Flow pipe hot, return cold - is this right

[pistonheadforum]

In the house there are two pipes running up to the 1st floor for central heating - a flow and return pipe. Both are 22mm and this is then tee-ed of to 8mm microbore for 4 radiators and a single radiator on 15mm piping.

The flow pipe is hot and the return pipe is much colder. Is this right? Does this mean the radiators have given up all the heat in the water (presumably good) and returning room temp water? Or should the water be going back with some heat in it so the boiler does not have to keep adding heat?

The radiators in the upstairs bedrooms are giving of a fair bit of heat so I'm assuming this is correct?

Or is this an indication that the boiler should be on a higher setting - it's currently on 1.

What is the most efficient temp delta between the water leaving the boiler and water returning?

Thanks in advance.

Edited by pistonheadforum on Sunday 28th November 12:48

[mrmistoffelees]

That's correct. You're typically looking for a difference of around 20 degrees between the two. The return should be much cooler than the flow.

[sherman]

Yes you want cold water going back to the boiler. If its still warm you are not usimg all the heat from the boiler.

[pistonheadforum]

Thanks very much - boiler currently set on 1 - never changed from the summer.

What do most people run the boiler at over winter - never adjusted it and somewhat scared of a huge gas bill? Maybe each individual system is unique and no one-size-fits-all.

[pistonheadforum]

Is it worth replacing microbore 8mm with 15mm if the rads are all hot? Leave alone or replace the pipework - 30 years old in a soft water area that's recentrly been powerflushed to clear. Planning to rewire so the floors are likely coming up at some point.

I'm guessing 15mm pipe would give of more heat to the pipework (larger diamater means more loss of heat on journey?) leaving less heat for the radiators to release into the room? Or is this not correct.

Would you lag the pipes under the floors or use that to advantage effectivly giving a modicum of underfloor heating?

Thanks in advance.

[Mr Pointy]

How old is the boiler & do you know if it a condensing type? If it is then the return water temperature needs to be below a certain number for the boiler to work efficiently - usually around 55°C.

https://www.viessmann.co.uk/heating-advice/what-te...

The flow temperature is usually around 20°C higher. Does the boiler HW temperature contorl have any markings on it?

[98elise]

pistonheadforum said:

Is it worth replacing microbore 8mm with 15mm if the rads are all hot? Leave alone or replace the pipework - 30 years old in a soft water area that's recentrly been powerflushed to clear. Planning to rewire so the floors are likely coming up at some point.

I'm guessing 15mm pipe would give of more heat to the pipework (larger diamater means more loss of heat on journey?) leaving less heat for the radiators to release into the room? Or is this not correct.

Would you lag the pipes under the floors or use that to advantage effectivly giving a modicum of underfloor heating?

Thanks in advance.

As long as the heat is radiating into the house it's being used so it's not costing you anything more if they are uninsulated.

I wouldn't replace the pipework if everything is working fine.

[98elise]

sherman said:

Yes you want cold water going back to the boiler. If its still warm you are not usimg all the heat from the boiler.

That's not right. If your return water is cold then your flows are too low. Ideally it's about a 20 degree difference.

[98elise]

pistonheadforum said:

Thanks very much - boiler currently set on 1 - never changed from the summer.

What do most people run the boiler at over winter - never adjusted it and somewhat scared of a huge gas bill? Maybe each individual system is unique and no one-size-fits-all.

Running it higher in winter shouldn't result in higher bills. The heat is still going into the house, just faster and more efficiently. If you run it lower your radiators effectively have a lower output.

[sherman]

98elise said:

sherman said:

Yes you want cold water going back to the boiler. If its still warm you are not usimg all the heat from the boiler.

That's not right. If your return water is cold then your flows are too low. Ideally it's about a 20 degree difference.

Cold is a relative term.
If its still toasty then you are wasting gas heating it up in the first place. You need to balance your radiators to slow the flow if thats the case.
You want it only warm on the way back maybe a better terminology to use.

[pistonheadforum]

98elise said:

Running it higher in winter shouldn't result in higher bills. The heat is still going into the house, just faster and more efficiently. If you run it lower your radiators effectively have a lower output.

Firstly, thanks to all for advice.

This bit has confused me but then I probably don't fully understand I'm afraid.

The boiler is an old non-condensing boiler rated at 80,000BTU. I've converted this and think it's a 23kw rated boiler. It's not particular effeciant with the specs showing SAP of 65% efficiency. Actually that's a horrible effeciency rating.

The boiler is old and not great I get that! I'm trying to make do with the best I have. It has no smart controls.

The setting is currently at 1 and it goes all the way up to a max of 6.

I have 10 radiators in the house over 4 bedrooms with the TRV set low on unused rooms.

Is 1 a bit low?

What does this number relate to? Temp of water? Time boiler is on? How high the flame - is it like a gas cooker oven setting?

Would increasing the number not surely result in a massivly increased gas bill?

Thanks in advance for any light you can shed.

[98elise]

sherman said:

98elise said:

sherman said:

Yes you want cold water going back to the boiler. If its still warm you are not usimg all the heat from the boiler.

That's not right. If your return water is cold then your flows are too low. Ideally it's about a 20 degree difference.

Cold is a relative term.
If its still toasty then you are wasting gas heating it up in the first place. You need to balance your radiators to slow the flow if thats the case.
You want it only warm on the way back maybe a better terminology to use.

You said if it warm you're not using the heat from the boiler?

The return will feel hot. Flow should be around 75 degrees so the return will be around 55 degrees. You are not wasting heat, it isn't disappearing anywhere. It just goes back through the boiler and gets reheated.

If you reduce flow too much you will will be lowering the efficiency of your radiators.

[Mave]

pistonheadforum said:

Firstly, thanks to all for advice.

This bit has confused me but then I probably don't fully understand I'm afraid.

The boiler is an old non-condensing boiler rated at 80,000BTU. I've converted this and think it's a 23kw rated boiler. It's not particular effeciant with the specs showing SAP of 65% efficiency. Actually that's a horrible effeciency rating.

The boiler is old and not great I get that! I'm trying to make do with the best I have. It has no smart controls.

The setting is currently at 1 and it goes all the way up to a max of 6.

I have 10 radiators in the house over 4 bedrooms with the TRV set low on unused rooms.

Is 1 a bit low?

What does this number relate to? Temp of water? Time boiler is on? How high the flame - is it like a gas cooker oven setting?

Would increasing the number not surely result in a massivly increased gas bill?

Thanks in advance for any light you can shed.

The number relates to the temperature of the water coming out of the boiler. If you turn it up, the rooms will heat up quicker so the boiler won't run for so long.

[pistonheadforum]

Thanks.

I've discovered the thermostat/timer is not connected to the system - turning the dial would make no difference and it's been set to over 30 degrees C. Looks like it's been completely cut out the loop and no longer does anything.

However, the boiler is turning it's self off - and back on - and providing heat. What would be doing this? Is this regulated by the boiler in some way and if so where is it sensing this temperature from?

Thanks for all the assistance.

[pistonheadforum]

I appreciate this is all being done blind and who knows how a random system is configured! I'm just keen to understand the principles which I can then apply to my own case.

Many thanks.

[essayer]

In theory without a room stat
The TRVs on each radiator will close as the rooms reach the desired temperature
The return temperature will get hotter until it’s close to the flow temperature
The boiler will be trying to heat already hot water, its thermostat will close and it will stop heating

In practice if not all rads have TRVs (towel rails) or the room never reaches TRV temperature the boiler might be running for longer than it really needs to, so probably worth checking. I recommend fitting a wireless stat which you keep in the room with you

[Mr Pointy]

pistonheadforum said:

I've discovered the thermostat/timer is not connected to the system - turning the dial would make no difference and it's been set to over 30 degrees C. Looks like it's been completely cut out the loop and no longer does anything.

However, the boiler is turning it's self off - and back on - and providing heat. What would be doing this? Is this regulated by the boiler in some way and if so where is it sensing this temperature from?

The boiler has an internal thermostat that cuts the boiler off when the return water temperature gets to a certain point, otherwise it would keep adding heat & boil the water.

If you don't have an external timer how do you control the heating to come on? The stat can be tested by turning it right down when the heating is running; if it's connected the heating will switch off.

[pistonheadforum]

Mr Pointy said:

The boiler has an internal thermostat that cuts the boiler off when the return water temperature gets to a certain point, otherwise it would keep adding heat & boil the water.

If you don't have an external timer how do you control the heating to come on? The stat can be tested by turning it right down when the heating is running; if it's connected the heating will switch off.

It's always on it seems! The hot water tank is wired like a radiator with no TRV so it's always keeping the hot water tank hot. The rooms are regulated based on the TRV (so they do not provide heat in the summer).

Does the temp setting (1) on the boiler determine the temp at which it will turn off and on?

[pistonheadforum]

They system seems to working at the moment but I feel at could all pack in at any moment!

I'm just trying to determine wha the best course of action is until the spring when I a look to get the system inspected and (most likely) the boiler swapped.

I don't want to adjust too much as the bills are managable but the rooms are a bit colder than most - currently 17c or so in the cold spell.

[AW10]

pistonheadforum said:

Does the temp setting (1) on the boiler determine the temp at which it will turn off and on?

The thermostat and timer settings determine when the boiler will start heating. The knob on the boiler sets the output temperature of the heated water flowing out of the boiler. When the returning water is with a set number of degrees of the outgoing water the boiler stops heating (but generally keeps the pump running) as it perceives there’s no place for the heat to dissipate to. This protects the boiler and prevents it from overheating and/or boiling the water.

Are you sure there isn’t a thermostat strapped to the hot water storage tank?