Gas Boiler Heat Exchanger Pressure Loss

[fixitflav]

You can't see the the Hx pressure loss, you have to derive it, as i said originally a 6M pump pumping 900 LPH will have a total head of ~ 4.5M (assuming a true CC pump), at 900LPH the residual pump head is 2.5M therefore the Hx head loss is (4.5-2.5) or 2.0M.
We are informed, below that when on curve H that the residual head is 1.7M @ 900LPH so of course its for a system that needs a lower residual head to give the same flowrate, one system might need 2.5M to circulate 900LPH, another may only require 1.7M to circulate 900LPH, the Hx head loss will remain the same at 900LPH, but because a 4.2M pump head will fall to ~ 3.2M when pumping 900LPM then the residual head is only (3.2-2.0), 1.2M (made a typo of 1.0M, above) or else a 4.2M pump will only fall to a total head of (1.7+2), 3.7M in pumping 900LPH. Its still amazing why they don't just specify the Hx head loss, all you need is the head loss at any one given flowrate, the rest can then be calculated very accurately because head is proportional to flow squared.

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The only place I can see a pump giving 900LPH at 4.5m is on one the curves in your #20. Is that where it comes from? Is it the same pump to generate Fig. 28? There are a lot more speed choices on Fig. 28! If the system specific resistance varies, any number of flow/head combinations can result.
For a heat-only boiler, the manual should give a graph of boiler headloss vs flow (or loss at a given flow, from which it's easy to produce the curve). You can then add the estimated system loss to get the required pump duty.
With a system boiler or combi, I assume the pump is based on a typical system loss, and available pump head is provided. If the installer can be bothered, he could estimate the system loss and check flow is adequate. I doubt if it's usually a problem, partly because if the system resistance is untypically high, it's likely to be due to fewer rads, so lower flow is OK. One I just looked at gives available pump head at just one flow, presumably at max pump setting. I agree it would be useful to have the pump curves (at various speeds) and the boiler headloss, to give a better idea of what's going on.

 

[Johntheo5]

OK, will give it another lash.
We don't know what exact pump is installed in the boiler but probably very like the one below, a 7M. The (what they call "Residual" head) pump head is 4.5M at 900LPM, the remaing pump head then available will depend on the boiler Hx head loss.
The remaining ("feed head of the pump" pump head is 3.5M which means that (4.5-3.5) 1.0M is the Hx head loss
This, (1633/900)^2 X 1.0, 3.3M is a bit less the eco tech plus 438 known Hx head loss of 4.05m at 1633LPH, but in the right ball park.
I would think that most systems would require ~ a 4M head loss through the system (net of Hx loss) to circulate 1633LPH, 27.2LPM. ( required to give a 38kw boiler a dT of 20C). Based on the above then that 7M pump will, by my calcs, circulate ~ 1146LPH, 19.1LPM with a (residual) pump head of 3.6M, 1.63M head loss through the Hx leaving 1.97M (remaining) head loss to circulate (1146LPH) through the system.
It highlights one of the reasons for a LLH as a head of 1.97M with say 20 rads would nearly require a magician to balance them? IMO.

 

[JonathanM]

OK, will give it another lash.

The calculations are a bit above my head, but I think this is the sort of graph you are saying you would like to see manufacturers provide. Shows combined pump data and head loss for three different size Intergas HRE system boilers, with a 7m head pump, rather than showing residual head.

 

[Johntheo5]

The one that most people would like to see I would think is the one below, very useful IMO for trouble shooting as systems do get choked up and are cleaned with the boiler in situ, why have they changed to these new incomphrehensible graphs?.
This one is from a 10 year old boiler I think.

 

[JonathanM]

The one that most people would like to see I would think is the one below, very useful IMO for trouble shooting as systems do get choked up and are cleaned with the boiler in situ, why have they changed to these new incomphrehensible graphs?.
This one is from a 10 year old boiler I think.

The type of graphs you want are still provided for heat only boilers. But when there is a built in pump, they usually seem to show those complicated residual head graphs.

The one I've posted from Intergas is more straightforward. It shows the same information as yours. But it is showing three different boilers on the same graph A, B and C. Presumably to avoid producing three different graphs. And then it also shows the graph for the built in pump, but the full head, not the residual head. Maybe they could have had a separate graph for the pump. But even I can understand this one. At least I think so!

EDIT: This is the graph from the heat only version, again with three different power boilers on the same graph.

 

[Johntheo5]

That's looks fine, any idea of the A,B&C boiler outputs?

 

[JonathanM]

That's looks fine, any idea of the A,B&C boiler outputs?

For the heat only boilers it is, 18.2, 23.1, & 26.6

 

[Johntheo5]

Maybe one last one for their combis with their embedded HW HXs
+ pump curve, if possible.

 

[JonathanM]

Maybe one last one for their combis with their embedded HW HXs
+ pump curve, if possible.

No problem. I enjoy doing this.

 

[JonathanM]

This is the equivalent graph for the combi version. I've been told that Intergas combis don't have a separate HX for DHW. Just a smaller pipe for water going through the main heat exchanger. And I think the combi version is identical to the system version, you just change a parameter in the menu to switch between the two. So, I'm guessing it will be identical to the first graph. Then below that I've got a DHW flow resistance graph, just in case that is of interest. Stated outputs for CH the same as heat only, so 18.2, 23.1, & 26.6. But I think there is extra power when they do DHW, of 24, 28 & 36. Did you ask for anything else?

 

[Johntheo5]

What are the units of the y axis, immediately above?

 

[JonathanM]

What are the units of the y axis, immediately above?

Y axis is litres per minute - of hot water to the taps I guess?. Versus Bar on x axis, which is presumably inlet cold water pressure?

EDIT: boiler A is restricted to 8 litres per minute

 

[vulcancontinental]

Going back to basics, why do you think you need to know this? (for central heating circulation I mean)

 

[Johntheo5]

If this is to me, need to know what??

 

[vulcancontinental]

If this is to me, need to know what??

No, beg your pardon, I was pondering why JM needed to know this in relation to his boiler change. Mass flow, head loss, as long as met by pump whether in or out of boiler, all is well.