Flow temperature on Greenstar junior

[vansbest]

Hi, I had a Worcester Bosch Greenstar Junior 24i installed 5/6 months back but the boiler has to be set to max to get enough heat around the house just now. I checked the temperature of the flow pipe from the boiler, at the boiler, and the highest it gets is 66/67 degrees. return is 10/11 degrees below this. Does the flow temp seem a bit low?.
thanks

 

[Dan Robinson]

A bit.

But e need more information about the building.


These Juniors are a bit on the pants side of things, but should manage to heat the house.

The most important question is "are you warm enough?"

 

[vansbest]

Hi, thanks for the reply, House is a small(ish) 3 bedroom mid- terraced house with Hall, stair and landing, lounge, kitchen diner, bathroom. Loft fully insulated, EWI insulation professionally installed, fully double glazed.

 

[Dean744PiN5]

what did you use to measure the temperature?

 

[Agile]

A very simple test of rad temp is how long you can keep your hand on the top of a rad!

At 60 C its a few seconds, at 70 C its about 1/2 second and at 80 C its hardly ant time at all!

Condensing boilers are designed to work best at a flow temperature of 70 C or less. But in very cold weather and if the rads are a little on the small side for a flow of 70 C then its quite acceptable to need to turn to higher than 70 C and most boilers will give around 80 C.

Tony

 

[D_Hailsham]

Hi, I had a Worcester Bosch Greenstar Junior 24i installed 5/6 months back but the boiler has to be set to max to get enough heat around the house just now.

Hasn't it been very cold up Scotland recently (-15C)?

I checked the temperature of the flow pipe from the boiler, at the boiler, and the highest it gets is 66/67 degrees. return is 10/11 degrees below this. Does the flow temp seem a bit low?

The installer may have deliberately oversized the rads so they give out the correct amount of heat when running at 66/67C. For example:

if a 24kW boiler is connected rads which add up to a nominal 30kW, they will give out 24kw (as that's the most the boiler can supply). But the flow and return temperatures will be 67C and 57C, even if the boiler stat is set to 80C. It's called thermal equilibrium.

 

[Agile]

if a 24kW boiler is connected rads which add up to a nominal 30kW, they will give out 24kw (as that's the most the boiler can supply). But the flow and return temperatures will be 67C and 57C, even if the boiler stat is set to 80C. It's called thermal equilibrium.

Would you like to explain that?

He says its a smallish 3-bed house which is well insulated. The heat requirement would typically be only about 10 kW at -1 C outside. That would be well within the power output the boiler is capable of.

In my view something is very wrong if that temperature is correct. The installer should be asked about it and if needed Worcester contacted.

It is not for us to suggest causes but a very simple test to give an idea if there is any problem with the gas supply to the boiler is to check that the gas supply pipe is 22 mm right up to just before the boiler. Also that a gas flame on a cooker does not change size if a hot water tap is turned on while heating is off.

However, I am well aware that some boilers do NOT give out the maximum flow temperature when the power required is close to their maximum.

Tony

 

[Gasguru]

Gas rate the boiler...see the FAQs.

Also consider whether the installer has installed the Low NOx code plug since that will limit the heating output to 13 Kw.

 

[Agile]

Thats a good point.

Although even 13 kW should still be quite adequate in a smaller well insulated house.

Tony

 

[Gasguru]

I had 10Kw in a one bed flat I once owned...it could barely cope when it was snowing outside...there was practically no insulation and single glazed windows.

 

[Agile]

Yes, but this house is well insulated!

 

[D_Hailsham]

if a 24kW boiler is connected rads which add up to a nominal 30kW, they will give out 24kw (as that's the most the boiler can supply). But the flow and return temperatures will be 67C and 57C, even if the boiler stat is set to 80C. It's called thermal equilibrium.

Would you like to explain that?

The output of a boiler is determined by the flow rate and the temperature differential: kW = litres/sec x ΔT X 4.18. Note that the actual temperatures are not relevant, so a boiler running at 80/70 will have the same output as one running at 60/50 provided the flow rate is the same. (I'm ignoring the increased output due to condensing). The flow rate for a 24kw boiler with a 10C differential is 34.45 litres/min.

If you connect a 24kW boiler to 24kW of rads and let it run without any temperature control it will heat up until the rad output exactly matches the boiler input.

The output of a radiator is determined by room, flow and return temperatures, which are:room temp = 20C; flow = 75C; return = 65C (i.e 10C differential).

Now connect 30kW of rads to the same 24kW boiler. The flow rate is the same as the pump has not been changed, so the boiler cannot produce more than 24kW. The question then is: at what temperature will the 30kW of rads produce 24kW of heat when the flow rate is 34.45 litres/min?

The answer can be found by referring to the temperature table, which can be found in any rad manufacturer's catalogue. This give the correction factor which must be applied when the temperature difference between rad mean temp and room temp is not 50C [((75+65)/2) -20].

The required correct factor for 30kW rads and a 24kW boiler is 24/30 = 0.8. The Stelrad table only shows factors in 5C steps: 40C = 0.748; 45C = 0.872. If we assume a linear relationship, 0.8 = 42C. So the mean rad temp is 20+42 = 62C, or 67C flow and 57C return.

 

[Agile]

Quite so.

But thats making an assumption of 30 kW of rads.

This is a smallish 3-bed mid terraced house with a heat load of perhaps 8-9 kW which is very small for a 24 kW boiler.

In that case the boiler would not be reaching any equilibrium caused by 24 kW being dissipated by the rads.

Much more likely the boiler output is limited to a too low value.

Tony

 

[Agile]

Quite so.

But thats making an assumption of 30 kW of rads.

This is a smallish 3-bed mid terraced house with a heat load of perhaps 8-9 kW which is very small for a 24 kW boiler.

In that case the boiler would not be reaching any equilibrium caused by 24 kW being dissipated by the rads.

Much more likely the boiler output is limited to a too low value.

Tony

 

[vansbest]

Hi, many thanks for all your responses. I will check the boiler flow rate to-morrow but can you explain what you mean by, "Much more likely the boiler output is limited to a too low value".
Thanks