15 litres per minute - enough for Unvented system?

[malcolmallan]

I am looking at putting in an unvented system but there are issues with water pressure. The water board have told me that there is three bar pressure at the main, so I can only presume that it is my pipe into the property that is restricting it. I get around 15-20l per minute.

I have just had the gas engineer from a large well known company tell me that he can't quote me as the flow rate doesn't meet the minimum that his company will work with. But he did give me a nod and a wink that there are companies that will do this. Although I have three bathrooms there is only myself and two young kids in the property so demand for lots of simultaneous water is low. I am keen to have the unvented system to get rid of header tanks, etc. I don't want to go to the additional expense of a new water main just now (around 2K I believe) but would like to put the system in with a view to upgrading the main later (before the girls become teenagers!)

Is this feasible/sensible???

 

[david17uk]

Thought about getting a combi?

That will get rid of your cylinders and tanks, and will give you mains pressure hot water

Edit: Also, pressure and flow rate are 2 different things. Make sure you're not getting them mixed up

 

[malcolmallan]

Thanks for the reply. Yes, I had a combi in my last place and was not massively impressed by it. When you opened the hot water tap it would run cold as soon as you opened it more than a little and it took 30 minutes literally to run a bath. I heard that an unvented system would sort that. It could be that the combi was undersized and that a beast of a combi would be the answer.

 

[david17uk]

Yea, sounds like it was undersized.

I'm not 100% sure of the flowrates & pressures required for a combi - But I'm sure this will be your best option.

 

[homeplum]

why dont you contact the makers of the unvented cylinders and ask what would be the min pressure to operate on
bob

 

[gas4you]

A 35 KW + combi will give at least 14 l/m hot water. If you have mainly baths the a 37/39 will give around 16 l/m.

 

[tonybhoy]

Last one I fitted it was 2.5bar or at least that was what the PVR that came with the tank was set to. You do need a really decent flow though.

WB Greenstar 42 CDi requires a minimum of 1.9 BAR and about 20 litres a minute,37CDi is about 1.6 BAR and 20 litres and a Greenstar 550 CDi needs about 25litres a minute flowrate.

Each boiler produces 17.1,15.2 & 25 litres per minute respectively.

All above are combi boilers.

BTW, you should be able to get the mains replaced for about £700, there is a firm in Livingston who do it with a 'worm' I believe

I did a boiler changed last Sept and local guy,who I recommend, replaced the mains for £700. Run was about 20mtrs and it was done in 25mm blue poly.

Don't know if he would go to Edinburgh but get in touch with me (details in my profile) and I'll pass on his details if you want.

Tony.

 

[malcolmallan]

Thanks for all the replies. The other slight complication that I should have mentioned earlier is that it is a large house with 14 radiators, some are 2.5-3KW in power so I guess that the combi might not cope with producing heating and water simultaneously? I was looking at a 37-40KW boiler plus store to give me the required power for both.

 

[gas4you]

Combi is either or, not both together

 

[tonybhoy]

A combi will cope with the heating though not WB combis all of them will only output 30KW (at most) to heating but there are other boilers with larger heating outputs however you do really have to consider the cold water main issue otherwise you will be severely limited in whatever you do.

Also the gas supply will no doubt have to upgraded to at least 28MM or even 35MM pipe with that sort of heating load.

 

[david17uk]

I agree with the 2 above posts, however, i think this will probably still work out cheaper and better than an unvented system.

 

[simond]

An unvented system will work at any flow rate, up to the maximum allowed by the valving eg: 55 litres per min, for example.

If you have just 15l/m, any unvented will work at least as well as a combination boiler capable of this flow rate.

The key issue is whether it is worth paying for a large cylinder and a separate boiler to achieve the same performance as what you would achieve from a simple combination boiler.

That is your decision. With an unvented, if you choose to upgrade the water supply later, it has the headroom to reward you with an improved performance.

The reason we in the trade are wary about fitting unvented cylinders to 'slow' water supplies is that the customer reads the manufacturer sales brochure and expects water gushing from all orifices - most don't appreciate all the motive energy comes from the water main. You only get out what you put in.