Understanding our gas-fired central heating

For a year we've lived in a semi-detached that has a combination gas boiler for the central heating and dish water heating (It's a Biasi Riva Advance). Yesterday, the landlord finally installed a thermostat for the central heating. Previously, I'd been going out to the boiler closet a couple times a day to adjust the heat setting (when it was warmer I used the built-in timer to turn off the heat at night). It seemed like a good idea to get the thermostat so that I wouldn't have to constantly go outside to adjust heat settings. What I discovered last night, however, is that when the thermostat turns the system on and off, it turns on/off not only the burner but also the pump, so that it's now a fairly noisy system. Previously, it seems to me that the pump ran continuously while only the burner turned on and off, so there wasn't such an audible jolt when the system changed states. Furthermore, when it's reached a point of equilibrium, it cycles on and off every three minutes. I'm a light sleeper, so this is not really welcome. So, my question is basically twofold: first, does it sound like the system is operating normally? and second, will this new behavior with the thermostat save us money?

cheers for any help to be offered

Ron

When it turns on..does it stay noisy for long? or just for 10 minutes or so..? 

In my house, I have the timers set on the boiler so that it turns off and on at certain times of the day when I am at home..but I switch it off completely in the summer. 

The thermostat is to switch the heating off once it gets to a certain temperature.  I tend to keep mine on about 20 / 25 degrees. So even if I have set the boiler system to run for 3 hours..if it gets to that temp before then..it switches off...rather that leaving the heating on constant for 3 hours..so in the end it does save money.

I am sure somebody else could explain this better than me 

It's normal for the pump to switch on and off with the thermostat, but not for it to cycle over such a short period. 

It sounds as though the heat anticipator in the thermostat may be adjusted incorrectly.  This is a tiny heating element on the thermostat's bimetallic strip which artificially raises the thermostat temperature very slightly when it is calling for heat.  The idea is that when the heating system shuts off, the room temperature will still continue to rise for a short time due to the residual heat in the radiators.  The thermostat thus "anticipates" this rise by shutting off just a fraction early.   

If the heat anticipator is set too high, it results in the thermostat switching off too soon, then the sensing element soon drops back to the ambient room temperature which has not yet attained the desired level, so the thermostat calls for heat again, and so the process repeats with the system cycling on and off every two or three minutes.    It's also not too uncommon for somebody to wire the terminals in the thermostat the wrong way round so that it still switches the boiler and pump on and off but the heat anticipator is running continuously.  That will result in the same sort of problem. 

Failing that, it could be down to a simple faulty thermostat where there isn't the correct amount of hysteresis between the on and off points.  I'm assuming you don't have something silly like the wall thermostat being used in combination with thermostatic radiator valves. 

As for the noisy operation, if the system is really making some thumps and clonking noises etc. each time the pump starts up, it might be worth getting the system checked out overall.  A pump service can sometimes cure that, or you might have a few partial airlocks in the system.    Many circulating pumps are two-speed and have a selector switch.  If you're currently running on high speed you might quieten the start-ups a little by switching to low, although obviously at the expense of it taking a little longer to get full heat through to the radiators.

Thanks for the replies. I called the company that sells these boilers in the UK, and the tech help guy told me that the pump should not be shutting off and that the thermostat was likely incorrectly wired. I can see some logic in either approach (pump stops or pump continues). It's hard to say which would lead to a faster cooling of the indoor temperature, since if the burners have stopped, the water being moved might be cooler than that which is just sitting there. It's also hard to say how much electricity the pump is using. In any case, the landlord's installer (his father) is coming today to check it out. Fortunately we have the installation guides showing the correct wiring (which I doubt he checked before doing the work).

I can see some logic in either approach (pump stops or pump continues).

One problem with a continuously running pump is that even if you turn the thermostat right down to shut off the heat, the pump is still going to be sitting pumping cool water around the system needlessly while the main controls have the heating "on." 

There are considerable variations in the way the system can be designed though, and some arrangements work better in some types of installations than others.   

It's also hard to say how much electricity the pump is using.

Again, obviously there are many variations in sizes of pumps for different systems, but most new-ish domestic systems of average capacity rarely have a pump larger than about 1/8 h.p., so typically under 100 watts when running.  On the lower speed, some draw only about 50 to 60 watts.