Rate of increase in boiler flow temperature

[Agile]

I get 288l now, which is a lot of water! Did I divide by 4 twice in my head? Oh well, so much for that.

Good, so the typical 70 litre capacity system will reach 70 C in about 15 minutes if it starts at 20 C so in 15-20 min it will have nearly reached the set 75 degrees.

Of course there is the heat loss which is somewhat proportional to the temperature difference between rad and room. That will only begin to be significant as it starts to get to about 50 C as below that there will be little temperature difference and little thermal convection currents which increase the heat output.

Back to the OP. If you are using significant hot water late at night and shortly before heating is timed off then increase the morning reheat to about 40-50 minutes as at 1700 30 minutes was not enough.

You need to have the hot water fully hot by the time the CH is required. Then have the hot water OFF for the first hour of the CH to enable the system to heat up. The HW can be timed to be back on after that hour because by then the house should be warm.

The boiler cycling will be a feature of an over powered boiler with a minimum power output of perhaps 10 kW so it will cycle a lot. Its important in your situation to set ip up in the best way. Can you time the on and off cycles on CH only, once its hot, so that we have an idea. Time it over about three cycles to get a better average.

Tony

 

[bw65587]

Back to the OP. If you are using significant hot water late at night and shortly before heating is timed off then increase the morning reheat to about 40-50 minutes as at 1700 30 minutes was not enough.

Hi Tony. I have had the CH & DHW on the same timed setting from 16:00 until 21:00. We don't use a lot of water in the evening so it is a waste heating water we're not going to use much of I suppose! I seem to remember a BG engineer telling me something like 'put them on the same timing because if you're heating the house, you must be in so would need hot water also'.

Can you time the on and off cycles on CH only, once its hot, so that we have an idea. Time it over about three cycles to get a better average.

It has separate timings for CH & DHW and it can be on & off every 15 mins all day so I believe so I'll do what you say.

 

[Agile]

There is a lot of misunderstanding about heating hot water.

Modern cylinders lose VERY little heat! Its often as little as 1 degree overnight or 1-2 kWh per day.

With an oversized boiler its probably actually more efficient to heat the hot water the evening before when the heating is on, rather than running the boiler on its own in the morning just for HW!

Tony

 

[bw65587]

Modern cylinders lose VERY little heat! Its often as little as 1 degree overnight or 1-2 kWh per day.

Hi Tony. I don't know whether you noticed in an earlier post that our cylinder is 162 litre 48" x 18" and says on it that heat loss would be 2.74 KW per KWH/24h which probably confirms what you're saying. I don't understand the actual realities of the calculation though! Neil

 

[D_Hailsham]

There is a lot of misunderstanding about heating hot water.

Modern cylinders lose VERY little heat! Its often as little as 1 degree overnight or 1-2 kWh per day.

With an oversized boiler its probably actually more efficient to heat the hot water the evening before when the heating is on, rather than running the boiler on its own in the morning just for HW!

I found this out recently when the boiler packed up (thermistor gone) and we had to rely on the immersion heater.

Originally I had boiler timer set with hot water running from 5.30am to 6.30am and then from 6.30pm to midnight. However the morning the boiler packed, we found that there was still enough hot water in the cylinder when we got up at 6.30am and only needed to turn the immersion on in the evening.

I now have the timer set for 9pm to 12.30am (which covers evening showers and leaves plenty for the next day) and 12pm to 1pm (tops it up, if required, for the lunchtime and evening washing up).

 

[bw65587]

I altered this evenings timed setting so DHW on from 17:00 to 17:30 followed by CH from 17:30 onwards.

In the half hour, the DHW rose from around 40 C to 48 C, not to the required 55 C. Additionally, the rads warmed up slightly?

Interesting stuff being put forward here. Another 30 mins would have done it. It's really making me think (sadly I know!) about when and how much water we use and when we should really be heating it!

Tell me, when I had the CH off in my scenario above, why was there heat felt at the radiators when they weren't due on until 17:30. Faulty zone valve or is this to be expected? Shouldn't they have stayed cold? Neil

 

[D_Hailsham]

I've calculated that I have 14,376 watts of existing radiators.

I also have a 48 x 18 DHW cylinder (162 litre) that I would need to add on. Is that 2kW to add on???

Now, an online whole house calculator comes out with 12kW.

Does the 12kW from the online calculator include 2kW for hot water - it will do if you have not deducted it. In which case, you only need 10kW for heating, so another 6KW for the HW cylinder is more than enough.

Your boiler is designed to work with a 20°C temperature differential. If you run with an 11°C differential the water flow rate has to be nearly twice as much. The graphs on page 16 of the Hxi Manual shows the pressure loss through the boiler. It's high enough already at a 20°C differential; at 11°C it would be horrendous.

However, with your boiler set to 16kW, the flow rate is lower and the pressure loss will be less.

16kW at an 11°C differential is a flow rate of 1253litres/min; 16kW at a 20°C differential is a flow rate of 688lmp. You can see from the graph the difference in the pressure loss.

There is however a downside in running with a 20°C differential - radiator outputs are lower. Fortunately your rads seem to be oversized - 14kW, when only 10kW is required - and this can compensate for the lower output.

Rads outputs are measured with flow temp of 75°C and return temp of 65°C (10°C difference). If you are running with a return temp of 55°C (20°C difference), the radiators would have to be oversized by 25% to provide the same heat. With 14kW of rads and only 10kW required you are 40% oversized, so you can easily run at 20°C differential (75°C/55°C).

I see you have tried this already. You should find that this will still allow you to heat the HW to 60°C. (55°C is too low as it does not provide the necessary protection against the Legionella bacteria).

 

[bw65587]

Does the 12kW from the online calculator include 2kW for hot water - it will do if you have not deducted it. In which case, you only need 10kW for heating, so another 6KW for the HW cylinder is more than enough.

The 12kW did include 2kW for the water. If the CH and the water are set to never be on at the same time, does that mean that the boiler can be down rated further because it will never be needing the 10kW & 2kW at the same time?

 

[D_Hailsham]

The 12kW did include 2kW for the water. If the CH and the water are set to never be on at the same time, does that mean that the boiler can be down rated further because it will never be needing the 10kW & 2kW at the same time?

In theory you are correct, but dont forget that the boiler will automatically adjust its output between 6.3kW and the maximum you set according to requirements. So it does not matter too much if you set the max higher, say 14kW (output of rads). This would give you spare capacity when the outside temperature drops below -1°C (this is used when calculating boiler and rad sizes).

 

[bw65587]

Apologies for not posting a reply for a week but have been trying to work out the behaviour of the system.

I have found that whilst the HW & CH are on at the same time and they are both calling for heat, all things are well. However, if HW is satisfied, boiler starts cycling even though CH is calling for heat. This happens whether the boiler is rated to 12kW, 14kW or 16kW.

However, if I increase the set boiler temp from 75 C to 82 C, I can get it to stop cycling.

I have repeated the above with HW on outside of CH timings and the same cycling happens when just CH on.

So, question is, do I put up with it at 82 C to stop the cycling?

When BG changed the pump the other week as they said it wasn't circulating properly, he asked me if I had altered a manual valve which I assume is a bypass. I hadn't so told him that. He said it had been either fully closed or fully open (can't remember which!) and said that it should be a quarter turn which he altered. Has this caused this situation?

Finally, read a 38Hxi manual last night and it says that the 38Hxi model doesn't need to be range rated as it is fully modulating. Why does it say that when ours would take the roof off if we had it set to 38kW? Or is that another topic altogether???

 

[D_Hailsham]

Rightly or wrongly, I wanted to see what would happen once the cylinder stat setting was reached so I turned it down slowly until it clicked at around 48 C. At that point the boiler went off despite the CH stat being at 25 C and the house was at 22 C according to a thermometer next to the stat. Maybe that's what it should do, I don't know?

I know I am going back to something you said on the first page, but nobody seems to have picked this up.

Just to clarify what happened:

CH and HW are both calling for heat, so the boiler is running.
You turn the HW stat down to stop demand for HW heat
The boiler stops, despite the continuing demand for heat from the CH.

Then something is wrong!

Possible causes:

Faulty motorized valve (how many do you have?)
Faulty Room thermostat
Faulty CH programmer.

 

[bw65587]

I altered this evenings timed setting so DHW on from 17:00 to 17:30 followed by CH from 17:30 onwards.

In the half hour, the DHW rose from around 40 C to 48 C, not to the required 55 C. Additionally, the rads warmed up slightly?

Tell me, when I had the CH off in my scenario above, why was there heat felt at the radiators when they weren't due on until 17:30. Faulty zone valve or is this to be expected? Shouldn't they have stayed cold? Neil

I don't think anybody actually answered my question above which I presume is now relevant to your diagnosis re: zone valve?

There are 2 Honeywell two port zone valves. Interestingly, I now remember BG replaced the CH zone valve a year ago when it broke after only 12 months. Maybe this has gone again for some reason. Tell me, should I be seeing the manual lever moving to and fro on the CH zone valve as it never seems to move yet the HW zone valve I know for a fact moves to and fro?

Cheers,
Neil

 

[D_Hailsham]

I altered this evenings timed setting so DHW on from 17:00 to 17:30 followed by CH from 17:30 onwards. In the half hour, the DHW rose from around 40 C to 48 C, not to the required 55 C. Additionally, the rads warmed up slightly?

Tell me, when I had the CH off in my scenario above, why was there heat felt at the radiators when they weren't due on until 17:30. Faulty zone valve or is this to be expected? Shouldn't they have stayed cold?

I don't think anybody actually answered my question above which I presume is now relevant to your diagnosis re: zone valve?

Yes, the rads should have stayed cold.

There are 2 Honeywell two port zone valves. ... BG replaced the CH zone valve a year ago when it broke after only 12 months. Maybe this has gone again for some reason. Tell me, should I be seeing the manual lever moving to and fro on the CH zone valve as it never seems to move yet the HW zone valve I know for a fact moves to and fro?

Yes, you should see (and hear) the CH zone valve move. It's unusual for a valve to fail after a year, particularly Honeywell, so we nned tpo look elsewhere for the cause. (Thought: did BG just assume the valve was a faulty or did they carry out proper diagnostic tests?)





Cheers,
Neil[/quote]

 

[bw65587]

Yes, you should see (and hear) the CH zone valve move. It's unusual for a valve to fail after a year, particularly Honeywell, so we nned tpo look elsewhere for the cause. (Thought: did BG just assume the valve was a faulty or did they carry out proper diagnostic test?)

Well this would be the second failed valve in as many years if this is the cause. Can't commit on what BG did. I just remember the boiler failing to stay on at all and they replaced this valve to fix. I've booked an engineer to visit tomorrow morning. I'll let you know outcome.

 

[D_Hailsham]

I've booked an engineer to visit tomorrow morning. I'll let you know outcome.

First he should check that there is 240v on the grey valve wire.
Then he should check that 240V is on the orange wire from the valve (this fires the boiler) when the programmer and room thermostat are calling for CH. If that's OK the valve is working, but there could be leakage past the valve, causing the rads to warm up.

If he does not get 240V on the orange wire he should check that the programmer is supplying 240V to the room thermostat when heating should be on. (If it doesn't the programmer is faulty).

If that's OK, he should next check that the valve is being supplied with 240V from the room thermostat when it is turned up - this is what opens the valve. (If it isn't, the wall stat is faulty).

If 240V is getting to the valve motor and it doesn't open the valve, then either the motor is faulty or the valve is sticking. He can check this by removing the actuator (box on the top) and manually rotating the valve shaft. He would also be able to see if the cog wheel of the motor is turning.