'm yet to have my own experiences. This should allow the use of a highflow combi.

 

[batsran]

I must try to make you understand that an open pipe flow rate of 14 li/min is ALL that can be used anywhere in your house!

Boilers dont suddenly dream up their own water flow! It all comes from the 10 li/min dynamic flow rate that you have. Thats for toilet flushing, showers, washing machine, dish washer, garden hose etc.

Tony

Hi, thanks for that. The dreaming bit was I didn't understand if unvented systems build up and store water at a higher pressure than the mains feed. Just terms like expansion vessels etc. What is the dynamic flow?, how is it calculated from the 14 l/min flow measured by the water board and how does it affect me?

I am wondering whether to pay for a megaflo (or worcester equiv) solution since I really need central heating before the weather gets any cooler. Then if the we find it not good enough, perhaps we can explore a new main pipe in the new year when the rest of the building works commence (since it will involve digging up the front garden and drive for a new soil pipe anyway.

This is unless there is anything flawed in this plan!

From what you've said I would see your options as thus:

1 Best option, upgrade your mains supply pipe to the road, I might be wrong but I think the water board are obliged to upgrade their pipework if its in 15 and you get your bit done up to the boundary line. and then have an unvented cylinder or if desired a highflow combi.
2 Fit unvented cylinder and accumulator(the 4 bar standing pressure should be sufficient to make the most of the accumulator).
3 Stick with what you have, maybe upgrade to larger cylinder and storage tank, and utilise shower pumps if required.
4 Look into the option of fitting a grundfos home booster, sounds like a decent bit of kit but I'm yet to have my own experiences. This should allow the use of a highflow combi.

Thankyou - it looks like the general opinion would suggest 14l/min @4bar is not enough to support unvented systems. Does megaflo or worcester specify requirements anywhere. I looked, but could not come across it anywhere.

 

[mickyg]

Does megaflo or worcester specify requirements anywhere. I looked, but could not come across it anywhere.

The requirement is irrelevant. They will still "work", but you will get out slightly less than you put in. So if you have 14/min, your megaflo or combi, will give you less than 14 l/min. So although it will work, it will be worse than what you already have. If you want 20l/min from your hot water tap, you need minimum 20 l/min from your cold.

 

[simond]

An unvented cylinder is really no more than a tin bucket. What you put in at the bottom (cold water) displaces hot water out at the top.

So if you have 14 litres per min, that is it for the whole house. No point at all in fitting one at this rate.

Disregard the plumber talking about the stored water in the pipe, he is typical of everything wrong with this industry regarding training and knowledge. If his brains were made of dynamite he wouldn't have enough to blow his head off.

Either upgrade the water main, having found out from your water supplier the likely flow rate improvement. Frankly what washers are fitted is minutae not worth worrying about at this stage.

Or, if you are not happy with the water board, fit an accumulator. Correctly installed and specified, they are better than any domestic water main I've yet to come across.

 

[gas4you]

Hi, thanks for that. The dreaming bit was I didn't understand if unvented systems build up and store water at a higher pressure than the mains feed.

This would need an accumlator fitted to acheive this, Cue Simond

 

[Agile]

This would need an accumlator fitted to acheive this, Cue Simond

Simon came in a minute before his cue!

He is really on the ball today ( or not very busy? )

Tony

 

[simond]

I had the afternoon off to meet my planning specialist!

Just written up the quote I saw this morning on windows remote desktop, it will be on their doormat tomorrow morning (my office staff will post it).

On Friday I won two boiler installs I saw and wrote up on Thursday - what recession?

 

[batsran]

Thanks everyone. I rang the water board (3valleys) and they have said the charge £280 to connect a new water main to the stop tap (that would be replaced as part of the process). They cannot however give a guide on what the flow rate would be other than 'improved'. When I mentioned a 35mm pipe she almost fell off her chair and asked if it was for commercial use or a farm! saying that typically 25mm should be plenty. I guess it depends on how much I can get someone to run the new pipe to the property. Its going to be about 6mtrs I think - and I would hope the builders would dig the trench.

On balance I guess I'm hoping that a new water main would be better than an accumulator - but again this is from a position of ignorance, assuming it would be another device that needs servicing and ultimately replacing in the fullness of time.

Does the accumulator only boost the feed to the mega flow?

 

[Agile]

Its usually cheaper to fit a new supply pipe than an accumulator. That all depends on the distance though.

My Mother had a 1 kM supply pipe and a static pressure about 3 Bar but a flow rate of about 4 li/min !

The 35 mm supply pipe is hardly any more than 25 mm to buy and it will make the most of whatever connection the S-T make to their main. They often refuse to make a connection at more than 25 mm but dont let that bother you.

An accumulator can be connected to serve whatever you want it to! Hardly needed for toilets though.

Tony

 

[stevebamford]

My Mother had a 1 kM supply pipe and a static pressure about 3 Bar but a flow rate of about 4 li/min !

Hey Tony - that must have been a long old garden !

Batsran - I'm by no means an expert (in fact about as far from an expert as you can get!) but when the guy did the flow test did he check that the two stopcocks were both fully open (the one in the road and the one in your house) as if either were partly closed it would restrict the flow?

Also, the tap he used presumably may also restrict the flow (the qualified people here may be able to give pointers on this) - or is this normally done by using an unrestricted pipe ?

My gut feel would be to upgrade the mains pipe for a permanent solution (I'm sure accumulators have their place but a pipe upgrade seems more logical if you can do it)

We've just had our place replumbed. The supply to the unvented was done in 22mm except for one 15mm section under he concrete floor which we will upgrade when we re-do the kitchen. Works fine for us as is but should be even better when we replace that last section of pipe.

One tip though. I'd get moving on the CH install asap. It took us a long time to find someone and as winter is approaching people get busy doing repairs etc and have less time for installs.