Electric versus LPG for hot water?

[Paul Goff]

On the face of it a simple question, but bear with me

I had a new boiler fitted in January, a Worcester Bosch 27i LPG Compact System boiler, this load modulates and can be weather compensated.
I noticed immediately that when set at the 60C (1st indent on the knob) point to allow maximum condensing it could not heat the contents of the hot water tank. The 20 year old previous boiler had no trouble with this when set to a 60C flow temp.
I was advised to try the 2nd indent (75C) and this improved matters but it really didn't work properly until set to maximum.

The boiler manual and the sales literature state that weather compensation is not possible with system boilers, but faced with the prospect of next to no condensing in my new boiler, I did a lot of research and was able to apply weather compensation, get it to work properly AND still heat the water.

I did this by using the hot water on demand signal from the tank controls to put a resistor in parallel with the compensation sensor. This dummies the boiler into thinking that the outside temperature is very cold, it therefore runs at maximum temperature until hot water demand is fulfilled and then compensated service is resumed. This works well

The disadvantage of this is that the central heating also runs at maximum boiler temperature until the central heating demand is satisfied, resulting in a large thermal overshoot in the house.

Here's the question (AT LAST! they all cried)
Given all the above, might it be cheaper to use the immersion heater, controlled by my existing timer via a suitable contactor, and just use the boiler for weather compensated central heating? The immersion would after all, just be heating the tank, not the pipe work and over heating the house.

Does anyone have any experience of making this comparison, or any learned sources I could look up?

Thanks in anticipation, Goffy.

 

[muggles]

Your best option would be to get the diverter kit installed on your boiler. This will allow you to run two different flow temperatures on your boiler - low-temperature for space heating (IE maximum efficiency) and a higher temperature for your cylinder. The boiler will heat the cylinder faster, and will also modulate down as the cylinder comes up to temperature, ensuring that it is putting just the right amount of power in. Installing this kit will vastly improve your boiler and give you the control and efficiency you're looking for

 

[Paul Goff]

Thanks for the reply Andrew,

Since the boiler installation, and reading the manual, I did conclude that may well be the case.
Unfortunately at the quotation stage it proved extremely difficult to engage the companies concerned in any sort of meaningful discussion about weather compensation, opentherm interfaces or anything else that smacked of modernity. Thus the opportunity to make the choice of a boiler with the diverter kit already already in the box (so to speak) was lost.
I got the impression that all that was underneath a rock and best kept there.

Is just the diverter kit needed, or will extra plumbing be required between the tank and the boiler? I don't really want to approach my installer (the least luddite of the four) about it without some idea of what could be involved. But at least I now have time on my side, the old boiler had been on a wing and a prayer for a few years

Thanks again, Goffy.

 

[muggles]

Very very few Worcester installers I've come across understand advanced technologies such as OpenTherm (which isn't supported at all by WB) and weather compensation (which is only vaguely supported). Hot water priority systems using the diverter valve I've suggested are available from WB to those who go looking for them, but don't seem to be advertised outside of the manuals. Even WB's top-level accredited installers aren't all aware of the availability of this diverter, but then that's because they get accreditation based purely on how many boilers they fit in a year. Their competence and technical knowledge doesn't seem to be of any interest to WB when choosing their accreditation level. It's one of the many reasons I don't recommend WB boilers - there are much better boilers out there, with better support, often for less money. Nevertheless, you can't easily go back now, so you must press on.

Installation of the WB diverter requires your existing motorised valve(s) to be removed, and the diverter serves that function instead. The cylinder sensor cable needs to be run from boiler to cylinder, and you may need to run an additional pipe between the boiler and cylinder as well. The hot water programmer is then put onto the hot water switch live connection on the boiler, and the heating programmer is put onto the central heating switch live connection. So, a bit of wiring and a bit of pipework.

 

[Razor900]

You seem really clued up Goffy. I'm surprised you picked that boiler!

Sadly I'm not so surprised that the installers didn't seem to understand what you wanted...

 

[Paul Goff]

Thanks again guys,

Prior to early retirement I worked in a strange little corner of the NHS as a Senior Maintenance Technician, primarily concerned with Building Management Systems, HVAC, lighting, alarms, CCTV, etc via the old Johnson Controls Metasis system and later changing over to Trend. We had 6 fitter/plumbers, 4 of them Gas Safe, they all did quite a bit of moonlighting and their consensus was that Worcester Bosch was the one to get. It was that simple really.

Faced with needing a new boiler now ish and thus time to winkle out all the relevant info being short, I went with WB. Short of down loading loads of boiler manuals and knife & forking through them it is very hard to find any info re' compensation, controls compatibility etc in sales literature. To even find out what Opentherm was/what it actually did involved an email to the Drayton Wiser tech desk!

Ultimately what is needed is Drayton to market an integrated smart hot water control thermostat as art of the Wiser range, when/if (Drayton will not be drawn into the subject) that happens I will get one of the Opentherm/WB protocol interfaces, properly link my controls to the boiler and remove my current fudge.

Meanwhile, back at the ranch, which is likely to be cheapest, run what I have now, the boiler running flat out to heat the water and thus the heating overshoot while it does so, OR heat the water electrically via Wiser and just use the boiler to provide temperature compensated central heating?

Cheers Everyone, Goffy.

 

[oldbutnotdead]

Not sure what LPG costs per kwh, your bill should have that info on it. That number (compared with your price per kwh of electricity) is the key- typically mains gas is a quarter of the price of electricity but everyone is on different deals. You can safely assume 99% efficiency from your immersion heater

 

[Paul Goff]

I have found a veritable mine of information
confusedaboutenergy.co.uk

In round figures,
LPG = £0.06/KWH
Mains Gas = £0.03/KWH
My electricity (OVO) £0.14+/KWH

In short, as I suspected, heating my tank using the fudge may not be the most efficient way, but with such a difference in cost, using the boiler on LPG has to be the best way!
And of course, any extra heat is going into the rooms anyway

Thanks once more chaps.

 

[oldbutnotdead]

Do check your gas bill to see what price you are actually paying- that 6p/kwh is prob for customers with big white tanks rather than small or large red bottles

 

[Paul Goff]

I have a big white tank 2250litres nominal capacity. I assume it's white, it is buried in the garden
But you are correct, bottled LPG is MUCH more expensive!