Small boiler - big bath!

[clittlewood]

The problem:

A 2 Bed Bungalow, 300ltr Bath - Small 12/14 oil fired combi boiler that wont keep up with the hot water demand for the shower after about 2 minutes. A 15mm feed from outside main restricts the flow rate even though we have 3.5 bar water pressure from this main feed. Its 15mm pipe to all outlets including the bath, but the bath takes ages to fill because we have to slow down the flow from the bath tap to get a hot bath! At the moment replacing the boiler is not a viable option.


We have had the following suggestions:


To improve the bath and shower available hot water and flow rate.

Fit a 300ltr vented hot water cylinder in the roof space - heated by the combi radiator side (like an additional radiator), so when the heating is on it also heats the water in this vented cylinder. Fit an immersion heater on a timer to ensure the water in this cylinder is hot enough when needed (even in the summer when the heating is off).

The hot water is then pumped to feed the bath and shower only (the other hot water outlets are still fed direct from the combi).

The main problem with this is the cost of running the immersion heater to maintain the water temp in the vented cylinder.

Does anyone have a better solution?

 

[Hugh Jaleak]

It is possible to fit the cylinder to be heated from the combi, using the heating output from the boiler on a 'Y' plan system. This would also have the advantage in the warmer months that the boiler could still be used to heat the cylinder without having the radiators on. Immersion would still be an advantage as a backup in event of boiler breakdown, but would not be required on a day to day basis in normal circumstances.

Smaller combi's can struggle in the colder months, the incoming mains water is at a lower temperature to begin with, the boiler can only put a limited amount of heat into that water as it passes through, so the resulting output of hot water from the boiler is often at a lower temperature than that desired by the user.

 

[oilhead]

Is your boiler a Worcester heatslave? If so, it should deliver 15 lts/min at 37degree rise for 90 lts. Slowing down the flow will improve temperature and delivery quantity. Experiment with a measuring jug to check your flow rate. A 3/4 tap will let a helluva lot of water through when only half open. If the water is hot enough at the basin tap, slow the bath fill match and wait for it to fill.
What setting do you have the hot water thermostat at?
Is the hot water flow as strong as the cold water flow?
Plumbing in and purchasing a mains pressure hot water cylinder will probably cost more than fitting a new boiler!!

 

[clittlewood]

Is your boiler a Worcester heatslave? If so, it should deliver 15 lts/min at 37degree rise for 90 lts. Slowing down the flow will improve temperature and delivery quantity. Experiment with a measuring jug to check your flow rate. A 3/4 tap will let a helluva lot of water through when only half open. If the water is hot enough at the basin tap, slow the bath fill match and wait for it to fill.
What setting do you have the hot water thermostat at?
Is the hot water flow as strong as the cold water flow?
Plumbing in and purchasing a mains pressure hot water cylinder will probably cost more than fitting a new boiler!!

Yes its a heatslave and the water stat is set to the highest temp. Both hot and cold flows are he same. It takes 15 to 20 mins to fill the big bath! because I have to slow the flow at the tap to maintain hot water temp. I want this fill time reduced by at least a third and I want a VERY POWERFUL shower. This boiler is only just up to the job, giving a less than perfect shower for a few minutes before we have to slow it down a bit to maintain the temp. I know a 30 or 40 kw boiler with a pump to increase the pressure by 1 bar would be the ideal solution, but I am exploring some alternatives that do not require a boiler change. Thanks tho!

 

[Charnwood]

Is your boiler a Worcester heatslave?

Blimey! Do you have this weeks jackpot numbers for the lottery?

 

[clittlewood]

Is your boiler a Worcester heatslave?

Blimey! Do you have this weeks jackpot numbers for the lottery?

Err, I don't get what you are on about, I have aspergers tho so that wont help . . .

 

[Burnerman]

Is it a daft idea to suggest that the burner is jetted for maximum output, or to at least check that it is indeed doing so?
John

 

[clittlewood]

Is it a daft idea to suggest that the burner is jetted for maximum output, or to at least check that it is indeed doing so?
John

The chap who services the boiler each year replaced it last March and he is very good, so I am assuming . . . (I know... never assume anything!) Good point tho and I will certainly check next month when he returns. Thanks.

 

[Burnerman]

I have a 12/14 Heatslave in my holiday place, and I must say I find it pretty impressive shower wise.....after all, the incoming water is bloomin' cold just now, but my rain head shower can still be too hot!
In the summer, the boiler can handle two showers at the same time.
Just for the record, mine is jetted .60 gph / 60 degrees EH nozzle - but that was in when I bought the place so I don't know what the output actually is.
John

 

[clittlewood]

I have a 12/14 Heatslave in my holiday place, and I must say I find it pretty impressive shower wise.....after all, the incoming water is bloomin' cold just now, but my rain head shower can still be too hot!
In the summer, the boiler can handle two showers at the same time.
Just for the record, mine is jetted .60 gph / 60 degrees EH nozzle - but that was in when I bought the place so I don't know what the output actually is.
John

Well that is a good reference for us, so thanks for that. Is it possible to fit a different sized nozzle? and if so, do you know what the highest output nozzle that is available to fit this boiler?

 

[Burnerman]

Its certainly possible to fit a different size of nozzle, but unfortunately I'm not the right guy to advise you on this one.
Keep bumping the topic up, and you'll get comments from people much more knowledgable than me!
Obviously there's a limit to nozzle size, as the boiler would overfire and burn the baffles away - but it would be nice to know what nozzle you are currently running.
John
Edit - On the limited info I have, it says for the full 14 kW output, its a .50 gph 60 deg ES nozzle, at 100 psi and 11.5% CO2.
Maybe the one fitted to mine is way too big!
J.

 

[oilhead]

0.50 x 60ES is the specified nozzle at 100 psi. Fitting a larger nozzle can overfire the boiler and cause damage especially as this boiler does not have a target plate.
We should trust the manufacturer to design his appliance correctly.

 

[clittlewood]

I have a 12/14 Heatslave in my holiday place...
Just for the record, mine is jetted .60 gph / 60 degrees EH nozzle - but that was in when I bought the place so I don't know what the output actually is.
John

So it would apear yours is operating at 120% of its design spec, have you (or whoever services it) noticed any damage at all?

I bet when this boiler was designed it was rated on the very safe side, so it may well be feesable to push the design spec by 20% with no damage. A bit like overclocking a cpu on a pc.

I would be interested in other readers thoughts on running it with a .60?

 

[Boilerman2]

The correct Nozzle size and oil pressure are noted on a green data badge which is on top of the grey control panel - you have to pull off the top casing panel to read it Worcester Combi's should be set at their maximum output in order to achieve best hot water temperature & rate

is the Grundfoss pump on the boiler set to Speed 3 ? if not set it to "3"

it could be that the Primary side of the plate heat exchanger is sludged up causing poor heat transfer to the secondary side

When set to "Hot Water Only" do the radiators start to heat up (even slightly) when a hot tap is operated? even a small heat leak around the radiators will dramatically reduce hot water performance if this is happening the Diverter valve will need replacing.

TBH the Mk2 Heatslave is one of the best performing oil combi's hot water wise!

 

[clittlewood]

The correct Nozzle size and oil pressure are noted on a green data badge which is on top of the grey control panel - you have to pull off the top casing panel to read it Worcester Combi's should be set at their maximum output in order to achieve best hot water temperature & rate

is the Grundfoss pump on the boiler set to Speed 3 ? if not set it to "3"

it could be that the Primary side of the plate heat exchanger is sludged up causing poor heat transfer to the secondary side

TBH the Mk2 Heatslave is one of the best performing oil combi's hot water wise!

Yes the pump is set to 3 and the Hot Water to the highest temp. The rads dont get any heat when its heating the hot water if the heating is off.

fyi I have just fitted a water softener to my system so this will quickly de-scale the pipework. The system has been powerflushed too as I thought that may well have been the case.