Central heating design causing cold rad?

[purplemonkey]

hi all, I've recently fitted a new central heating system and all has been great until the latest two radiators, they don't appear to heat up like the others. I've attempted to balance the system but doesn't appear to help much. I'm wondering if maybe my pipe design could be causing the issues, as I'm not a heating engineer and have taken this on myself. I'm just looking for some advice and thoughts on the system I've installed and if the design could be the cause, or maybe what else could be the cause?

See the attached picture

Untitled

• purplemonkey
• 31 Dec 2013
• 1

for a overview of the design in the building, (red flow, green rtn, blue=boiler and rads, black=walls). Radiators 5 & 6 that are being problematic. all the pipework is 15mm, and is combination of plastic and copper. There are two manual air release valves above the main branch for rads 5&6.

The boiler is brand new Valiant 838.

The problem radiators used are http://www.ebay.co.uk/itm/400485619...eName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649 the others are a mix of standard b&q dbl steels, towel rads and a upright designer similar to the ones in the link above but its a single.

All rads have TRV's and as mentioned above I've tried balancing to the point that all rads other then 5&6 now have only a quarter turn on the LockShield. All rads are set to level 5 on the TRV side.

It appears now that rad 6 heats up at the start of the heating system coming on, but then cools over time, so doesn't maintain temperature.

My next stage of investigation is to remove the TRV's off 5&6 to eliminate them, although on bleeding they do fill up fine.

cheers for any assistance. and let me know if you want more info.[/url]

 

[denso13]

Do the problem radiators heat up with the others turned off? Is it 15mm straight from the boiler?

 

[ALEC1]

as a rue of thumb 15mm is only good for about 4kws, and it would be good practice to put only two small rads on 15mm pipe, the rest being 22mm.

the problem is lack of flow caused by small pipes...

 

[purplemonkey]

With all other rads turned off there seems to be little difference. i ave 22 from boiler to end of first room then 15mm from there. I have been wondering if to extend this. it would be straight forward to extend to the start of the big rooms, i.e for 3,4,5&6 rads. extending further would require a larger commitment of work.

I have seen these rad systems with really tiny copper pipes, how do they work if one needs 22mm throughout?

 

[ALEC1]

mico bore does one radiator per pipe..teed off 22mm in a manifold

 

[denso13]

I have been wondering if to extend this. it would be straight forward to extend to the start of the big rooms

Try it and see. You'll have to do it as a minimum anyway. Plastic also has a thicker wall than copper therefore a smaller pipe bore.

 

[ALEC1]

the other thing to consider is pump speed...by default on a vaillant the pump is slow in heating...raise it in diagnostics...

 

[purplemonkey]

the other thing to consider is pump speed...by default on a vaillant the pump is slow in heating...raise it in diagnostics...

I will take a look at this, not sure how to do this yet, but will give it a go.

I will also upgrade the 22mm run to be closer to the large room and this will leave 4 rads on the 15mm from this junction. and the other rads will then be t'd off the main 22mm. so hopefully all that will help.

cheers all.

 

[ALEC1]

it's in diagnostics of the boiler...all explained in the book..

 

[Terry lambert]

as a rue of thumb 15mm is only good for about 4kws, and it would be good practice to put only two small rads on 15mm pipe, the rest being 22mm.

the problem is lack of flow caused by small pipes...

15mm Copper pipe will service 11kw approx for a system design of a 20 delta T ( condensing boiler HE)

6 KW for a 11 Delta T (non condensing SE)

Allow a 10 % margin for pipework/ fittings

15mm plastic will carry 9.2 kw

Edited to show plastic load

 

[Blagard]

With all other rads turned off there seems to be little difference. i ave 22 from boiler to end of first room then 15mm from there. I have been wondering if to extend this. it would be straight forward to extend to the start of the big rooms, i.e for 3,4,5&6 rads. extending further would require a larger commitment of work.

I have seen these rad systems with really tiny copper pipes, how do they work if one needs 22mm throughout?

Make sure you really have got only the two problem rads fully open if no improvement you have a blockage or airlock there somewhere.

The actual amount of heat carried by the pipes varies depending on the flow rate and as a ballpark, I use 10kw for 15mm similar to terrydoh. Higher pump speeds and flow rates lead to higher pipe noise. If you had balanced the system properly and flow rate was a problem, all your rads would be running the same but cooler than before. With such a small system I would suggest you go back and rebalance the whole system leaving the distant rads wide open and shutting down the ones by the boiler.

In particular shut off the towel rail completely. Once you get the rest of the system going just open the valve a fraction to get it hot otherwise it may act like a bypass depending on the valves fitted.

While ALEC1 has mentioned pump speed, it should by default be on auto for your system. It can be overridden and set to a specific speed, so I would try that before increasing any pipework size.

 

[purplemonkey]

thanks for all the tips, I've restricted all other LS to less then 1/4 turn, they are noisy now, but it appears heat distribution is better. next time I drain the system I will upgrade the available pipe work and this should improve the flow to the final 4 based on the KW ability of the 15mm.

cheers all