What Does A Modulating CH Pump Do?

[Norcon]

To much hassle all that and expense.
Compensating the flow from the boiler in order to prevent a high temperature in the first place is easier.
Then blending/mixing at T's or inside stores ,buffers, by pass flow etc raising return temp isn't a problem.

Thermal stores/buffers integrated or what ever are unecessary with modern controls.

You weren't reading were you ?

You're beginning to sound like that other loon.

 

[mickyg]

not compatible with many modern domestic boilers, such as those with the gianonni heat exchanger, as the boilers require quite a high min flow rate, which the modulating pumps may not constantly provide.

The flow rate has nothing to do with the type of heat exchanger.

it has alot to do with that type of heat exchanger. Just because it doesnt say so in a book doesnt mean thats not the case.

 

[SimonH2]

To much hassle all that and expense.
Compensating the flow from the boiler in order to prevent a high temperature in the first place is easier.
Then blending/mixing at T's or inside stores ,buffers, by pass flow etc raising return temp isn't a problem.

Pray explain how all that works then when the boiler shuts down with insufficient flow ?

Thermal stores/buffers integrated or what ever are unecessary with modern controls.

If you want to believe that.

You're beginning to sound like that other loon.

That's a bit of an insult

 

[D_Hailsham]

Actually the problem is that whatever the boiler, it will have a minimum flow rate for it to work properly/at all.

So it pure coincidence that the "minimum flow rate" for, say a Vaillant 418, just happens to be the flow rate necessary to produce the stated output (18kw)with the stated temp differential (20C)?

From the MI: Minimum flow rate 12.9 litre/min (Page 8, table 2.1)

(18kw x 60) ÷ (4.18 x 20) = 12.92 litre/min.

You can do this for any boiler and the answer is always the same: the "minimum flow rate" is that required to meet the specified output with the specified temperature differential. If the boiler modulates down, say to 12kW and the flow rate stays the same then the differential will drop from 20C to 13C. If you want the differential to remain constant at 20C, the flow rat has to reduce to 8.6 litre/min. The same is true if you range rate the boiler down to 12kW, the flow has to be reduced.

 

[Onetap]

Drivel

The return of Dr. Drivel/Water Systems/Gayton Tonner/Thermal Man/Thermal Man 1/Thermal Man 2/Thermal Man 3/Thermal Man 4!

 

[SimonH2]

Actually the problem is that whatever the boiler, it will have a minimum flow rate for it to work properly/at all.

So it pure coincidence that the "minimum flow rate" for, say a Vaillant 418, just happens to be the flow rate necessary to produce the stated output (18kw)with the stated temp differential (20C)?

From the MI: Minimum flow rate 12.9 litre/min (Page 8, table 2.1)

(18kw x 60) ÷ (4.18 x 20) = 12.92 litre/min.

You can do this for any boiler and the answer is always the same: the "minimum flow rate" is that required to meet the specified output with the specified temperature differential. If the boiler modulates down, say to 12kW and the flow rate stays the same then the differential will drop from 20C to 13C. If you want the differential to remain constant at 20C, the flow rat has to reduce to 8.6 litre/min. The same is true if you range rate the boiler down to 12kW, the flow has to be reduced.

Your statement is entirely correct (though I haven't checked the actual numbers you've quoted), but doesn't actually disagree with what I wrote if you take it in context. You are looking at a different aspect of the system to what is being discussed.

Taking the boiler you've quoted figures for, can you tell me what it does if you reduce the flow rate to (say) 1 l/min ? Why does it mater ? Well if the heating load is such that the TRVs only pass 1/min through the CH circuit then unless you fit a bypass circuit somewhere, then that's the only flow you'll get through the boiler.

Say your heating flow temp was 60, and return was 30, and flow rate was 1 l/min (that's 2.1kW). By the time you've mixed 7.6l/m of hot water at 60˚ with 1 l/m of water at 30˚, then your boiler return is now at 56.5˚. So you are running the boiler for extended periods when it's not condensing.

The question was not what the minimum flow rate is to maintain a given delta-T for whatever power the boiler is able to range up/down to, but what is the minimum flow rate before the boiler stops functioning properly/at all. Clearly it should be low enough that you can get min power out while still maintaining a sensible delta-T - below that then it's case of a) will the boiler shut down because of a low sensed flow rate, b) will it suffer from overheating (or even localised boiling) and shut down.

Historically we've designed the rad loop to maintain a significant flow rate and the discussion hasn't been much more than a curious question to most - and the return temperature issue wasn't an issue in those heady PC days (Pre-Condensing). Now we are expected to fit TRVs and IMO a modulating pump goes well with that - but the question of what the low flow rates do to the boiler gets neglected by most plumbers (I refuse to call most of them heating engineers as some of them like to call themselves).

IMO the ideal setup is to decouple the flows and buffer the boiler output - so you can have the boiler run intermittently at a good flow rate commensurate with it's capacity, and have the CH loop run at a flow rate commensurate with demand.
Just plucking some numbers out of the air, suppose you had 20l of active buffer (the actual tank would be bigger), and you kept the top at around 60˚ and stopped heating when the bottom reached 45˚, and further assume that the boiler will start ranging down when the return exceeds 40˚ (for a delta-T of 20&#730, and we'll assume we avoid mixing. To heat 20l of water, through 30˚ (from 30 to 60) with 18kW would take in the region of 70s. To take that heat out again at 2.1kW would take around 10 minutes.
So in this (vary simplified) scenario - we have no problem of low flow rate through the boiler, we have no issues with flow rate through the CH loop (it can be fully TRV), and the boiler will fire on full power for a bit over a minute every 10 minutes and then completely shut down. When the boiler is firing, it will always be condensing.

Obviously these are all figures plucked from the air and we could argue until the cows come home about what if this, what if that, etc, etc. The simple fact is that unless your boiler can continue running with very low flow rates, then you can't match it properly to a TRVd CH loop without either a bypass or a buffer. A bypass will frequently result in temperatures that don't allow for condensing, and that drops efficiency dramatically. Add in the current fad for fitting grossly oversized boilers in order to try and provide more than a dribble of DHW from a combi, and things are even worse. 18kW is not enough for even a small flat as a combi.

 

[D_Hailsham]

Actually the problem is that whatever the boiler, it will have a minimum flow rate for it to work properly/at all.

So it pure coincidence that the "minimum flow rate" for, say a Vaillant 418, just happens to be the flow rate necessary to produce the stated output (18kw)with the stated temp differential (20C)?

Your statement is entirely correct (though I haven't checked the actual numbers you've quoted), but doesn't actually disagree with what I wrote if you take it in context. You are looking at a different aspect of the system to what is being discussed.

I was replying to this:

[Variable speed pumps are] not compatible with many modern domestic boilers, such as those with the gianonni heat exchanger, as the boilers require quite a high min flow rate, which the modulating pumps may not constantly provide.

The discussion has now moved away from the original topic on to thermal stores, heat buffers etc.

I haven't ignored the remainder of your post, but I need to digest what you are saying.

 

[D_Hailsham]

Say your heating flow temp was 60, and return was 30, and flow rate was 1 l/min (that's 2.1kW). By the time you've mixed 7.6l/m of hot water at 60˚ with 1 l/m of water at 30˚, then your boiler return is now at 56.5˚. So you are running the boiler for extended periods when it's not condensing.

Where you get the 7.6l/m from?

The question was not what the minimum flow rate is to maintain a given delta-T for whatever power the boiler is able to range up/down to, but what is the minimum flow rate before the boiler stops functioning properly/at all. Clearly it should be low enough that you can get min power out while still maintaining a sensible delta-T - below that then it's case of a) will the boiler shut down because of a low sensed flow rate, b) will it suffer from overheating (or even localised boiling) and shut down.

I agree with that, but does it mean the flow through the boiler has to remain constant? At full 18kW output it requires a flow rate of 12.9l/min for a 20C differential, but at minimum 5kW the flow rate only needs to be 3.6l/min.

Historically we've designed the rad loop to maintain a significant flow rate and the discussion hasn't been much more than a curious question to most - and the return temperature issue wasn't an issue in those heady PC days (Pre-Condensing).

In the days of Fahrenheit and cast iron boilers systems were designed for a flow temp of 180°F (82°C) and return of 160°F (71°C) because they were considered "safe" temperatures, i.e. it was unlikely that someone would get burnt if they touched the radiator. The required flow rate then follows automatically from the accepted differential of 20°F (11°C).

IMO the ideal setup is to decouple the flows and buffer the boiler output - so you can have the boiler run intermittently at a good flow rate commensurate with it's capacity, and have the CH loop run at a flow rate commensurate with demand.

Do you mean the boiler should run in on/off mode all the time - there is no need for modulation?

So in this (very simplified) scenario - we have no problem of low flow rate through the boiler, we have no issues with flow rate through the CH loop (it can be fully TRV), and the boiler will fire on full power for a bit over a minute every 10 minutes and then completely shut down. When the boiler is firing, it will always be condensing.

Again you appear to be saying that a modulating boiler is unnecessary.

The simple fact is that unless your boiler can continue running with very low flow rates, then you can't match it properly to a TRVd CH loop without either a bypass or a buffer. A bypass will frequently result in temperatures that don't allow for condensing, and that drops efficiency dramatically.

The high "minimum" boiler outputs is also a contributory factor. In the UK, boilers need to be able to modulate down to about 1kW. Installing a 10kw-30kW boiler in a house which only requires 12kW in the coldest weather is ridiculous. The heat losses from typical new build houses can be only 7 or 8kW, yet many boiler cannot even modulate that low.

Going back to the original question about the use of variable speed pumps, it would be interesting to know the views of boiler manufacturers and companies such as Gianonni.

PS I agree about "plumbers" and "heating engineers".

 

[SimonH2]

Say your heating flow temp was 60, and return was 30, and flow rate was 1 l/min (that's 2.1kW). By the time you've mixed 7.6l/m of hot water at 60˚ with 1 l/m of water at 30˚, then your boiler return is now at 56.5˚. So you are running the boiler for extended periods when it's not condensing.

Where you get the 7.6l/m from?

8.6 - 1 = 7.6, the 8.6 came from your example boiler.

The question was not what the minimum flow rate is to maintain a given delta-T for whatever power the boiler is able to range up/down to, but what is the minimum flow rate before the boiler stops functioning properly/at all. Clearly it should be low enough that you can get min power out while still maintaining a sensible delta-T - below that then it's case of a) will the boiler shut down because of a low sensed flow rate, b) will it suffer from overheating (or even localised boiling) and shut down.

I agree with that, but does it mean the flow through the boiler has to remain constant? At full 18kW output it requires a flow rate of 12.9l/min for a 20C differential, but at minimum 5kW the flow rate only needs to be 3.6l/min.

Now you've changed the boiler - before the minimum was 12kW, giving a minimum flow rate of 8.6l/m.

Historically we've designed the rad loop to maintain a significant flow rate and the discussion hasn't been much more than a curious question to most - and the return temperature issue wasn't an issue in those heady PC days (Pre-Condensing).

In the days of Fahrenheit and cast iron boilers systems were designed for a flow temp of 180°F (82°C) and return of 160°F (71°C) because they were considered "safe" temperatures, i.e. it was unlikely that someone would get burnt if they touched the radiator. The required flow rate then follows automatically from the accepted differential of 20°F (11°C).

IMO the ideal setup is to decouple the flows and buffer the boiler output - so you can have the boiler run intermittently at a good flow rate commensurate with it's capacity, and have the CH loop run at a flow rate commensurate with demand.

Do you mean the boiler should run in on/off mode all the time - there is no need for modulation?

Not quite. The boiler can modulate down, but it doesn't need to. A boiler the size you quoted WILL be running in on-off mode (it will be short-cycling and working as a fan coil to blow hot ait out of the flue for part of the time) in the scenario I propose anyway - there's a big gap between the minimum output of 8kW and the load of 2.1kW. So why not let it just fire up, burn for a bit, and then completely shut down for 5 or 10 minutes (more with a larger buffer) - it's going to be it's most economical mode of operation.

So in this (very simplified) scenario - we have no problem of low flow rate through the boiler, we have no issues with flow rate through the CH loop (it can be fully TRV), and the boiler will fire on full power for a bit over a minute every 10 minutes and then completely shut down. When the boiler is firing, it will always be condensing.

Again you appear to be saying that a modulating boiler is unnecessary.

The simple fact is that unless your boiler can continue running with very low flow rates, then you can't match it properly to a TRVd CH loop without either a bypass or a buffer. A bypass will frequently result in temperatures that don't allow for condensing, and that drops efficiency dramatically.

The high "minimum" boiler outputs is also a contributory factor. In the UK, boilers need to be able to modulate down to about 1kW. Installing a 10kw-30kW boiler in a house which only requires 12kW in the coldest weather is ridiculous. The heat losses from typical new build houses can be only 7 or 8kW, yet many boiler cannot even modulate that low.

And that is the fundamental issue. Add in the "combi" factor, where for a house that might need 12kW heating, you'll be wanting a big combi, I wouldn't be too surprised to see 45kw or more. My flat has a 28kW combi (min 9kW) and I measured (by running on immersion heater - another advantage of having the thermal store - and watching the electric consumption) the heating load at something in the order of 2kW during cold weather. It also has quite a significant minimum flow rate - with only the direct connections to the thermal store, one sizeable TMV, and about 5m of 22mm copper pipe, the flow is insufficient if I turn the pump down to speed 1. At speed 2 it's giving about 25˚ delta-T on full output (it doesn't modulate in CH mode !). I make it about 16l/min from that, which is a crazy sort of flow rate to need.
OK, it is a 16 to 18 year old Vokera, and I guess things probably have improved a bit over the years

When it comes to boiler replacement, I'll be asking manufacturers to recommend a model that will modulate the flow rate and the burner output so as to maintain a set output temp with a return temp that could vary from 20˚ (possibly less under certain types of usage) up to probably no more than 50˚ - ie at very low return temps it will modulate the flow rate at max burner output, and at higher return temps it will modulate the burner at max flow rate. Somehow I doubt if I'll be offered one, and I'll have to leave the TMV in place to guarantee a minimum return temp

 

[CareerChangeChancer]

This is becomming hugely technical. Beginning to wish I hadn't asked!

 

[transam]

some of this is all very well ? if u have complete new install , or are designing something with a blank sheet of paper ect ect , but in many cases some are installing boilers on old /existing systems , 10mm or 8mm , flow & returns to radiators that are undersised poor flow rates so on so forth , in the ideal world we would repipe the whole place ect but the reality is this is not possible !

changing an old SE fixed rate boiler for a new modulating condenser can result in radiators not getting as hot , resulting in customer complaints ect
same go's for modulatimg pumps , taken several out , I beleive Vaillant make a combi with a modulating pump , which can be over ridden (fortunaetly) !

Plumbers , heating engineers hmm

vision technicians , hygene consultants , human resources managers !!

 

[D_Hailsham]

Where you get the 7.6l/m from?

8.6 - 1 = 7.6, the 8.6 came from your example boiler.

I agree with that, but does it mean the flow through the boiler has to remain constant? At full 18kW output it requires a flow rate of 12.9l/min for a 20C differential, but at minimum 5kW the flow rate only needs to be 3.6l/min.

Now you've changed the boiler - before the minimum was 12kW, giving a minimum flow rate of 8.6l/m.

Not really. The 12kW was just a mid-point, i.e an 18kW boiler rated down so the max was 12kW. The absolute minimum is 5kW.

The point I was making is that if you reduce the maximum output, you must logically reduce the flow rate to maintain the same differential.

 

[SimonH2]

This is becomming hugely technical. Beginning to wish I hadn't asked!

That's OK, sometimes what seems like a simple question can invoke some interesting and/or passionate debate.

The simple summary is that modulating pumps have their place, but the system as a whole needs to be designed (or re-designed) for it. Just replacing a standard fixed speed pump with a modulating pump may well cause various problems - including in the worst case, damaging the boiler (or at least causing it to shutdown for self-preservation).

some of this is all very well ? if u have complete new install , or are designing something with a blank sheet of paper ect ect , but in many cases some are installing boilers on old /existing systems , 10mm or 8mm , flow & returns to radiators that are undersised poor flow rates so on so forth , in the ideal world we would repipe the whole place ect but the reality is this is not possible !

changing an old SE fixed rate boiler for a new modulating condenser can result in radiators not getting as hot , resulting in customer complaints ect
same go's for modulatimg pumps , taken several out , I beleive Vaillant make a combi with a modulating pump , which can be over ridden (fortunaetly) !

Summary - changing one bit without considering it's place in, and effect on, the rest of the system often results in poor performance. I would not argue with that, nor would I argue that a modulating pump is the correct choice in the majority of installations without also doing other design work to integrate it.

Where you get the 7.6l/m from?

8.6 - 1 = 7.6, the 8.6 came from your example boiler.

That's OK, I had to go back and double check some of the figures - and then correct my post

I agree with that, but does it mean the flow through the boiler has to remain constant? At full 18kW output it requires a flow rate of 12.9l/min for a 20C differential, but at minimum 5kW the flow rate only needs to be 3.6l/min.

Now you've changed the boiler - before the minimum was 12kW, giving a minimum flow rate of 8.6l/m.

Not really. The 12kW was just a mid-point, i.e an 18kW boiler rated down so the max was 12kW. The absolute minimum is 5kW.

The point I was making is that if you reduce the maximum output, you must logically reduce the flow rate to maintain the same differential.

I agree, but then you can't do so indefinitely as many (most ?) boilers have a minimum flow rate. I'd argue that designing to a set differential is rather outdated. As you say, it used to be the norm to design for a set CH flow rate, flow temp, and return temp (or delta-T), and design for the boiler to match those three figures. Given that you can now have TRVs for very little money, and a modulating pump for (in the grand scheme of things) very little money, then I'd argue that designing for any specific differential was the wrong approach.

IMO I'd suggest the ideal for a new system (or a redesign) would be to pick a CH flow temp, and size the rads based on the flow temp, the heat load for the room, and a notional max return temp (ie what you'd get with the TRV open and the rad at full design O/P). That's going to set a max flow rate and max return temp (or min differential). At anything but max demand both the flow rate and return temp will be lower. Yes we're still designing to a differential - but now it's a minimum and under most condition the actual value will be higher.
At the boiler end, the ideal would be a boiler that can range down to "almost nothing" and simply control the flow temp whatever the return temp or flow rate. Since I doubt that there are many of those, a simple way of decoupling supply and demand is a buffer as discussed.

it would be interesting to compare the efficiency of two different setups :
1) A boiler (lets say 12kW just to pluck a number) that's ranged down to just 1kW
2) The same 12kW boiler in a system engineering with a buffer so if could fire at the full 12kW for 1 minute every 12 (or 2 in 24 or whatever it works out at for the size of buffer). A system could be arrange to allow the boiler to range down for part of the time, and only shut off when that fails - it would depend on how the boiler controls it's output, and what temperatures you set the buffer stats at.
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.

Do many manufacturers publish efficiency vs load graphs ?

 

[SimonH2]

Looks like a glorified dunsley neutralizer.

It does rather, but then it's a fairly basic idea and only so many ways of arranging it. On the other hand, if you are going to do it with a closed system, then it's going to come under the rules for pressurised hot water storage (might explain the £600 price tag) - it's going to have more than 15 litres of water if it's going to be useful.

I dare say you could do it fairly easily (at least for an open vented system) with whatever small cylinder you can buy cheaply at the plumbers merchants.

 

[D_Hailsham]

I agree with that, but does it mean the flow through the boiler has to remain constant? At full 18kW output it requires a flow rate of 12.9l/min for a 20C differential, but at minimum 5kW the flow rate only needs to be 3.6l/min.

The point I was making is that if you reduce the maximum output, you must logically reduce the flow rate to maintain the same differential.

I agree, but then you can't do so indefinitely as many (most ?) boilers have a minimum flow rate.

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

I'd argue that designing to a set differential is rather outdated. ... I'd argue that designing for any specific differential was the wrong approach.

I tend to agree. I came across the suggestion that it is perfectly OK to balance a radiator to a higher/lower differential so the actual rad output met the required heat loss. So the output of a "1kW" rad can, in theory, be adjusted between 150W and 1100W for a flow temp of 75C by varying the differential across the rad.

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.

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