What Does A Modulating CH Pump Do?

[SimonH2]

Can you give me an example where the MI's say that the minimum flow rate must be maintained all the time?

Well for the Vokera in my flat I can tell you that is simply won't turn on if there's too low a flow<period> I admit that I've got very limited experience in such matters and rather assumed that was normal.

Just dug the manual out, and there's a minimum flow rate specified - 350l/hr, or getting on for 60l/min, or 1l/second. It has a flow switch (differential pressure across the heat exchanger I assume).

Oh there's that "a" word again - making an "ass" out of "u" and "me". Or just me in this case

Now I expect I'll get a series of "but there are no boilers left on the market that are that <insert derogatory term of choice here> any more" statements. I'd hope not, but I'd not be surprised to find there are.

And for good measure, it only modulates the output when supplying DHW and not CH. When it trips on the CH flow temperature setting, it then has a 3 minute lockout before it will fire again. And in the meantime, it still has the fan running (albeit on low speed) while it pumps warm water through the heat exchanger. Hmm, just like sticking a Myson fan-coil outside Lets just say, that I've learned a lot from this boiler about features to ensure aren't on any future replacement.

 

[ianniann]

My suspicion is that it's a significant design challenge to design a boiler that will range down to very small fractions of it's full rated o/p and still maintain efficiency.

That's not a suspicion, that's a fact. There are all sorts of issues that affect efficiency as the firing power changes. Some can be designed around, some can't. The better solutions are essentially multiple boilers.

Some Geminox boilers can modulate down to 1kW. I don't know about their efficiency at the lower end.

That boiler only has a maximum output of 10kW, although that's still a 10:1 modulation range. 5:1 is more typical and 3:1 quite common in domestic boilers. Efficiency losses are typicalyl a few percent at these sorts of modulations but drop off rapidly when you go beyond certainly design parameters. I imagine no manufacturer would sell a boiler that could operate at those levels. This is all complicated by the fact that condensing efficiency increases as burner output drops, all other things being equal (and operating temperatures being within the condensing range), so you may actually get higher efficiency at a lower modulation.

 

[transam]

Can you give me an example where the MI's say that the minimum flow rate must be maintained all the time?

Well for the Vokera in my flat I can tell you that is simply won't turn on if there's too low a flow<period> I admit that I've got very limited experience in such matters and rather assumed that was normal.

Just dug the manual out, and there's a minimum flow rate specified - 350l/hr, or getting on for 60l/min, or 1l/second. It has a flow switch (differential pressure across the heat exchanger I assume).

Oh there's that "a" word again - making an "ass" out of "u" and "me". Or just me in this case

Now I expect I'll get a series of "but there are no boilers left on the market that are that <insert derogatory term of choice here> any more" statements. I'd hope not, but I'd not be surprised to find there are.

Plenty of boilers (combis & systems boilers) have used the pressure diff /primary flow switch method of proving water flow across the heat or that the boiler actually has water in it most (all) use pressure sensing devices now ! One exeption I know of is the Atmos combis that have nothing to protect the heat exc , dry firing is apparently not a problem for them , min flow rates for them is 200lh at 7 kw 750 lh 26 kw 20 deg diff

 

[SimonH2]

Plenty of boilers (combis & systems boilers) have used the pressure diff /primary flow switch method of proving water flow across the heat or that the boiler actually has water in it most (all) use pressure sensing devices now !

Thanks for that confirmation. I would have been very surprised if it were otherwise given how much "self protection" some boilers have these days - like a sensor to detect a blocked (frozen) condensate drain !

One exeption I know of is the Atmos combis that have nothing to protect the heat exc , dry firing is apparently not a problem for them ...

Is that an official statement, or just experience ? Given warnings I've seen on other stuff along the lines of "dry firing will cause the end of the world", I'd be surprised to see a manufacturer buck that trend and admit that "nah, it's no big deal". On the other hand, with sensors in the right place, and a well designed heat exchanger, it should be possible to deal with it - at a cost.

 

[D_Hailsham]

Well for the Vokera in my flat I can tell you that is simply won't turn on if there's too low a flow<period> I admit that I've got very limited experience in such matters and rather assumed that was normal.

Just dug the manual out, and there's a minimum flow rate specified - 350l/hr, or getting on for 60l/min, or 1l/second. It has a flow switch (differential pressure across the heat exchanger I assume).

Which model Vokera do you have?

 

[SimonH2]

Which model Vokera do you have?

It's an old Excel 80 SP

 

[D_Hailsham]

Which model Vokera do you have?

It's an old Excel 80 SP

OK

This boiler's output ranges from 8.9KW to 23.7kW and they recommend a differential of 20°C. A minimum flow of 350lph is equivalent to an output of 8.12kW, which is below the minimum modulation level. If the boiler is going at max output the flow rate would have to be 1020lph for a 20°C differential. If it was only 350lph the differential would be 58°C!

I can understand the reason for a minimum of 350lph; it's when the so-called "minimum" is actually the maximum that concerns me.

Incidentally, the current Vokera Linea HE boilers also quote a minimum flow rate of 350lph.

 

[SimonH2]

This boiler's output ranges from 8.9KW to 23.7kW and they recommend a differential of 20°C. A minimum flow of 350lph is equivalent to an output of 8.12kW, which is below the minimum modulation level. If the boiler is going at max output the flow rate would have to be 1020lph for a 20°C differential. If it was only 350lph the differential would be 58°C!

I can understand the reason for a minimum of 350lph; it's when the so-called "minimum" is actually the maximum that concerns me.

You've lost me now ! I don't recall if the manual specifies a maximum - probably not as there wouldn't be much point.

I don't follow what you mean when you say it's when the so-called "minimum" is actually the maximum that concerns me,

BTW - the boiler doesn't modulate when heating. It runs at the preset power until the output reaches the setpoint and then turns the burner off until the temperature drops again (and with a 3 min recycle lockout timer). It can be ranged down manually - but it's a fixed setting.

I'm assuming mine's on full output - it's had the control board changed and knowing the people that did it, I cannot imagine they even thought about whether any adjustments were needed Anyway, now it's connected to the thermal store, it runs flat out at something like 30&#730; differential - which would suggest it's running at around 650 to 700 lph.

Incidentally, the current Vokera Linea HE boilers also quote a minimum flow rate of 350lph.

So also quite incompatible with a CH loop that's fully TRV with a modulating pump.

 

[SimonH2]

Must stop hitting quote instead of edit

 

[transam]

Plenty of boilers (combis & systems boilers) have used the pressure diff /primary flow switch method of proving water flow across the heat or that the boiler actually has water in it most (all) use pressure sensing devices now !

Thanks for that confirmation. I would have been very surprised if it were otherwise given how much "self protection" some boilers have these days - like a sensor to detect a blocked (frozen) condensate drain !

One exeption I know of is the Atmos combis that have nothing to protect the heat exc , dry firing is apparently not a problem for them ...

Is that an official statement, or just experience ? Given warnings I've seen on other stuff along the lines of "dry firing will cause the end of the world", I'd be surprised to see a manufacturer buck that trend and admit that "nah, it's no big deal". On the other hand, with sensors in the right place, and a well designed heat exchanger, it should be possible to deal with it - at a cost.

yep its official ! its no big deal !!

 

[D_Hailsham]

BTW - the boiler doesn't modulate when heating. It runs at the preset power until the output reaches the setpoint and then turns the burner off until the temperature drops again (and with a 3 min recycle lockout timer). It can be ranged down manually - but it's a fixed setting.

That's not what the MI's say! The boiler does modulate - read paragraph 2.2 in the MI's. It can also be range rated, which mean the maximum output can be reduced.

From your description the heating requirement of your house is quite low, so the boiler is unable to modulate low enough. It therefore has to operate in on/off mode all the time.

This boiler's output ranges from 8.9KW to 23.7kW and they recommend a differential of 20°C. A minimum flow of 350lph is equivalent to an output of 8.12kW, which is below the minimum modulation level. If the boiler is going at max output the flow rate would have to be 1020lph for a 20°C differential. If it was only 350lph the differential would be 58°C!

I can understand the reason for a minimum of 350lph; it's when the so-called "minimum" is actually the maximum that concerns me.

You've lost me now ! I don't recall if the manual specifies a maximum - probably not as there wouldn't be much point.

I don't follow what you mean when you say it's when the so-called "minimum" is actually the maximum that concerns me.

Let's see if I can clear up your confusion.

The Vokera quotes a minimum flow rate of 350lph. Assuming a 20C differential, this means the boiler will be producing 8.13kW, which is lower than the minimum for your boiler. When your boiler is running at max output (23.7kW) the flow rate for a 20C differential is 1020lph. So the flow rate, for the same differential varies from 350lph to 1020lph.

What some contributors seem to be saying is that the flow rate should never drop below that required for max output - in your case 1020lph, even when the boiler is running at minimum output. If that was done, the differential across your boiler would be about 58C. A differential this large would do serious damage to any heat exchanger.

I think that manufacturers are at fault as they only show the flow rate at max output, so everybody assumes that is the minimum. A few just say that the differential must not exceed X degrees.

 

[SimonH2]

Let's see if I can clear up your confusion.

The Vokera quotes a minimum flow rate of 350lph. Assuming a 20C differential, this means the boiler will be producing 8.13kW, which is lower than the minimum for your boiler. When your boiler is running at max output (23.7kW) the flow rate for a 20C differential is 1020lph. So the flow rate, for the same differential varies from 350lph to 1020lph.

What some contributors seem to be saying is that the flow rate should never drop below that required for max output - in your case 1020lph, even when the boiler is running at minimum output. If that was done, the differential across your boiler would be about 58C. A differential this large would do serious damage to any heat exchanger.

I think that manufacturers are at fault as they only show the flow rate at max output, so everybody assumes that is the minimum. A few just say that the differential must not exceed X degrees.

I'm with you now - apart from the slight error. I think what you meant to say is that if you insist on maintaining 1020lph on minimum output, then the differential will be very low. Such a low differential would pretty well guarantee to avoid condensing mode on a modern boiler.

 

[SimonH2]

BTW - the boiler doesn't modulate when heating. It runs at the preset power until the output reaches the setpoint and then turns the burner off until the temperature drops again (and with a 3 min recycle lockout timer). It can be ranged down manually - but it's a fixed setting.

That's not what the MI's say! The boiler does modulate - read paragraph 2.2 in the MI's. It can also be range rated, which mean the maximum output can be reduced.

I stand corrected - dunno how I managed to read that as not ranging on CH

From your description the heating requirement of your house is quite low, so the boiler is unable to modulate low enough. It therefore has to operate in on/off mode all the time.

It's a one bed flat, and average load in cold weather is (I estimate) about a 1/4 of the boiler's minimum output - less in warm weather.
That's one of the reasons I dislike combi's - you have to have a boiler that's grossly oversized for the CH load, and (certainly in this case) it's still undersized for the DHW load. Worst of everything.

 

[D_Hailsham]

The Vokera quotes a minimum flow rate of 350lph. Assuming a 20C differential, this means the boiler will be producing 8.13kW, which is lower than the minimum for your boiler. When your boiler is running at max output (23.7kW) the flow rate for a 20C differential is 1020lph. So the flow rate, for the same differential varies from 350lph to 1020lph.

What some contributors seem to be saying is that the flow rate should never drop below that required for max output - in your case 1020lph, even when the boiler is running at minimum output. If that was done, the differential across your boiler would be about 58C. A differential this large would do serious damage to any heat exchanger.

I'm with you now - apart from the slight error.

Not so slight.

A flow rate of 1020lpm at an output of 8.9kW gives a differential of less than 7.5°C. A flow rate of 350lpm at 23.7kW gives a differential of 58°C.

I think what you meant to say is that if you insist on maintaining 1020lph on minimum output, then the differential will be very low. Such a low differential would pretty well guarantee to avoid condensing mode on a modern boiler.

That's right.

As you only have a 1-bed flat, it might be worth using the Boiler Size Calculator to find the heating requirement. The boiler can then be range-rated down to that by a Gassafe registered person. Instructions are in para 6.7.9.

Your boiler will give a hot water flow rate of just under 10 litres/min at a temperature rise of 35°C.

 

[SimonH2]

As you only have a 1-bed flat, it might be worth using the Boiler Size Calculator to find the heating requirement. The boiler can then be range-rated down to that by a Gassafe registered person. Instructions are in para 6.7.9.

No need - the thermal store isolates heating demand from boiler output very nicely However, the calculator says 2.87kW - just over a third of the boiler's minimum output.

Your boiler will give a hot water flow rate of just under 10 litres/min at a temperature rise of 35°C.

Which is "not very much" when you're used to a house with a proper hot water tank and sensibly sized pipes. It's even less when you consider just how useless a 35&#730; rise is - it's not hot enough to run a decent bath as the water will go cold faster than the bath fills !