Boiler flow rate below minimum but no cycling

[petegj]

Is this OK or am I doing damage to my boiler somehow? The set up is:

• British gas 330+ (min flow rate of 600l/ph quoted for 10kW lowest modulation)
• Grundfos 15-60 130 super selectric pump (on lowest setting, max 2.2 metres head)
• Flow temperature - 71c
• Return flow - 50c
• 8 radiators
• 3 bed detached
• Have balanced the system - all radiators get up to temperature at the same rate with roughly the same temperature drop

I have been trying to estimate the flow I'm achieving from looking at the pump curve, but to do this I understand I need to know the system's total pressure loss. The boiler manual states that it has 0.5 metres pressure loss at the min. 600 l/ph, but I don't know how to estimate the rest of the system's resistance. Is there a rule of thumb I could use?

If I instead work backwards from the temperature difference and the gas used - a 21c rise with approximately 11kW gas going in (according to smart meter), the flow rate is 450 l/ph (450/60 mins*21 c temp rise/100*7). That's well below the 600l/ph minimum rate specified for the lowest modulation possible according to the installation manual, so my questions are:

1) Is the boiler still condensing? It sounds as though it wouldn't need to based on the heat output - is heat being wasted somehow?
2) Is exceeding the minimum flow rate causing damage to the boiler?

As I say, there's no cycling, the boiler just burbles away on what I presume is it's the lowest setting. The downside is that the system takes rather a long time to get up to temperature, but moving the pump speed up a notch reduces my return flow temperatures to 55+. Lowering the boiler temperature to compensate not an option as the house fails to warm up.

Big thanks to everyone here for sharing their expertise - I have learnt quite a lot over the last couple of weeks trying to optimise my system.

 

[Hot&Cold]

When is the boiler going to be serviced ?

 

[petegj]

Wasn't planning on having it serviced for at least a year

 

[ScottishGasMan]

You shouldnt be getting a 21*c rise on the lowest rate.

The 20*C rise would normally be looked for when its on its highest rate (and only on heating up not ticking over), so yes I would agree that your getting no where near the flow rate through the appliance that it was built for if your above that on minimum rate.

You say increasing the pump speed drops the return temperature, what effect does it then have on the flow temperature?

Its taking a long time to heat the system up as youre not moving enough water through the boiler to take the heat out, have you worked out what the heat loss is in the house? (as a rough approximation how many radiators are you heating?)

Condensing comes into question depending on return/flue gas temperatures. If your house doesn't heat up with it set at a lower flow temperature then unfortunately there will have to be a compromise of it not condensing as much as it could do, but that doesn't mean its not efficient.

The correct question as you asked is it damaging the boiler. in short yes, it may be putting excessive heat strain (thermal shock) on the main heat exchanger, thats why manufactureres stat a minimum flow rate. On condensing boilers like this they also do not light on minimum rate, they will fire around 60% of the maximum 18-20kw in this case which for a short period may cause a more notable difference between flow and return temperatures.

Is the boiler set to run at 10kw all the time? or is it trying to run a higher rate then dropping down? Have you tried opening the automatic bypass valve a bit to allow more water to flow through the appliance?

 

[Dan Robinson]

Oh dear.... A massively oversized boiler installed in a house with an occupier with a little bit of knowledge and far too much time on his hands...

Wasn't planning on having it serviced for at least a year

Yip

 

[dilalio]

(according to smart meter)

And therein lies the instigator.
Thousands (becoming millions) of home owners with theses devices, trying to beat the clock rather than live in comfort levels they desire from their heating system.

 

[petegj]

Hi ScottishGasMan,

Thanks for your help. I'm surprised that the unit doesn't just shut down when there's a risk of thermal shock. Anyway, I've increased the pump speed to medium and lowered the temperature to 65. I'm now getting 65 out and 51 return. Still looks like it has modulated to it's lowest firing rate though.

The boiler is limited to 18kW. It does indeed usually start off firing at a higher rate but then quickly settles down.

Bypass valve is set to just below maximum head for the pump on medium, which is 4.5m. I don't really want this open unless it needs to be as it reduces efficiency.

...with a little bit of knowledge and far too much time on his hands...

Err.... you are the one with 55k posts under your belt! Happy trolling

 

[Dan Robinson]

I'm right though aren't I?

 

[Notch7]

Err.... you are the one with 55k posts under your belt! Happy trolling

But trolls arent known for getting over 9,000 thanks........

 

[petegj]

I'm right though aren't I?

About the knowledge, sadly not the time!

Can you help and explain thermal shock to me. I would have thought that low firing rate with a low flow rate is equivalent in terms of temperature rise to high gas rate with a high flow rate. Why would this cause damage to the heat exchanger?

 

[bernardgreen]

But trolls arent known for getting over 9,000 thanks........

Thanks that are received for posts in the combustion chamber should not appear in the public areas.

 

[durhamplumber]

Thanks that are received for posts in the combustion chamber should not appear in the public areas.

Why not?

 

[bernardgreen]

Why not?

To be blunt .... mates hand out "thanks" like candy to their mates in the CC.

Thanks that have been earned by actually helping members of the public are the only thanks that should appear in the public areas of the forums.

 

[Gasguru]

But to be fair we rarely get thanked by the public on here...in the CC we appreciate each others knowledge a whole lot more.