Replacement radiator is colder at bottom than it is at top.

[fluff29]

Hi Guys

Can we forget all my previous posts on temperatures, etc. The thermometer that I had was naff. Here are the proper readings.

- Alpha CB28X boiler

- Boiler is in the kitchen. Standard 2 floor house. Kitchen leads to dining room then living room (divided by an archway). Hall, bathroom and 3 bedroom upstairs.

- Boiler on full, trv's all removed except for the spare bedroom as this TRV was not shutting off the other week when plumber tried to remove rad (TRV closed fully).

*Boiler - Flow F87.8 Return R43.7
*Kitchen - F79.3 R49.7
*Dining Rm - F81.9 R41.9
*Living Rm - F84.0 R50.3
*Bathroom - 80.3 R74.5
*Hall - F82.2 R49.1
*Daughters room - F80.7 R56.0
*My room - F79.7 R51.3
*Spare - did not balance this as the TRV does not turn off when I open or close LS.

When I set the LS to the above temps, I was getting good heat in every rad except for the problem one in the dining room. When I had all rads off except for the kitchen and the dining room, the boiler was pumping the water but the hot water never went to the bottom of this radiator. My daughters rad is a bit like this but it has improved more than it was before (ie a bit warmer at the bottom but not boiling).

As you can see from the temperatures above, there is a big difference in temperature between the flow and returns. If I take the living room rad for an example. There is 33.7 of a difference but the radiator was roasting warm at this point and I saw no need to open the LS any further as surely this would take the flow from the other rads?

I got the above readings on friday night. When the heating came back on on saturday night, I found that some of the radiators were not heating up all the way to the bottom. Ie I had to open the LS on my bedroom, bathroom and hall. Otherwise, the rooms were not heating up.

Also, the dining room (problem rad) LS is fully open and yet the rad does not heat up properly to the bottom, even on start up when the room is cold. I also took surface temperatures from the radiator on friday. All temps taken in the centre of the rad length. Top 63.1, Middle 57.6, Bottom 48.5.

I am really confused as to what could be the problem.

I really appreciate everyone time and effort taken to try and help me with the problem. The one good thing is that I am beginning to understand a bit more about how the system works.

Fluff

 

[D_Hailsham]

Alpha CB28X boiler

This boiler is designed for a 20°C temperature difference, so you need to use this temperature when balancing.

Boiler on full, trv's all removed except for the spare bedroom as this TRV was not shutting off the other week when plumber tried to remove rad (TRV closed fully).

Some TRVs do not shut the boiler off completely. The lowest setting, often show as *, is a frost setting. If you want to close the TRV valve completely, you either have to use a "decorators cap" or insert a coin inside the head so it allows the pin to close completely.

Did you remove the head and check that the pin would go down and spring up?

Not sure why you did not remove the TRV head in the spare room as the instructions say you have to remove all TRV heads when balancing (this is to stop the TRV operating).

*Boiler - Flow F87.8 Return R43.7

That's a 44°C difference! Which is much too high. Set the pump to a higher speed.

Spare - did not balance this as the TRV does not turn off when I open or close LS.

You still need to balance this rad, otherwise the rest of they system will not be balanced. You must remove all TRV heads when balancing.

As you can see from the temperatures above, there is a big difference in temperature between the flow and returns. If I take the living room rad for an example. There is 33.7 of a difference but the radiator was roasting warm at this point and I saw no need to open the LS any further as surely this would take the flow from the other rads?

This does not happen as the flow through a radiator is very much lower than the flow through the main pipe. To use a crude analogy, if the main pipe run is equivalent to a motorway, a radiator is like a C road. One car leaving the motor way to go down the C road will not affect the flow of traffic on the motor way.

To put it into numbers, a 28kW boiler has a flow rate of 20 litres/min. A 1kw radiator will have a flow rate of 0.7 litres/min.

the dining room (problem rad) LS is fully open and yet the rad does not heat up properly to the bottom, even on start up when the room is cold. I also took surface temperatures from the radiator on friday. All temps taken in the centre of the rad length. Top 63.1, Middle 57.6, Bottom 48.5.

There is always the possibility that the radiator is full of sludge, which will settle on the bottom. Have you removed the rad and flushed it through with a hose?

 

[fluff29]

Hi D_Hailsham

I will try to answer some of the points that you raised:-

The plumber who fitted a new rad in my spare bedroom had to drain the pipework because when he turned off the LS and TRV, water was still coming out of the pipe when he disconnected the rad to replace it. He told me that the TRV was goosed - I was therefore not sure if it would leak when I removed the TRV. When I first tried balancing last week, I closed the LS on this rad & just fully opened up the TRV. When I did this, the rad was pumping out heat on this rad at full pelt, therefore the LS was not closing off the TRV. When I balanced the whole system on friday, I turned the LS on this rad fully off and also kept the TRV and set at 0 otherwise, if I removed the TRV, I am assuming that the heat would still have came through. (This is only my stupid woman assumption, please forgive me if I am wrong).

Am I able to adjust the speed on the pump easily or does it involve taking the pump out and things. I would be comfortable taking on the cover and turning a screw but not taking a pump out.

What if the pump is already at full speed? Could the pump be broken?

Thanks

Fluff

 

[kevplumb]

fluff29 wrote:
Alpha CB28X boiler

Quote:
*Boiler - Flow F87.8 Return R43.7

That's a 44°C difference! Which is much too high. Set the pump to a higher speed.

leave the pump alone the cb28x is a COMBI

 

[D_Hailsham]

Can we forget all my previous posts on temperatures, etc. The thermometer that I had was naff.

What type of thermometer are you using?
What made you suspect it was naff?
How was it tested?

I am very suspicious about the high temperature differentials you are getting.

 

[fluff29]

Hi there

the first thermometer I used was a very old one from work. It is at home at the moment so don't know the name. The one I am now using which is the 2nd set of temperatures that I reported is a brand new Hanna Instruments yellow thermometer with a corresponding probe which I push in slightly against the pipe to get a reading. The actual m/c has an on/off button, reset, etc.

When I tested for flow and return readings at the boiler. I waited until the boiler switched on and started pumping the hot water, I held the probe on the flow pipe until it reached it's hottest temperature. Much the same with the return. But I was unsure when I should have taken the reading with this, so I waited until the next time that the boiler kicked in again and then I gave the reading of when the return pipe became the hottest.

Oh no. Have I been doing it wrong??

I thought this would be around the correct temps as the boiler book says that the boiler has a flow of 82 when on full and I got a temp of 87.8 with the new probe.

The reason that I thought that the first probe wasn't correct was because it took that long to heat up/cool down that the boiler had stopped pumping water before the thermometer had reached a constant temperature figure. The 2nd hanna probe is quite instant on it's readings.

My apologies if I have been wasting everyone's time.

The good thing was that the rads seem a lot better all over the house apart from the problem one still in the dining room.

However, I had the heating turned up to 7 on the boiler stat last night. (it was frosted in Scotland - around 2 deg). The house was generally warm enough (comfortable with a jumper on) but not roasting. My concern is come winter I will need to have the stat at 9 and have to keep the boiler running all the time to obtain a comfortable temperature in the house.

This boiler is the same size boiler with approximately the same size and double panels as my previous house. My other house was slightly bigger. I have a Worcester Bosch 28i junior in my other house and yet I never ever had the stat above half way even on a very cold winter day. Yet with this alpha boiler the house isn't as warm except when stat up to almost full. This is the part that I don't understand. Why both 28kw boilers don't give out the same heat.

Could it be that the Alpha boiler is generally not very good? or is it that there is a possible problem with either the system or the boiler?

If this is the capacity of the boiler then I will need to either put up with it or get a new one.

If there is a problem, then I will need to get a heating engineer in to look at at. I think I have got beyond my capabilities.

I really appreciate all this good advice that everyone is giving me. I am starting to understand how things work a bit better.

Fluff

 

[D_Hailsham]

Thinking about it, I am wondering if your system is properly sized for the heatloss.

Can you do two things and post the results?

1. Use the Boiler sizing wizard to find out what size boiler you need to heat the house. Your combi will be much bigger but don't worry, that's because it has to heat the hot water from cold virtually instantaneously. Post the result

2. Measure up all your radiators (height x width in mm) and post a list. There are three basic types of rad: K1 - single panel with fins; P+ - two panel with fins on one panel; K2 - two panel with fins on both panels. (I am assuming that you have the standard flat panel rads, not fancy designer ones.) Please state the type of each rad.

 

[fluff29]

Hi there

Please find the info requested.

1. 12kw boiler required.

2. Rad sizes:-

Kitchen H700 x W550mm - K2
Dining Rm H600 x W1600mm - K2 (this is the rad which won't fully heat even when room not up to temp)
Living Rm H600 x W600mm - K2
Hall - H700 x 750mm - K2
Bath - H700 x W400 mm - K2
Bed 1 - H530 x W1540 mm - K1
Bed 2 - H600 x W1200 mm - K2
Bed 3 - H600 x W1100mm - K2

Speaking to my boss last night - electrician to trade. His thoughts were possibly the pump or the by-pass had a fault. He couldn't understand why the big rad in the dining room would not fully heat even on the start up of the boiler. Also, why the boiler needs to be at number 7 to get a reasonable heat into the house.

Thanks again for looking at this for me.

Fluff

 

[D_Hailsham]

12kw boiler required.

Assuming you have not deducted the 2kW it allows for a hot water cylinder, your heating requirement is 10kW

The rad sizes add up to approx 13kW, which is OK as they are not producing that amount of heat. The reason for this is the way radiators are tested. This is done with a flow of 75°C, return of 65°C, i.e a 10°C difference. Your boiler works with a 20°C difference, so the radiator outputs will be lower. If you set the flow temp to about 72°C, the return should be about 52°C and your 13kw of rads will produce about 10kW of heat.

One problem is the size of the boiler. It produces between 11.2kW and 28kW; you only need 10kW. That means the boiler will come on, initially at full output and quickly reduce output to the minimum. But that is still to much, so the boiler shuts down for a few minutes and tries again. (This will go on indefinitely and will be even worse when the weather is warmer.) That means that there is not a steady flow of hot water through the rads, so the house will take a long time to heat up. The dining room radiator is the largest, so it will take the longest to heat up.

The ideal solution is to either change the boiler to a combi which can modulate lower than 11.2kW, or get rid of the combi and install a regular 12kW boiler and hot water cylinder.

This is obviously a drastic, and expensive, solution, so I would first try dropping the flow temp to 72°C and balance the rads to have a 20°C difference. They will feel slightly cooler, but the dining room one should give out more heat.

He couldn't understand why the big rad in the dining room would not fully heat even on the start up of the boiler. Also, why the boiler needs to be at number 7 to get a reasonable heat into the house.

It's due to an incorrectly sized system as explained above.

 

[fluff29]

sorry, but I don't understand.

My previous house, approx same size, same amount & size of rads with a worchester 28i junior. I never had this problem with the boiler being oversized before. Why am I now getting this problem with this boiler?

If i am reading your post correct, i can never achieve 13kw from my rads because the boiler is switching on and off because the boiler reaches temperature but this means that my rads never get a constant flow of hot water and the big rad will therefore never get enought hot water flowing through it. this makes sense.

what do you mean by drop the flow temp to 70degs? do you mean at the boiler? how do i do this?

thanks again

fluff

 

[kevplumb]

Assuming you have not deducted the 2kW it allows for a hot water cylinder, your heating requirement is 10kW

on a combi

 

[D_Hailsham]

My previous house, approx same size, same amount & size of rads with a worchester 28i junior. I never had this problem with the boiler being oversized before. Why am I now getting this problem with this boiler?

The WB 28i Junior CH output modulates between 7.2kW and 24kW. Assuming your other house needed 10kW, this means the boiler can modulate low enough. Your boiler can only modulate down to 11.2kW, so it will always be providing more than the rads can handle.

what do you mean by drop the flow temp to 70degs? do you mean at the boiler? how do i do this?

Turn the boiler temperature down by using the right hand "thermostat" dial. Try one division at a time and check the flow temperature.

Assuming you have not deducted the 2kW it allows for a hot water cylinder, your heating requirement is 10kW

On a combi?

Yes. I asked the OP to use the on-line boiler sizing wizard to calculate the size of boiler she needed. This wizard assumes a regular boiler, so it includes 2kW in the result for the HW cylinder. I was checking if the OP had left the 2kW in her reply or deducted it, as it was the CH requirement I was interested in.

 

[fluff29]

Thanks for that - fully understand now re my old house. I thought all boilers were the same if they were the same size.

I will need to test the temperatures tonight after turning back the therostat dial. The figures that I quoted after the 2nd time that I balanced with the new probe were down with boiler on full.

I am currently operating the boiler at around number 6 (out of a possible 9). I am unsure what this is giving as a flow and return at the boiler.

What I do know is at this setting of 6, the problem rad is still not heating to the bottom. Also, at this setting, the house will take a long time to heat up on really cold days.

Is it possible on cold days because of this problem that I will have to leave the boiler on alot longer? Normally I would have my boiler on in my old house in the morning for 1 1/2 hours and then in the evening from 4.30 - 9.30 ish. I am thinking if I kept the same pattern with the boiler, then the house will not heat up to the same temperature because the dining rm rad is not heating to the bottom.

If I have picked this up correctly, it means that either:-

1. I get a new boiler

or

2. I leave the heating on for alot longer at the setting which gives out 70 deg or possibly all day?

Is it going to cost me alot of money if I have to keep the heating on all day? It may be cheaper to get a new boiler?

Thanks again for all your help.

 

[fluff29]

Sorry, just thought of another question.

I turned my boiler down to number 2 the other night. When I did this, the rads were only luke warm at the top and that was with every radiator fully open and it did not heat up the room. If number 1 give 11.2. kw output and my rads come to 13kw with them all turned on fully, why were the rads only luke warm?

I am just trying to understand this and I know that I am looking at this simplisticly and there will be some other factors to take into account.

Just want to try and find the most cost effective way to use my boiler but still stay warm.

Everything that you are saying seem logical and I can see that this is why my system is doing this.

Thanks again

Fluff

 

[D_Hailsham]

What I do know is at this setting of 6, the problem rad is still not heating to the bottom. Also, at this setting, the house will take a long time to heat up on really cold days.

Altering the temperature using the dial on the boiler is only a compromise. There is no getting away from the fact that the boiler is massively oversized for your heating requirement.

I turned my boiler down to number 2 the other night. When I did this, the rads were only luke warm at the top and that was with every radiator fully open and it did not heat up the room. If number 1 give 11.2. kw output and my rads come to 13kw with them all turned on fully, why were the rads only luke warm?

The numbers round the temperature knob on the boiler relate to flow temperatures, not the boiler output. When you set the boiler to a particular number/temperature, the boiler automatically adjusts the gas flow, and so the heat produced, to maintain the set temperature. The problem in your case is that the boiler cannot adjust the heat produced low enough. The boiler comes on, set the flow to the minimum, and the water temperature quickly rises to the set value. Because the boiler cannot reduce its output any lower, the boiler thermostat turns the boiler off. The boiler stays off until the temperature has dropped sufficiently. The boiler then starts up and tries again. This cycle goes on continuously. Every time the boiler stops working the pump stops, so hot water is no longer pumped through the radiators. Hot water will rise in the rads, so the bottom of the rad will feel cooler.