Heating & Hot water function

[Big G]

Hi, a bit of background first. I Have a Gloworm Ultimate Balanced Flue boiler. The heating part is pumped whilst the hot water is gravity fed. I’ve mentioned in other posts that the plumbing is a bit dire and that my intention is to stop using the boiler to provide hot water and use the immersion instead.

The above plan is in action. It is a bit of a pain having to remember to switch the immersion on to get a tank of hot water but it’s a small price as it takes about 20 minutes to get enough water to bath the kiddies – the gas used to just keep firing for as long as the system was on and even then, the water temperature was variable.

Last night, as the weather has turned slightly cooler, I put the “Heating and Hot Water” function on to twice. The room stat is set at about 16 degrees and the temperature within the house is about 22 degrees (the stat clicks at 22, anyhow). If my understanding is correct, this means that the stat is not calling for heat.

Despite the fact that the stat is set lower than the room temperature, the boiler is still firing as soon as the time switch kicks in. The pump doesn’t operate until the room stat is turned up. Is this normal?

I would have thought that the “Heating and Hot water” function would heat the central heating while the room stat is calling for heat. As a by-product, convection within the boiler would send hot water up the gravity flow and this would heat the hot water. Is my interpretation wrong of how a gravity system works?

The programmer is a Honeywell, BTW. It has no model number on it but it is digital and is about 4 inch square. It has a slider for “Heating and Hot Water” and one for “Hot Water only”

I don’t intend to use the gas boiler for hot water ever again unless I convert it to fully pumped. I don’t have the wiring diagram for the programmer but, is there any harm in taking the power supply for the boiler from the same feed as the power supply for the pump?

Any advice would be greatly received. It’s not a problem at the moment as we don’t need the heating on but, in winter, if we go away we tend to set the room stat fairly low just to keep the house aired. I don’t want to be heating a cylinder (poorly lagged at that) of hot water for the sheer fun of it.

Cheers

G

 

[Agile]

You have correctly explained how it works in your Para 5.

I therefore do not understand your concern.

It may help you to say that when heating the hot water the gravity flow is very slow and the boiler will cycle on/off to maintain the set temperature. It will take about 90 minutes to heat the water.

I would still suggest you set the timeclock to allow at least an hour of heating by the boiler at a time when the cylinder will be cold.

You can use the electric to top up the temperature later.

Tony

 

[Big G]

I would have thought that the “Heating and Hot water” function would heat the central heating while the room stat is calling for heat. As a by-product, convection within the boiler would send hot water up the gravity flow and this would heat the hot water. Is my interpretation wrong of how a gravity system works?

Sorry, I may not have explained myself too well. What I am saying is that I would expect the above to happen.

What is actually happening is that, regardless of whether the room stat is calling for heat or not, the boiler is still firing up. This means that if I set the room stat to, say, 10 degrees C and go away on holiday; regardless of the actual temperature within the house, as soon as the programmer is ‘active’ (what’s the correct terminology for when the little red light comes on?), the boiler fires up. Turn the room stat so that the stat is saying, say, 30 degrees C, and the pump then kicks into life.

Does that make sense?


Cheers

G

 

[Agile]

Yes thats correct too !!!

The boiler comes on to provide gravity hot water and the pump comes on if both the central heating is selected AND the room stat calls for heat.

Tony

 

[Agile]

Perhaps you have difficulty understanding the requirement which causes the pump to run for CH ?

Perhaps they dont teach simple logic in schools any more? Its what all digital systens are based on. Any teachers care to comment?

Your heating system is an "AND" gate!


The AND gate is so named because, if 0 is called "false" and 1 is called "true," the gate acts in the same way as the logical "and" operator. The following illustration and table show the circuit symbol and logic combinations for an AND gate. (In the symbol, the input terminals are at left and the output terminal is at right.) The output is "true" when both inputs are "true." Otherwise, the output is "false."


AND gate

Input 1 Input 2 Output

0 0 0

0 1 0

1 0 0

1 1 1

In this example the time control for CH is "Input 1" and the room stat is "Input 2". The pump is controlled by the "output".

Tony

 

[Big G]

Perhaps you have difficulty understanding the requirement which causes the pump to run for CH ?

Perhaps they dont teach simple logic in schools any more? Its what all digital systens are based on. Any teachers care to comment?

Your heating system is an "AND" gate!


The AND gate is so named because, if 0 is called "false" and 1 is called "true," the gate acts in the same way as the logical "and" operator. The following illustration and table show the circuit symbol and logic combinations for an AND gate. (In the symbol, the input terminals are at left and the output terminal is at right.) The output is "true" when both inputs are "true." Otherwise, the output is "false."


AND gate

Input 1 Input 2 Output

0 0 0

0 1 0

1 0 0

1 1 1

In this example the time control for CH is "Input 1" and the room stat is "Input 2". The pump is controlled by the "output".

Tony

Thanks for the replies. I’m not sure whether they teach ‘AND’ gates at school any more. They certainly didn’t when I was at school 20 odd years ago; maybe I’ve forgotten it though .

I have since learnt all about AND operators and use them regularly in my profession. Mainly Conditional AND operators but sometimes Boolean AND operators. If in doubt I use the Boolean AND but this does have an overhead. I could go on about the OR operator, whether that is the Conditional OR, Boolean OR or the Exclusive OR operator.

Anyhow, not bothered about the pump, I’m bothered about the power to the boiler. Using the example with Input 1 and Input 2 but where the output is the wire that goes to the boiler. Not sure what this does – probably operates a solenoid to keep a gas valve open.

Input 1 Input 2 Output

0 0 0

0 1 0

1 0 1

1 1 1

In this example the time control for CH is "Input 1" and the room stat is "Input 2". The boiler is powered by the "output". As you can see, this does not fall into either an AND or an OR gate.

I do not want to heat up my water using the central heating boiler, unless it is as a by-product of the radiators being heated.

Why? Two reasons:
1. The domestic hot water side of my system is extremely in-efficient. Over the summer it has cost £40 a month to heat a 4 inch deep bath of water once a day. The immersion heats enough water in the cylinder in 15 minutes to give a bath. Assume a 3kw immersion and, for the sake of ease, it takes 20 minutes. That’s 1 KWh of electricity – about 14 pence. That’s over a pound a day less than currently.
2. When I go to Oz for two months over the winter, I will set my programmer to continuous and the room stat to ten degrees. I would expect the heating to fire up occasionally during the day and night. Whilst it is heating, I’m not too bothered that the cylinder is also getting heated. However, I do not want the cylinder to be getting heated when the room stat is not calling for heat.

Does that make sense?


Many thanks in advance

G

 

[Agile]

No, sorry, there are incorrect logic statements there!

Just considering the boiler on its own, the supply goes via the boiler stat and is only applied to the gas valve when the boiler is below the set temperature.

The supply to the boiler is taken DIRECTLY from the timeclock ! The boiler comes on when water is selected and stays on when Water + Heating is selected. Selecting heating enables the pump via the room stat.

Selecting HW only for a period of an hour will be reasonably efficient.

Leaving the heating on 24/7 means that the boiler is powered all the time and when it reaches the set temperature the burner goes off and it then acts a a radiator and loses heat through its flue. THAT is massively inefficient!

Tony

 

[ChrisR]

G the solution is just a simple bit of rewiring.
Just wire the "CH ON" output from the timer in series with the room stat, then to the boiler "ON" input. You won't need the HW output from the timer.