heating CH water from cold

Andy From Sligo

sorry, I know I am a pain for keep asking the questions but I am just curious.

Outdoor Boiler. If you heat up cold water in the boiler is it less efficient than heating up warm water?

Say a scenario like this if you fit another thermostat to keep the water in the boiler/system constantly at say 21c 24hrs and you had your main boiler thermostat at 60c - would it be more economical because you are heating the water from 21c to 60c ... or would it use more oil in the long/short run than just heating up your water from cold to 60c

I understand if I am peeing people off with all my questions and i understand if you dont want to answer. And no, I am not planning on doing it/modding it before you ask, it just got me thinking, and also thinking if some people have set their boiler up to work like this?

K.Flyer

My oil boiler is outdoors in a boiler-house with 4 adults using showers and to be honest I experimented with this same notion.
Long story short, keeping the boiler "warm" 24 / 7 used a lot more fuel as it still ramped up to the where the boiler stat was set at.
My system is split HW / Heat, and I did this during the summer when the only requirement was hot water.
In the end I found the time getting the coil on the cylinder hot was a little quicker with the warm boiler, but the difference was the wasted fuel burning for short spells keeping the boiler warm at all other times outside of demand.

K.Flyer

I also experimented with the leaving the boiler on all the time for hot water, definitely much higher consumption.
And I experimented with the theory of the immersion being left on, thereby keeping the cylinder "topped up" with hot water, that was another expensive lesson.

TheChizler

Think of it this way; when the boiler is warm it's losing heat to the elements. If you keep it constantly at 21 degrees and sometimes heat to 60 it'll always be losing some heat and sometimes lose lots of heat. If you only heat it when you need it at 60 it's only sometimes losing lots of heat.

Andy From Sligo

how about this kind of setup - would this be more efficient or less efficient?:

Here is our current setup (the way the boiler is wired up at moment):



Roomstat calls for heat - zone valves open up - cold water goes to return of boiler - CH pump turns on as does burner (until it reaches 60c where burner turns off) so cold water flows through rads gradually rising up to 60c.

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How about this as an alternative - could this be more efficient? :



CH pump off - boiler heats up to 60c then cuts off - water in boiler is stored at 60c - when room is cold, roomstat calls for heat - sends live to CH pump - pump and required zone valves are on - takes the 60c stored water as opposed to pumping cold water until boiler reaches 60c.

The only things are that the water will be stored in outside boiler and heat could dissipate , but then again these outdoor boilers are insulted very well.

number 2 issue what about the rads being cold one minute and then getting 60c of hot water pumped into them (or rather dragged through them because the CH pump in our system is on the return and not on the flow side) could this cold one minute and 60c the next stress the metal, leading to metal fatigue?

the way i look at it is that if you have to heat up a boiler from cold , you would obviously want to heat up the water in the boilers water channels as quickly as possible wouldnt you? and how can you do that if that when the burner is running cold water from the radiators keeps returning back to the boiler from the rads whilst the burner is trying to heat up the water in the boiler? - it would be like trying to boil a kettle whilst keep adding cold water all the time wouldnt it, and then taking much longer to heat up/boil than it should do.

DGOBS

Keeping the water at 21ºc in the boiler would cause condensation in the primary heat exchanger and rot the boiler.

Andy From Sligo

DGOBS wrote: »

Keeping the water at 21ºc in the boiler would cause condensation in the primary heat exchanger and rot the boiler.

ah right thank you , never thought of that. - condensation not go out through flue or condenser trap outlet then no?

DGOBS

if your return water hits the heat exchanger at too low a temperature, hence a cold heat exchanger (ferrous) the flue gases will condensate there rather than in the secondary heat exchanger, and on out the condensate trap.

Andy, when I see your posts coming up, I always have to start from the point of 'why you wouldn't do that', rather than the merits of them. One day, you'll hit on an idea that (if it doesn't kill you) will potentially make you millions, but as you disclosed it here first, one of us will beat you to it!! lol

jimf

I can see the headlines tony

andy the brain picker from Sligo makes millions

Andy From Sligo

Oh please - I'd really love to be a millionaire, or half a millionaire .... or any aire!

jimf

ever think of taking up oil boiler servicing andy I think u would be good at it at this stage

Andy From Sligo

jimf wrote: »

ever think of taking up oil boiler servicing andy I think u would be good at it at this stage

Thank jim - how many years training are we talking? 4 years is it. and I be no good on theory or exams only practical .

ive done my schooling a very long time ago, dont fancy going back to school/college

back permitting (got a couple of slipped discs that give me jip a lot of the time) do you know I most probably would like it, Im fascinated in it anyway