Vaillant 438 with 3 zones, frequent S53

[Dean744PiN5]

the first number is actually the diameter of the port hole on the pump,
so
15/50= 15mm port hole and 5 meter head
25/55= 25mm port hole and 5.5 meter head

the larger the diameter of the port the higher the flow rate through the pump.
Simples

 

[Onetap]

the heat exchanger taking amost all the head from the 15/50 and then there is 24 rads, I dont think w/c is going to overcome undersized pump/s in this setup.

The pump sizing is a separate issue which weather compensation isn't intended to deal with. Installing a constant-flow primary circuit with variable flow secondary circuits will raise the temperature of the return and reduce the boiler efficiency for most of the heating season and is probably unnecessarily complicated.

 

[picasso]

The op is looking for a quick/cheap fix so I reckon llh or w/c is a moot point.

 

[Agile]

Installing a constant-flow primary circuit with variable flow secondary circuits will raise the temperature of the return and reduce the boiler efficiency for most of the heating season and is probably unnecessarily complicated.

I also take this view and you are the first that I have read who also rather agree with my thoughts!

 

[fezster]

So would it be fair to say the 26/50 is better for flow than the old 15/60 was? Just trying to satisfy myself I didn't waste £300 on this pump, though it sounds like a 25/80 may have been a better choice.

Given the d/s with a second pump (15/60) is ok, is the cheapest fix here perhaps to add a second pump to the u/s or to move the second pump to the return to boost flow on both circuits?

What is the best way to range rate this boiler to cater for each zone independently and all at the same time, or is that just a case of trial and error and perhaps never getting a perfect balance?

Finally, I know the pipe layout of my entire install. Would you be able to provide me some rough numbers to work out the resistance and thus pump needed, or point me to some literature to work it out?

Thank you all for taking the time to respond.

 

[scoobydo123]

Can you re-cap when you have a relatively large system calling for heat on all zones everything is fine for the first fire from cold? But it is when zones close and eventually the boiler cycles it then probably refires after waiting a period of time it then fires at an inflated rate and short cycling occurs. This is particulary prevalent this time of year when only hw is calling for heat. What is your pump overrun timer set to? I would also set the anticycle timer to 60 to see if you can get bit more heat away after a cycle in conjunction with an inflated pump overrun.

In answer to the question about will a vr470 enable different KW rating between CH and DHW generation the answer to this is yes all features however are limited to using the unistor unvented but the NTC for the cyclinder that comes in the VR65 pack will need pitting on the megaflo with a change to the wiring to insure all safety features are working correctly.

Yes the revised PCB has a positive impact to simple on off controls.

 

[hazetimesfive]

In laymans terms, could you explain at a high level the difference between the following models:

26-50
25-80
25-100
25-120

15/60 = 150mm (end to end)/6 meter head
26/50 = 260mm/5m head (guess it's a 25/55?)
25/80 = 250mm/8 m head
25/100 = 250mm/10 m head
etc etc.

Guess you must be altering pipework a fair bit.

OP asked for laymen terms, writing a full page essay would be tiresome.

err, its just plain wrong thats all.

 

[fezster]

Can you re-cap when you have a relatively large system calling for heat on all zones everything is fine for the first fire from cold? But it is when zones close and eventually the boiler cycles it then probably refires after waiting a period of time it then fires at an inflated rate and short cycling occurs. This is particulary prevalent this time of year when only hw is calling for heat.

The d/s zone works ok. This has a secondary pump.

The h/w zone's return temperature rises too quickly. This means on first fire it runs for a fairly long time, but then the flow / return temperature differential is too small, and it repeatedly short fires until the cylinder stat is satisfied. I think this worked ok (or better than present) when I had an extra rad on the h/w zone, so I'll be adding this back in soon.

The u/s zone's return temperature rises too slowly. It has no secondary pump. This results in s53's when firing from cold, but once the return temperature gets to around 40, the boiler fires normally.

I believe that if all three zones are open, then the high return temp of the h/w zone and low return temp of the u/s zone kind of balance out and the boiler fires normally, albeit the u/s zone takes a lot longer than d/s to heat up.

 

[D_Hailsham]

hex resistance on the 38 is around 3.5 m head which leaves around 1.5 metre for pipework losses/resistance.

It's actually 4.05m at 38KW with a 20C differential. If the system is running with an 11C differential the resistance is way higher.

However the OP says that the boiler has been range rated down to 26kW, which is a flow of approx 1200 litres/min at 20C differential, giving a head of just under 2m. It's still off the scale if running with an 11C differential.

 

[fezster]

Sorry, bit of a tangent - but how is the pressure of a sealed system determined ? My original installer told me 1.5 bar, but reading earlier that a higher pressure means more resistance got me thinking whether reducing the system pressure could help with flow. Obviously, I dont want to attempt this until I know it's safe to do so. How is the system pressure calculated, and is it possible 1.5 bar is too high ?

 

[D_Hailsham]

reading earlier that a higher pressure means more resistance

Where did you read that?

 

[Agile]

The h/w zone's return temperature rises too quickly. This means on first fire it runs for a fairly long time, but then the flow / return temperature differential is too small, and it repeatedly short fires until the cylinder stat is satisfied. I think this worked ok (or better than present) when I had an extra rad on the h/w zone, so I'll be adding this back in soon.

I would aim to correct that by reducing the flow rate through the cylinder coil.

However, that in itself can create other problems because the flow through the boiler can become too low and require an auto bypass to open which would increase the return temperature at the boiler.

To correct the situation the boiler max power output would need to be reduced ( which would need a control which dynamically varies the max power ).

But in your case with such a powerful boiler you are probably up against the absorbtion of the heating coil being less than the minimum output power of the boiler and particularly when then cylinder water is nearly at the set temperature. Such are the problems of an oversized boiler!

Tony

 

[fezster]

But in your case with such a powerful boiler you are probably up against the absorbtion of the heating coil being less than the minimum output power of the boiler and particularly when then cylinder water is nearly at the set temperature. Such are the problems of an oversized boiler!

Tony

Correct, if I reduce the flow through the coil, the auto bypass kicks in, raising the return temperature.

Why do you say the boiler is oversized ? I know from reading (many, many past posts!) that your argument for an oversized boiler is often a valid one, but in this case, given my heat loss calc is 34kw and the total kw rating of all 20+ radiators is around 40kw ? I believe my boiler should need to run at full output, but because of the zone arrangements, I am having to range rate it lower to achieve the right balance of being able to run zones independently or all together.

 

[fezster]

reading earlier that a higher pressure means more resistance

Where did you read that?

I thought I read it earlier in this thread, but maybe in another post. Cant find the reference now.

I'm still intrigued though as to how the system's pressure is determined, and how my original installer came to the conclusion it needed to be pressurised to 1.5 bar ?

 

[Agile]

Why do you say the boiler is oversized ? I know from reading (many, many past posts!) that your argument for an oversized boiler is often a valid one, but in this case, given my heat loss calc is 34kw and the total kw rating of all 20+ radiators is around 40kw ? I believe my boiler should need to run at full output, but because of the zone arrangements, I am having to range rate it lower to achieve the right balance of being able to run zones independently or all together.

I think your boiler is probably oversized because:-

1. You don't know if, or if so what figure, an allowance was made for water heating was used if the heat loss calculation. The recommendation is that just 2 kW is used ( but many installers use far higher figures! )

2. 40 kw rad output would be very high for about 20 rads, most average about 1.0 - 1.5 kW but obviously I have never seen your house!

3. You have already said that its currently set at just 26 kW. If that's adequate for heating your house then a 38 kW boiler is grossly oversized when a 28 kW boiler would suffice! There is NO advantage oversizing boilers and it just reduces efficiency and causes problematic short cycling!

Tony