HRM Wallstar oil fired boiler 12/14 Short Cycling or Normal?

[Robbot]

HRM Wallstar Oil fired Boiler 12/14: Short Cycling or Normal?


Hi

I’ve recently moved into a property that has an oil fired Wallstar Boiler (12/14 model ?) that’s around 20 years old. It has a good service record and ‘appears’ to be working.

When it is being used for heat, the boiler fires up and heats up the rads quite quickly. Once the rads are hot, it then cycles on and off (approx 2 mins on and two mins off). The pump is running all the time, there are no lights other than the amber mains neon indicator and the heat valves (S-plan system) is on. The rads stay hot all the while.


Having just moved from a house that had a new gas boiler, it too cycled when the heating was on, but not this regularly.


My first thought is the boiler has reached temp and it is waiting for the water to cool before switching on again. It does appear quite effective in warming the rads quickly. I've looked all through the manual and an unable to find any information on the way it operates normally.

 

[oilhead]

What controls do you have on the system? Room stat? TRV's?

 

[Robbot]

The room stat is a Honeywell Smartfit. All the rads apart from the bathroom have TRVs which are fully open.

 

[Robbot]

This may also be worth knowing. The pump I think is variable? Grundfos Alpha 15-60 130 with a Honeywell DU145 auto bypass valve.

I suppose at some point I will have to explore how to set the DU145 up with a variable speed pump, but that's for another day.

​

 

[Exedon]

I would suggest your boiler is operating normally.
Unlike modern gas boilers most oil boilers do not modulate down as they reach temperature.
They burn at a fixed rate, modulating burners have now been introduced but are still relatively rare.
We have replaced a few older wallstars due to hex leaks make sure to have yours serviced by a competent oil engineer.

 

[Johntheo5]

This may also be worth knowing. The pump I think is variable? Grundfos Alpha 15-60 130 with a Honeywell DU145 auto bypass valve.

I suppose at some point I will have to explore how to set the DU145 up with a variable speed pump, but that's for another day.

​

You can't set up a ABV using proportional pressure (PP) control but you can set it, to a fashion, using constant pressure (CP) control, and even though these A rated pumps have "traditional" 3 speed or constant curve (CC) settings, these behave partly in CP mode, right at the point where you require the ABV to open. I've never found out why they are designed in this way..
Its a bit unusual to install a ABV with a oil fired boiler as the heat exchanger contains 20/25 litres of water and will soak up the residual heat on boiler cut out without the Hi limit stat locking out the burner.

You can use the cycling as a reasonable indicator of the house heat requirements once you know the boiler output, by just monitoring the on/off cycle times for two or three cycles. If the average on time is, as above, 2 mins on, 2 mins off, the total cycle time is 4 mins, the on time is 2 mins so heat demand/boiler output is 2/(2+2), 50%, a 14kw boiler will be running at a average of 7kw, if the on time is 1.5 mins with 4.5 mins off, boiler average output is 1.5/(1.5+4.5) or 25%, heat demand/boiler output of 3.5kw with above boiler.

 

[Robbot]

Thank you for the words of advice

@Exedon With my rudimentry knowledge of heating systems, the only conclusion I could reach was it was operating correctly. Its been serviced by a local guy for over twenty years (nearly all oil boilers in this area) who has a good reputation, so I’ll use him for the next service as it was last carried out a month before we bought the property.

@Johntheo5 The current setup was put in from new back in 2001. Out of the 18 houses built, nearly 14 - like mine- still have the original heating system (Wallstar, Smartfirt S plan, Grundfos Alpha 15-60 130 and Honeywell DU145 auto bypass valve). The system comes on for between 25-35 mins constant, then cycles 2 min on 2 min off. It heats up the house very quick, thereafter it takes a good few hours to drop. I’ve never lived in a house that retained heat as this one.

 

[Johntheo5]

Do you mean that after the initial heat up period that it then cycles on/off every 2 minutes for the remaing heating period?, if so thats a average boiler output of 7kw which is fairly high, have you any thermostatic radiator valves (TRVs) installed, my 20kw firebird will cycle to give a average required output as low as 2.5kw when all the TRVs have throttled in. You can save alot of energy even if you only had a roomstat installed in your living room.

 

[Robbot]

Do you mean that after the initial heat up period that it then cycles on/off every 2 minutes for the remaing heating period?, if so thats a average boiler output of 7kw which is fairly high, have you any thermostatic radiator valves (TRVs) installed, my 20kw firebird will cycle to give a average required output as low as 2.5kw when all the TRVs have throttled in. You can save alot of energy even if you only had a roomstat installed in your living room.

Yes, that’s correct. If the room is at 15 degrees, and I set the room stat to 21 degrees. For the first 30 mins the boiler comes on and stays on, then it cycles. At this point the room temp may be around 19 degrees, so the boiler cycles until 21 degrees is satisfied.

It’s a 2 bed bungalow with 6 rads. One rad is off (TRV on frost), another TRV set between 2 and 3, and the remaining rads have their TRVs set fully open. All rads heats up equally, and all reach the same temp at the same time. The cycling can be as low as 1min on 2 min off up to 3 min on 4 min off. Room stat is in hall; however we keep the door open between hallway and living room.

I will carry out a check later on and time them.

 

[Robbot]

The weather here has been very mild, so have had no need for the heating. I did look back over some notes I took when I just moved in and the pattern was as follows:

Boiler set between min and max, five rads with their TRVs fully open, and one on number 2. Wall stat, at 18 degrees

After initally turning on the heating it ran for 20mins and reached 19.5 degrees.

Then:

Off 1:30

On 2:24

Off 2:16

On 2:12

Off 2:43

On 2:04

Wall stat reached 20 degrees

Turned if off at that point as it was warm enough for us, but if I had set the stat for 21, that pattern would have followed until it was satisfied.

That is very typical of how if functions

 

[GoferBlue1947]

I have done a search on oil boiler cycling in the forum and this appears to be the latest thread on this topic. I have the same problem with my Grant Utility Pro 15-21 oil boiler. Our C/H system has 15 radiators: 6 singles, 2 towel rails and 7 doubles. All bar one (one towel rail) have TRV's. The towel rail has both inlet and outlet valves fully open at all times. The total nominal output of the rads is 18.4 kW.

Since we moved in we have massively increased the amount of insulation (both loft and IWI), and reduced the draughts, so much so that the 3 single rads upstairs are permanently off as the heat rising from the ground floor is sufficient to keep the bedrooms at around 17C on the coldest days e.g. today with daytime ambient temperature around 3C and 0C tonight.

What I am finding is that after the initial warm-up phase, but while the room stat is still calling for heat, the boiler cycles as follows: 133 seconds off and 111 seconds on (these are the average of 10 consecutive readings). Using the calculation suggested by Johntheo5, this equates to a boiler output of an average of approx 9 kW (111/233 x 21). So presumably this means our boiler is hugely over-sized? I repeated the exercise with the three upstairs rads open and the cycling changed to 115 seconds off to 111 seconds on. 111/226 x 21 = 10.3 kW.

From the boiler installation instructions I see that it is possible to fit three different size nozzles to this boiler: 0.45/80/EH, 0.55/60/ES and 0.60/60/ES, to achieve 15, 18 and 21 kW respectively. So is it sensible that we should try fitting the 0.45/80/EH to give 15kW which on paper is ample output for our system?

 

[GoferBlue1947]

Do you mean that after the initial heat up period that it then cycles on/off every 2 minutes for the remaing heating period?, if so thats a average boiler output of 7kw which is fairly high, have you any thermostatic radiator valves (TRVs) installed, my 20kw firebird will cycle to give a average required output as low as 2.5kw when all the TRVs have throttled in. You can save alot of energy even if you only had a roomstat installed in your living room.

When your boiler is giving an average output as low as 2.5kW, what do the on/off times look like please?

 

[Johntheo5]

When your boiler is giving an average output as low as 2.5kW, what do the on/off times look like please?

Around 1.5/2mins on & 8.0/10.0 mins off, I think the main reason that the (oil fired) burner can stay off for so long is the the TRVs throttle right down to give a flowrate of ~ 1 to 2 LPM (no ABV installed) so the 25 litre Hx takes this longish time to drop the boiler temp from 75C to 65C and burner refire. I have the circ pump set to PP control.

 

[GoferBlue1947]

Around 1.5/2mins on & 8.0/10.0 mins off, I think the main reason that the (oil fired) burner can stay off for so long is the the TRVs throttle right down to give a flowrate of ~ 1 to 2 LPM (no ABV installed) so the 25 litre Hx takes this longish time to drop the boiler temp from 75C to 65C and burner refire. I have the circ pump set to PP control.

OK, thank you. Our TRV's also throttle right down but I have no idea what the flow rate is. Sorry, I don't know what PP control means. Our circulation pump is a bog-standard Wilo jobbie with no fancy electronics, so all I can do is alter the speed from 1 to 3, currently it's on 2.

Did you ever think about fitting a smaller capacity nozzle?

 

[Johntheo5]

My pump is also a Wilo but only 4 years old so in addition to the standard 3 speed settings additionally has proportional pressure (PP) and constant pressure (PP) modes which saves a bit of power on TRV throttling down.

No, have no intention of reducing the nozzle size as I get a reasonably fast system heat up in the morning, the rads reach full temperature within 25/30 minutes.