Top man, micky.
I'll give that a go. If I get a warm house, you're all invited around for a beer !
Rads slow to heat - bigger pump required ?
- Thread starter yoda
- Start date
I'll give that a go. If I get a warm house, you're all invited around for a beer !
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Thanks for clarifying
I thought you might get a shock at the price!
Probably
Your boiler modulates, so it may not be running at the full 38kW. I know it's a bit late now but the Sedbuk Whole House Boiler Calculator will tell you what size boiler you really need.
In addition, you can find out the output of your radiators - assuming they are standard flat panel convector, using the data in Stelrad Elite Catalogue You should multiply the results by 0.85 as the Stelrad data assumes a flow-return temp difference of 11°C and your boiler works with a 20°C difference. This will tell you what your boiler will need to deliver (add 2-3kW for hot water).
If you boiler is actually delivering much less than 38kW you may not need such an expensive pump as the head loss within the boiler varies with the flow, which depends on the required output.
This got posted while I was writing my reply.
This setting reduces the max output from 38kW to 24kW. At 24kW the required flow rate is 1000 litres/hour and the boilers resistance is 1.5 metres. A Grundfos 15-60 can supply 1000 litre/hour against a head of 4.5 metres, which leave 3 metres (4.5-1.5) for the radiator circuit; which should be adequate.
Result - no £1000 pump!
PS if you use the Stelrad data to find the rad output and then add the 2-3kW for hot water, you can then set the boiler output correctly.
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Yes, it modulates between 38kW and 6.3kW.
The problem is that the boiler starts off at 38kW, but the pump cannot provide sufficient flow to remove the heat. So the boiler thermostat cuts the gas and the water cools down. When it starts up again, the boiler again tries to supply 38kW and we are in a vicious circle. Result - the boiler is continually switching on and off and nothing gets hot.
Reducing the boiler's maximum output to what is really needed will solve this problem.
Quite an understatement..
Unfortunately, we are where we are with the boiler. Its all bought and paid for and changing it (unless I really had to) would seem a bit daft..Probably
Your boiler modulates, so it may not be running at the full 38kW.
(and more useful boiler / radiator stuff)
So irrespective of it being rated 50% higher than is maybe needed, I quite like mickyg's idea of downgrading the output of the boiler via the control panel (assuming that was what he was instructing)..
Thanks again for the the help and advice, guys - its very much appreciated..
[/i]In itself, I can see your logic. What baffles me is that a 15-60 can not shift the heat fast enough for the boiler to modulate down. I have seen plenty of those working fine for many years on a non-modulating 25 kW. Internal resistance of the worm must be many times more than normal.
28mm in and out from the boiler and throughout the HW circuit, dropping down to 22mm round both CH loopsfiver says the F&R pipes are in 22mm.
fat chance of shifting 38kw through there
if you want a really easy way of figuring out what output to set your boiler, measure the width of your rads, add them all up and the number in metres is the amount of kw you need.
this works pretty much spot on for 600mm high; different heights is straight conversion
this works pretty much spot on for 600mm high; different heights is straight conversion
To shift 38kW with a 20°C temperature difference requires a flow rate of 1600 litre/hour. The Grundfos 15-60 can deliver this against a total head of 3.5 metres. But the boiler by itself has a head of 4 metres at that flow rate. which is more than the pump can handle. So the flow rate decreases and consequently the heat is going into the boiler faster than water can remove it. The result is that the boiler quickly overheats.
I agree that the resistance of the boiler is very high, but it is exactly the same as the Vaillant ecoTec plus 438, which shows they use the same heat exchanger. Of course the graphs in both boiler installation manuals could be wrong as they are identical.
By contrast, the Baxi Solo 30HE only has a resistance of 1.25 metres at the same flow rate.
That's only true for single panel (K1) convectors. The multiplier for P+, K2 and P1 radiators is different.
true, but 90% percent of rads are k1 and k2. I guess op can work out that 'doubles' count double.
the tiny bit of difference is not important as it won't work under laboratory conditions anyway.
couple of kw under or over won't be a problem; it is only a setting.
the tiny bit of difference is not important as it won't work under laboratory conditions anyway.
couple of kw under or over won't be a problem; it is only a setting.
I agree.
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