How to set flow rate through HW cylinder coil

[Notch7]

You could set the thermostat on your HW cylinder to 45c and then set the temperature on the boiler to 55c when feeding the HW cylinder then the boiler will condense. You just need to increase the cylinder temperature to 60c (and of course the boiler temperature to say 70c) periodically to kill off any bacteria. Another way if you boiler can't increase the water temperature would be to set an immersion heater to come on using a timer.

Many thanks, that’s certainly worth thinking about.

It occurs to me the heating will be off for 6 months, so hot water priority is a bit pointless and if the hot water priority settings are kept the same, the boiler won’t condense for the 6 month period. In that situation perhaps doing something as you suggest might well work.

I suppose I need to experiment to find out what the typical difference between flow and return temps are on the boiler when heating the cylinder

 

[Mister Banks]

Many thanks, that’s certainly worth thinking about.

It occurs to me the heating will be off for 6 months, so hot water priority is a bit pointless and if the hot water priority settings are kept the same, the boiler won’t condense for the 6 month period. In that situation perhaps doing something as you suggest might well work.

I suppose I need to experiment to find out what the typical difference between flow and return temps are on the boiler when heating the cylinder

Your Ideal Vogue boiler is good for hot water priority, this video may help. The practical bit is about 8 minutes in……

 

[Johntheo5]

There have been discussions before on here about whether boilers modulate based on flow temperature or return temperature. I have always thought flow temperature was the more obvious. Others seem to disagree. Is there any hard info?

Most boilers control the firing rate based on the target (setpoint) flow temperature up to the point where the heat demand is less than the boiler's minimum output, at which point the return temperature will start to rise, leading in turn to a flow temperature increase, the burner will then cut out when the actual flow temp reaches the target temp+5C, the circ pump will continue to run and the burner will refire when the anti cycle time has elapsed and if the flow temperature has reduced to the target temperature-5C. Vaillants can be run on return temperature control by changing the D parameters but I've rarely seen anyone running in this mode. The newer models have a adjustable dT, 10C to 20C, with flow temperature control.

 

[MNW67]

I’ve had a hot water priority system fitted with Ideal vogue boiler and an ultra fast recovery cylinder.

Have you tried running off all the hot water and then timing how long the cylinder takes to reheat? That's the sort of thing I'd be itching to have a go at if I had a spanking new system!

 

[Johntheo5]

@Notch7 has hot water priority so there should have been no gate valve installed but it was so should be fully opened, the higher the flow temperature and the greater the flow rate will give the fastest recovery.

 

[SimonH2]

standing loss doesnt seem great its quoted as 2.27kw loss in 24hrs

Tsk, that would be 2.27kWHr kW is a measure of power flow, kWHr is a measure of energy. It's a very common mistake. And 2.27kWHr over 24 hours is about 95W.

But, some combi boilers can have higher standing losses. About 13 years ago now I fitted a thermal store in our rental flat, and had just bought the house next door. Having both empty I was able to take some measurements with identical weather conditions. My thermal store (which I built a lagged cupboard around in the garage) had a standing loss of about 80W (measured by running for several days on immersion heater and taking reading), while the BG badged WB combi in the house was around 160W if the eco mode wasn't turned on (with eco off, it fired up periodically to keep the DHW loop warm for faster hot water from taps).
And yes, any insulating of pipes and the cylinder cupboard will help reduce losses.

Given that any gas boiler doesnt go into condensing mode when heating hot water

It should be able to if the system is well set up.
IF you are able to set the flow rate through the coil such that the boiler loop delta-T is at the boilers max of 20˚C, then you'll be condensing up to a flow temp of around 74˚C. That means, by the time the boiler stops condensing, the top end of the cylinder is going to be up into the high 60s or low 70s temperature wise, but before you reach that stage, your cylinder stat will probably have turned the boiler off.

Most boilers control the firing rate based on the target (setpoint) flow temperature up to the point where the heat demand is less than the boiler's minimum output, at which point the return temperature will start to rise, leading in turn to a flow temperature increase, the burner will then cut out when the actual flow temp reaches the target temp+5C, the circ pump will continue to run and the burner will refire when the anti cycle time has elapsed and if the flow temperature has reduced to the target temperature-5C. Vaillants can be run on return temperature control by changing the D parameters but I've rarely seen anyone running in this mode. The newer models have a adjustable dT, 10C to 20C, with flow temperature control.

And what about those that cut back on the flow temperature if they see the return temperature rise ?
As I read some of the comments around, it seems a lot of boilers do this on the basis that as the heating system (TRVs) throttle back, the bypass returns a higher proportion of the flow back to the boiler return, so while the radiator outlet temperature drop, the actual return flow temperature seen by the boiler reduces. So the boilers back off on the flow temperature in an attempt to better match the heating demand.

 

[Johntheo5]

And what about those that cut back on the flow temperature if they see the return temperature rise ?

I'm not aware of boilers that actually do this, when the return temperature starts to rise, so too will the flow temperature tend to rise so the boiler just modulates down to maintain the (target) flow temperature.

As I read some of the comments around, it seems a lot of boilers do this on the basis that as the heating system (TRVs) throttle back, the bypass returns a higher proportion of the flow back to the boiler return, so while the radiator outlet temperature drop, the actual return flow temperature seen by the boiler reduces. So the boilers back off on the flow temperature in an attempt to better match the heating demand.

Re TRVs throttling back and the return temp obviously falling, then because the flow rate through the boiler also falls the flow temperature will again tend to rise so boiler will again modulate down, if it can, until it cant reduce the power any further, see example below.
Assume 10kw of T50 rated rads operating with a heat demand of 6.88kw (at 20C rooms temp) and assuming a target flow temperature of 65C, the boiler will operate (assuming no bypass), flowtemp/returntemp/dT/flowrate, 65C/50C/15C/6.574LPM and now assume the heat demand falls to 5.6kw, the flowtemp/returntemp/dT/flowrate, will now be 65C/39C/26C/3.086LPM, if for some reason or other the boiler couldn't or didn't modulate down then the dT would rise to, 6.88*860/60/3.086, 31.96C resulting in a flowtemp of 39C+31.96C, 70.96C and burner shutdown, any bypassing will not stop this occuring since the bypass is at the flow temperature.

 

[Notch7]

@Notch7 has hot water priority so there should have been no gate valve installed but it was so should be fully opened, the higher the flow temperature and the greater the flow rate will give the fastest recovery.

Thats exactly what I was seeking an answer to. I think my heating engineer is more used to traditional S plan systems.

My logic, rightly or wrongly is that if the boiler will never go into condensing mode when heating water, it makes sense to make recovery time as minimal as possible.

 

[Notch7]

Tsk, that would be 2.27kWHr kW is a measure of power flow, kWHr is a measure of energy. It's a very common mistake. And 2.27kWHr over 24 hours is about 95W

That’s very useful, I’ve been trying to work out the maths from that but struggling to work it out.

And yes, any insulating of pipes and the cylinder cupboard will help reduce losses

Great thanks for confirming

 

[Notch7]

Re TRVs throttling back and the return temp obviously falling, then because the flow rate through the boiler also falls the flow temperature will again tend to rise so boiler will again modulate down, if it can, until it cant reduce the power any further, see example below.
Assume 10kw of T50 rated rads operating with a heat demand of 6.88kw (at 20C rooms temp) and assuming a target flow temperature of 65C, the boiler will operate (assuming no bypass), flowtemp/returntemp/dT/flowrate, 65C/50C/15C/6.574LPM and now assume the heat demand falls to 5.6kw, the flowtemp/returntemp/dT/flowrate, will now be 65C/39C/26C/3.086LPM, if for some reason or other the boiler couldn't or didn't modulate down then the dT would rise to, 6.88*860/60/3.086, 31.96C resulting in a flowtemp of 39C+31.96C, 70.96C and burner shutdown, any bypassing will not stop this occuring since the bypass is at the flow temperature

I’ve noticed over the last couple of days, my boiler is short cycling.

The weather comp has been showing 11deg, room stat is set at 20 deg.

When the room stat calls for heat the boiler fires up and it seems to settle down to a flow temp around 45 deg

But after a while the boiler stops firing but continues pumping and then the boiler fires again but sometimes for only a minute or so….and the flow temp at this time has dropped down. I’m guessing the boiler can’t modulate down enough, but I’m not sure if there is any mechanism to reduce short cycling. The upstairs TRVs were all on low, around 1.5 so I’ve turned them up, maybe that will help (not sure why).

The boiler I have is the Ideal Vogue gen 2 18kw with a modulation ratio of 7:1 it’s a typical 1960 build semi with 8 rads, not sure if heat loss

I did wonder if the 15kw model would’ve been better, although I’m doing a garage conversion with 3 more rads this year so.

 

[MNW67]

The boiler I have is the Ideal Vogue gen 2 18kw with a modulation ratio of 7:1 it’s a typical 1960 build semi with 8 rads, not sure if heat loss

I did wonder if the 15kw model would’ve been better, although I’m doing a garage conversion with 3 more rads this year so.

There's not much difference in the minimum output between those two boilers. In nominal terms anyway. I suppose proportionally, yours is 20% higher.

 

[Johntheo5]

Are you sure it has modulation ratio of 7:1, I've seen a min output of 5.9kw?
Edit: yes, think it is ~ 3.5kw.

Do you know what weather compensation curve its on?, it may be curve 20?

 

[Notch7]

Are you sure it has modulation ratio of 7:1, I've seen a min output of 5.9kw?
Edit: yes, think it is ~ 3.5kw.

Do you know what weather compensation curve its on?, it may be curve 20?


View attachment 332563

It is set on curve 15, I think, the menu isn’t all that clear.

If I understand correctly I need to have a higher curve number to raise the flow setpoint.

Judging by the graph there needs to a compromise between keeping flow rate set point at highest outside temp sufficient to minimise short cycling and the flow rate set point at the minimum temp staying low enough so boiler return temp is below 56deg so boiler condenses.

 

[Notch7]

There's not much difference in the minimum output between those two boilers. In nominal terms anyway. I suppose proportionally, yours is 20% higher.

View attachment 332562

yes I can’t see that making a huge difference.

I’ve got plenty of insulation improvements to make which will have a far greater impact on energy usage than minor tweaks.

I am very glad I chose a hot water priority system, the house has been a nice gentle warmth with the rads just being warm not hot to the touch. My wife works from home until around 3.00pm every day somI think it will work out great having the heating on most of the day just trickle feeding at low flow temps.

 

[MNW67]

I am very glad I chose a hot water priority system, the house has been a nice gentle warmth with the rads just being warm not hot to the touch. My wife works from home until around 3.00pm every day somI think it will work out great having the heating on most of the day just trickle feeding at low flow temps.

Indeed. I think sometimes people overlook the comfort advantage and concentrate purely on the efficiency.

As part of your set up, do you also have a room thermostat. If so, which one?