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No you do, most don't.
plumber competence reprised
- Thread starter achrn
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No you do, most don't.
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This discussion has been rather missing the point!
The boiler output should be in 28 mm up to the motor valves.
From the motor valves to the cylinder the pipework can ( in my view ) be in 22 mm as that can adequately carry 20 kW.
However, the motor valves offer a significant restriction to the flow.
Even some 28 mm valves have the same internal orifices as their 22 mm valves!
Tony
The boiler output should be in 28 mm up to the motor valves.
From the motor valves to the cylinder the pipework can ( in my view ) be in 22 mm as that can adequately carry 20 kW.
However, the motor valves offer a significant restriction to the flow.
Even some 28 mm valves have the same internal orifices as their 22 mm valves!
Tony
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As I understand it:
By the table in BS EN 12828 at the flow necessary to match the boiler output the available pump head is 'used up' between boiler and valves (plus the equivalent length of return). There's no head 'spare' to drive water through coil or the radiator pipework, so flow rate will actually be lower and the boiler won't run at full power.
Whether the boiler needs to run at full power is a different question. Nominally the radiators I have installed can't deliver that much power to the rooms, and teh house heat loss doesn't require it anyway.
Why does flow velocity matter in the pipes between boiler and motorised valves? It's below 1.5 m/s in both size of pipe. Velocity (or time) in the radiators (and associated pipework) is not affected by velocity between boiler and valves, surely? The radiators won't care if the water got from boiler to valves at 0.8 m/s or 0.5m/s, will it?
G
gaswizzard
See this on a daily basis whereby circulator head is an issue , easy way out is to install a secondary circulator in series with the existing boiler circulator , job done.
In the generality of course thats true.
But we are dealing with a specific boiler with apparently only 2m pump head left.
Tony
Noise.
If the velocity gets too high the water can be heard flowing though the pipe. 1.5m/s is just an empirical value, though many think 1m/sec is safer.
You said yesterday that the EST calculator worked out at 18.2kW (with 2kW allowed for water). I assume that means the 2kW is included in the 18.2kW, so your heating requirement is 16.2kW. You also said that your rads added up to approx 23.3kW.
Radiator output varies with temperature, so it would be useful to know what temperatures are quoted in the manufacturer's catalogue. The 'standard' is 75C flow, 65C return and 20C room, which is BS EN442. If your system has been designed for a 20C drop, i.e 75C flow and 55C return, the radiator outputs will be reduced to 85% of the BS EN442 figure, i.e your 23.3kW rads will only produce 19.8kW, which is closer to the 16.2 kW actually required.
If you set the max boiler output to, say, 20kW, the loss through the boiler heat exchanger will be less (it varies with the flow rate), so there would be more than 2m left at the pump. The head loss in a 22mm pipe (flow and return) will be only 0.8m. So there should be enough head left to pump the water through the rads.
G
gaswizzard
That will be dependant upon loading/tube length/diameter.
Is that a sensible proposition?
It adds a failure-prone component, so will add to the long term maintenance burden, but presumably not by much.
More significantly, will the boiler pump be able to pump through the second pump when the boiler us trying to over-run and the second pump is off? There doesn't seem to be a connector for an external pump shown in the boiler instructions, so over-run would be the internal pump only. Even if there is a signal on the boiler, a wire would need to be run, so it doesn't eliminate intrusive work.
If that won't work it will need a bypass with a non-return valve, presumably? I've had headaches with a non-return valve before, so I'm not keen on that.
The 'technical data' in the boiler manual says "Available pump head at 21C system temperature rise" is 2m.
Agreed. I calculated it, as a worst case, from information given earlier in this topic.
The simplest way if you really don't want to dig the finished area up is to get the plumber to install a small LLH in the cylinder cupboard, and then run the heating and hot water from there with a second pump.
Some may suggest a small buffer tank would do the same job.
Yes you would need an auto by-pass but will save a lot of disruption.
Some may suggest a small buffer tank would do the same job.
Yes you would need an auto by-pass but will save a lot of disruption.
[quote="D_Hailsham";p="2700211"
Agreed. I calculated it, as a worst case, from information given earlier in this topic.[/quote]
Thank you.
I think the calc you've done is two thirds the flow I had (ie, you've used 0.24 litre/s) through the 18m of pipe and 14 elbows on my pipework.
But if I understood correctly, this was on the basis of not only 20kW boiler output (ie, two thirds) but also a temperature differential of 10C. If I'm running at two thirds the power but half the differential does that not mean I actually need higher flow rate, not lower?
For 20kW at a differential of 10C, I think I need 480 litre/sec, and in 22mm pipe that's exceeding 1.5 m/s flow, I think.
Am I missing something? Was the 20kW still proposing running at a 20C differential at the boiler?
Agreed. I calculated it, as a worst case, from information given earlier in this topic.[/quote]
Thank you.
I think the calc you've done is two thirds the flow I had (ie, you've used 0.24 litre/s) through the 18m of pipe and 14 elbows on my pipework.
But if I understood correctly, this was on the basis of not only 20kW boiler output (ie, two thirds) but also a temperature differential of 10C. If I'm running at two thirds the power but half the differential does that not mean I actually need higher flow rate, not lower?
For 20kW at a differential of 10C, I think I need 480 litre/sec, and in 22mm pipe that's exceeding 1.5 m/s flow, I think.
Am I missing something? Was the 20kW still proposing running at a 20C differential at the boiler?
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