Balancing hot water/central heating (to activate radiators)

What is 'havent always been getting hot' mean? What temperature are they getting to? Be aware, most humans cannot detect temperature above around 47 deg. C, it just hurts beyond that, so there is no way a (normal) human can detect a difference between rads at 55 and 65 degrees. And what temerature are the others getting to? Is the auto bypass set too weak, so it lets the heated water back through before it hits the rads? The boiler will be putting out the same temperature heated water whether the HW or CH was on, its only if the cylinder coil was absorbing more heat than the CH system , that the flow temperature to the CH would be lower, even then, its doubtful that could happen, as they are S valves, so the same amount of water should, in theory, be pushed around the system, but if the HW circuit is smaller, and the cylinder water needs heating, then that could take the bulk of the heated water, thus reducing radiator temperatures. Why is the HW on? Dont you have 2 set times each day for it? Having it set on, with the cylinder stat calling for heat whenever it drops a degree or so is quite inefficient, the boiler will be firing up for short periods regularly if that is the way it is set.

Two of the downstairs rads don't come at all and some others only get lukewarm. The heating and hot water both come on and off at the same time on a basic programmer. We sometimes turn the hot water on when lots of baths/showers going on and also it keeps the kitchen warm as that's where the boiler and a rad are (the rad gets hot even without the heating on). So turning the hot water off solves the cold rads and I take from what you say the cylinder is diverting hot water but can't see why it would when the cylinder is up to temperature as I thought the thermostat just shuts off the valve but maybe it's faulty.

Have a look at your pump speed settings (if any). It may be that your pump is failing and doesn’t have enough “oomph” to push water around your HW tank and CH circuit simultaneously.

You may get away with turning your pump speed up but if it is failing, a replacement may be on the cards.

Tim

The boiler - a Vaillant EcoTec Plus - is only 3 years old but we have a warranty service scheduled so will ask the tech to look at the oomph setting.

What temperature is the cylinder stat set to?

Should have also asked what temperature is your boiler set to?

So one heating zone and one DHW zone?

Chances are the DHW loop has significantly less flow resistance than the CH circuit, so with both zone valves open, you may get the bulk of the flow through the DHW circuit. Even if the cylinder is up to temp, it could still divert flow from the rads.

Much depends on how the system is setup. Ideally you want the DHW zone to close when the cylinder is at or reaches set point temp.

Since it is probably a very fast recovery cylinder that can absorb close to the full output of the boiler, operating in DHW priority mode might make more sense - schedule DHW reheat at times the CH is not on. You might also find that your boiler is capable of split temperature operation - i.e. where the boiler knows when it is heating DHW rather than CH, and can run different flow temperatures. This allows you to get better condensing efficiency in the less heating intensive months since the rads can run cooler, but the DHW is still heated to an adequate temperature to ensure sanitisation of the water.

You could also take that further and use full weather compensation:

Details on S plan configuration here:

Water will always take the path of least resistance which in this case is likely to be the path through the hot water tank. Pushing up the pump oomph will not necessarily solve the problem.

What also could be happening is that most of the water is circulating through the hot water tank coils and at speed that return temperature to the boiler rises close to the flow temperature. The boiler can only modulate down so far and with a high return temperature it has to cut off or else the flow exceeds the configured flow temperature. To prevent short cycling the Vaillant EcoTec Plus will now inhibit the burner firing for perhaps 5 minutes. During this 5 minute period the flow and return temperature may drop by 10C or more. The lesser amount of water flowing through the radiators will now be cooler.

Once the hot water has reached the correct temperature the valve should close and water only circulate through the radiator system. How fast it does this will depend on the boiler flow temperature and the tank thermostat temperature. If for instance the boiler flow and the tank thermostat temperature have both been set to 60C then the hot water may take a very long time to reach 60C, perhaps never, especially if the scenario above is true. In my experience the hot water in the tank will only reach 10C below the boiler flow temperature after an hour or so when selecting hot water.

Try setting the hot water tank thermostat 10c cooler.

The solution may be to reduce the flow through the hot water path so the flow resistance through the hot water path equals that of the radiator path. Balancing radiators via the lockshield valves involves open up some and CLOSING DOWN further the valve on the radiator where most of the water is flowing. This doesn't balance the system if most of the water is flowing through the hot water path.

Perhaps a controller where it can be set to select hot water only before the heating comes on, or a least for multiple shorter periods where both are on together.

The software on these boilers is utter s*it and cannot be fixed by adjusting the settings. I know, I have one.

In this case I suspect the problem is not the boiler but more to do with the rest of the system where a couple of work-arounds have been suggested.

This does not sound like a software problem!

Thanks John - that all makes sense. I think we need a new programmer - it's this:

What would be a good alternative that offers more flexibility. I'll also look at possible split temp operation but presumably that depends on getting a better programmer at least for months when we do use the CH.

Thanks - we need a nw programmer I reckon as the one we have can't cope with differential CH/hot water settings. I will try turning down the tank stat.

programers know nothing about temperature. That the job of thermostats.

Yes, but our programmer can't switch the CH and HW on at different times, only both at the same time unless one is set not to come on at all although it does have the function of leaving one on and the other off til the second on time but that can be most of the day.

The thermostat on the tank should though be shutting off the HW but doesn't always so I will turn down the temp.

Then get one that can.

-- Truth welcomes investigation because truth knows investigation will lead to converts. It is deception that uses all the other techniques.

A sticking valve seems more likely

The problem could be that the DHW and CH are on at the same time and the DHW is taking a very long time to reach temperature. The flow in the DHW is not balanced with the CH. The OP has stated that his controller doesn't allow DHW times to be set differently to the CH times.

A better controller with half a dozen independent timers would allow the DHW to come on in the morning and then switch over to CH an hour later so the water is only then flowing through the CH (which appears to let the radiators get hot as the OP has found when he turns of the DHW heating) later in the morning. During the day a top up of hot water could be obtained by using multiple short timing periods

Programmable thermostats know about temperature as well as time ...