Programmable Thermostat?

[richchad]

I have a British Gas 330+ boiler with a Honeywell ST9400C Programmer and a wireless Drayton digital thermostat.

I was also thinking of changing the existing almost unreadable digital thermostat for the wireless Honeywell CM927 but am not quite sure if this is a good move considering the TPI control switching the boiler on and off, I understand there is no way to deselect this option.

Also I wondered how the thermostat would operate if I still used the programmer to turn on/off the boiler for heating as well as for the hot water?

 

[D_Hailsham]

I was also thinking of changing the existing almost unreadable digital thermostat for the wireless Honeywell CM927 but am not quite sure if this is a good move considering the TPI control switching the boiler on and off, I understand there is no way to deselect this option.

I wouldn't worry about it - there must be many thousands of houses with CM927's and this supposed problem has never been raised before. In any case Honeywell will have tested the 927 with many boilers before putting the device on the market.

Also I wondered how the thermostat would operate if I still used the programmer to turn on/off the boiler for heating as well as for the hot water?

The heating side of the old timer needs to be set to permanently on. Then only the CM927 controls the heating.

 

[richchad]

Thanks for the reply D_Hailsham, I am sure you are right, my only other worry about the TPI is that British Gas fitted a Bypass valve next to the boiler only last week as we kept getting boiler faults and they diagnosed that when no heat or hot water called for it occasionally tripped as nowhere for water to go to cool down in flow and return.

However since fitting the valve have noticed that it is quite noisy when water is routed through the valve for a few minutes when system is in overrun, my worry is will I get this up to 3 or 6 times an hour when TPI kicks in as I assume there will be an overrun every time the boiler gets a no heat signal.

 

[D_Hailsham]

What type/make bypass valve did BG fit?

How many motorized valves do you have? They will look like one of these:

View media item 5946 View media item 11762
Was the system rebalanced when the boiler was installed?

 

[richchad]

They fitted an inline Tacotherm bypass valve next to boiler which diverts water around about 3-4 metres of pipework including a Magnaclean.

With regard to motorized valves, there are 3, one in airing cupboard for hot water, and two below boiler, one for a separate annex radiators and the other I assume for main house heating.

Not sure what you mean by rebalanced when boiler installed?

 

[D_Hailsham]

If you have separate motorized valves to control each zone (two CH and one HW) then an autobypass should have been installed when the boiler was put in.

Normally the Magnaclean should be installed on the return, before the boiler.

What is the setting on the Tacotherm?

Not sure what you mean by rebalanced when boiler installed?

Balancing is adjusting the flow through each radiator so they all receive the required share of the heat. You can read how to do it at How to balance a CH system. Changing the boiler will upset the balance as the resistance of the heat exchanger will be different to the old boiler.

 

[richchad]

Okay, yes I think they did balance the radiators after fitting boiler, although it was over a year ago.

The Tacotherm is 15mm mounted vertically and I can see the screw head is set between A & B. which is about an 8 o clock position (if that makes sense)

However have just read paperwork that came with it, and it says check boiler flow rate and BG have written 21 L/M on sheet, but the instructions state a 15mm one for 3-12 litres per minute and a 22mm one for 8-30 litres per minute, does this mean it is too small for the flow rate and hence the noise?

The BG guy also told me he had increased the pump flow to maximum.

The Magnaclean is located in the return to boiler.

 

[D_Hailsham]

The Tacotherm is 15mm mounted vertically and I can see the screw head is set between A & B. which is about an 8 o clock position (if that makes sense)

However have just read paperwork that came with it, and it says check boiler flow rate and BG have written 21 L/M on sheet, but the instructions state a 15mm one for 3-12 litres per minute and a 22mm one for 8-30 litres per minute, does this mean it is too small for the flow rate and hence the noise?

Is there a glass tube with a float on the Tacotherm? If so what number is the bottom of the float against? It should be 21 as written on the instructions. 21 L/m (litres per minute) is correct.

15mm is much to small for a 30kw boiler; it should be 22mm. So the Tacotherm should also be 22mm.

Sorry if I misunderstood your original description of the Magnaclean location, but you seemed to be saying that it was on the bypass pipe. As long as it is on the main return, after all rad, cylinder and bypass connections, that's OK.

May I ask why you have a 30kW boiler? Do you have a very large house?

 

[richchad]

Yes there is a glass tube with numbers 3 - 12 and it is set on about 12.

I assume the 22mm one will have numbers 8 - 30 and if it should be set to 21 this confirms it is too small....

I guess we have a 30kw boiler as it is a 5 bedroom house with an annex and is similar to what was installed originally.

The Magnaclean is on the return and is included in the bypass circuit, which I presume would help dissipate some of the heat.

 

[D_Hailsham]

Yes there is a glass tube with numbers 3 - 12 and it is set on about 12.

I assume the 22mm one will have numbers 8 - 30 and if it should be set to 21 this confirms it is too small....

Does seem like it, but BG may have adjusted the max output of the boiler to be less than 30kW. If they had, however, I would have expected the flow rate to have been less than 21l/m.

I guess we have a 30kw boiler as it is a 5 bedroom house with an annex and is similar to what was installed originally.

But have you had the insulation (cavity wall, loft, double glazing) improved since the orgininal boiler was installed.

Use the Boiler Size Calculator to find out what size boiler you should have. You will have to do the main house and the annex separately; the common wall between the two is not counted.

The Magnaclean is on the return and is included in the bypass circuit, which I presume would help dissipate some of the heat.

But if it is on the same pipe as the Tacotherm there will be no flow through the magnaclean when the Tacotherm is closed. As it is at the moment, with the 15mm Tacotherm, that does not matter as the Tacotherm will be open most of the time!

Time to get onto BG and ask them some questions.

 

[richchad]

Have just read the boiler is set to give a 24kw output and have just done quick calculation and boiler size may be a touch over but not by much and my calculations were a bit rough.

Looking at the boiler manual the flow rate chart shows 21 L/m is for 30kw and I guess somewhere between 15 -20 for 24kw.

Nothing has changed as far as insulation etc. the house is only 15 years old.

I am probably not being very clear regarding the Magnaclean, it has always been on the return just before the boiler, it is that the bypass now circulates through it as part of the bypass circuit.

Anyway as you say I will be raising some questions about it, as we need to reduce the noise regardless of whether we fit a programmable thermostat or not.

Thanks for all your input, it has been most helpful.

 

[D_Hailsham]

Have just read the boiler is set to give a 24kw output and have just done quick calculation and boiler size may be a touch over but not by much and my calculations were a bit rough.

Looking at the boiler manual the flow rate chart shows 21 L/m is for 30kw and I guess somewhere between 15 -20 for 24kw.

The flow rate for a 24kW boiler is 17.5 litres/min. That is with a 20°C difference between flow and return temperatures. If the difference is less, the flow rate is proportionately higher.

The problem with a small differential is that the resistance of the boiler heat exchanger increases as the flow rate increases (see the graphs in the manual). This means that your pump has to work harder - higher number, faster flow - which means the difference will be lower. It's a catch 22 situation, particularly with fixed speed pumps. That is assuming you have a pump with three fixed speeds, not an auto-variable speed one.

 

[richchad]

Yes it is a fixed 3 speed pump.

So are you saying a slower pump speed is unlikely to reduce the noise but a larger size bypass valve and 22mm pipework would?

I suppose I could easily try a slower pump speed after perhaps speaking to BG as it may then also require a larger valve if not successful.

 

[D_Hailsham]

So are you saying a slower pump speed is unlikely to reduce the noise but a larger size bypass valve and 22mm pipework would?

I suppose I could easily try a slower pump speed after perhaps speaking to BG as it may then also require a larger valve if not successful.

The pump has to be set with the bypass closed to give the correct differential with the radiators balanced. The bypass is then adjusted. See How to balance a CH system

As I explained, the problem with fixed speed pumps is that you are unlikely to achieve the desired results. The rads will all get hot but the differential will be too low, or the differential will be too high and the further rads will not get hot.

 

[richchad]

Hmmm looks like I need BG to check it is set right and see what they can do to reduce the noise when boiler is in overrun.

Thanks again for your help...