Larger pump required ? 25-80 ?

[fezster]

I've been scouring these forums for quite some time, and finally decided that posting my question may yield a better answer to my particular problem.

I have recently had a new central heating system installed which consists of a Vaillant 438 with 250l indirect Megaflo. It is a 3 zone system, with separate upstairs, downstairs and hot water. There are 18 radiators currently installed, rated at 33KW (9 rads upstairs, 9 downstairs). I have range rated the boiler to 30KW due to struggling with S53. This is an aside question, but I am still confused as to how to range rate the boiler to cope with one zone open (eg. upstairs) or all three zones open.

On the whole, the system is working ok, but there is a single radiator upstairs which does not get as hot as the others. Possible one or two others upstairs not as hot as the others either. They are probably the furthest away from the pump. I have a 15/60 pump before the zone valves on setting 3, and have added a second 15/60 pump on the downstairs circuit on setting 2, after a rad downstairs was struggling to warm up also. I've been advised by a couple of RGI's that I should have a single 25-80 pump installed, and that should be sufficient for the whole house. Having made the mistake of spending money on bad advice previously, I thought it'd be prudent to do some research into how to calculate pump size, and found an excellent post here by ChrisR:

//www.diynot.com/forums/viewtopic.php?t=19661&start=0


Im not sure exactly how I work out what my "head" is.

Btw, Ive tried to balance my system over the last couple of weeks, tweaking it here and there, which has resulted in every radiator warming up much better, but still that single radiator upstairs struggles.

My question is, what steps should I take to try and resolve this. Finding someone as knowledgeable as some on this forum to come and advise me what to do has proven impossible. Every RGI ive called seems disinterested in doing any calculations, or justifying why they recommend a 25-80 instead of a 25-55 or 25-100. It doesnt instil much confidence, but having said that, their "gut feel" could prove to be precisely right. The Grundfos 25-80 seems to sell for almost £300, plus I need somebody to remove my second pump, and replace my existing 130mm pump to accomodate the 180mm. Hence I want to be sure this is the right approach.

Thanks for any and all advice.

 

[bengasman]

If the system was recently installed, why don't you get the installer back to finish the job?
What does it say on the benchmark sheet under F&R temperature?

 

[ALEC1]

The trouble is that to work out the head you need to know the pipe sizes and fittings and route to the index circuit which on a retrofit boiler is very tricky, if not impossible.

The best solution really is to install a header to decouple the boiler from the heating circuit...each with its own pump, that for the boiler correctly sized (probably a 15-60) and the one for the heating educated guess work...you probably knew what the old pump size was..assuming that it worked properly... that would be ideal....especially if the old boiler was a cast iron one which would have had almost zero resistence

 

[fezster]

The original installer is less than interested, having been fully paid.

The installation is 99% new. All new pipework, new boiler location. Only a few original pipes have been used (literally one or two - quite why he didnt just change them also, I dont know). I believe it's 28mm out of the boiler, and then 22mm running right down the middle of the house, with 15mm pipes to each radiator, and 15mm drops to the downstairs radiators (concrete floor, so all rads dropped from upstairs).

The "index circuit" Im presuming is the longest run ? This is the downstairs rads, but they heat up properly, presumably because of the second pump. The longest run to the upstairs radiators is the to the one which is not getting completely hot. It's l/s valve is fully open. Is the "head" basically the length of the longest run, or does it depend on the number of bends and the size of pipework also ? If so, how do you calculate it ?

Could you explain to me what a header is, and whats involved in fitting it and decoupling the boiler and heating circuit ?

Given this alternative solution, does that mean to say you believe the 25-80 pump will not work ?

With regards to F/R temperatures, the difference on the vaillant display is roughly 20 - 24 degrees. Eg. 66/44 last night after approx half an hour running.

 

[Agile]

This is an aside question, but I am still confused as to how to range rate the boiler to cope with one zone open (eg. upstairs) or all three zones open.

Thanks for any and all advice.

As far as I am aware there is no solution to the problem of setting the max power out when operating the boiler into individual zones.

The manufacturers will say the boiler software will do what is necessary.

I gather that some recent Vaillant boilers have new improved software which is designed to overcome some of the shortcomings. Others may be able to advise if that includes the 400 series and if so if that includes your model.

Tony

 

[dcawkwell]

Two pumps like you have would be enough to drive about 26 radiators.
My guess would be a balancing problem near the boiler and pumps.
I would suggest a low loss header tapping off for two pumps and then
to the zone valves. If you shut the other radiators off does you last radiator get nice and hot?

Balancing of the system shouldn't be too much of a problem with thermostatic radiator valves. As the rooms heat up the valves should shut down and force water to other radiators. Provided they are set at reasonable valves not fully open at position 5.

Check your hot water tank thermostat is shutting the down when hot and therefore the water will divert to the central heating fully.

 

[fezster]

The second pump is only on the d/s circuit though, and all the rads d/s get piping hot. The u/s rads get hot too (except one), though perhaps not as much as the d/s (subjective). I wonder whether the second pump d/s is forcing less flow u/s, hence would quite like to just have a single pump drive the entire system.

The u/s radiator which doesnt get fully hot doesnt even when all the other rads are piping hot. Ive not tried closing them all down, will do so this weekend. I suspect the water is just not circulating enough through this last rad.

@Agile, thanks that makes sense. Im not sure if I've set my d0 too low. Am I right in saying that if all rads are getting piping hot (with exception of the one mentioned) that d0 is probably set sufficiently ? I.e. if d0 was too low, what would the symptoms be ?

 

[fezster]

Balancing of the system shouldn't be too much of a problem with thermostatic radiator valves. As the rooms heat up the valves should shut down and force water to other radiators. Provided they are set at reasonable valves not fully open at position 5.

Just read this properly, and at the moment all the thermostatic valves are fully open ("Max"). I will try reducing them one notch so that they shut down when radiator is hot - perhaps this will force water to the lukewarm rad.

 

[snb]

what setting is the automatic bypass set at? That is if you have one fitted!

 

[hazetimesfive]

What does it say on the benchmark sheet under F&R temperature?

 

[fezster]

Turned all of the other TRV's to 0 upstairs, and heard a gurgling sound on the luke warm rad. Within seconds, it was piping hot.

Does this mean that:

a) I definitely need a bigger pump to cope with all of the rads at the same time ?

b) The pump I have is sufficient, and this is simply a balancing issue (Ive spent a long time trying to reduce flow to the other rads, but can never leave them hot enough and get this one hot).

 

[fezster]

what setting is the automatic bypass set at? That is if you have one fitted!

When trying to solve my S53, I did have a look for an automatic bypass, as I believe it can be used to reduce the F/R temperature differential. All I have after my pump (before the zone valves) is a PRV (i think thats what it's called). Its got a red knob on it, and it says "remove screw to adjust", and it set to just below 1.5 (which is the pressure my system runs at). I believe this only comes into play in case of excess pressure ?

Currently my F/R return temperature differential is around 20 - 24. If I want to reduce it (to prevent the S53s which sometimes occur), do I open l/s valves or close them ? And do I do so on the rads closest to the boiler or the one's furthest ?

 

[Agile]

I really think you would benefit from a competent engineer!

Tony

 

[fezster]

Fancy coming round?

 

[fezster]

Any other advice ?

Just to be clear, my intention IS to use a competent engineer, but to evaluate just how competent he is by having an idea of what it is I should be expecting. Unfortunately, not all registered "competent" persons are actually that competent.