Gas central heating issue with radiators

[WindowWanker]

I don't know if anyone can help. We are in the final stages of buying a house and we put the gas central heating on today to check it worked. Some of the rads upstairs get hot at the top but only warm at the bottom and downstairs they were all cold even when opened fully.

I'm assuming this is a flow issue - can it be rectified relatively easily? Like flushing the system or something.

Think I'll tag @pigletjohn

Any idea if they have been bled recently? Could be as simple as air somewhere in the system.

I don't know but I thought usually radiators felt cold at the top not the bottom when they need to be bled.

probably poor flow
Or maybe you did not run the boiler long enough
Or maybe there is a completely cold cylinder taking priority and using the available heat
Or maybe the previous owers have got a frost protection setting that you did not find

but what do you mean by "opened fully?"

they should NOT be opened fully at both ends.

@WindowWanker

I don't know but I thought usually radiators felt cold at the top not the bottom when they need to be bled.

yes that's right

they should be hotter at the top than the bottom

but temperature differences top, bottom, middle, sides all have different meanings.

Sounds like they need a flush through with a cleaning agent, apparently all sorts of crap and silt deposit on the bottom of the radiators over time. The plumber flushed ours through when we had new boiler fitted. It made a massive difference to the temperature in our radiators.

The house has been empty for at least 3 months, and rented before. I don't know when the previous tenants vacated the property.

@pigletjohn Opened fully - I mean the valve on the right hand side. Is that wrong? What happens if you open them fully at both ends?!

The system was running for about an hour. Is it likely it would prioritise heating up the cylinder over the downstairs rads?

How much does it cost to resolve poor flow (ballpark)

Ours were cold at the bottom - there was a layer of sludge at the bottom apparently. We got the whole system flushed and that sorted it.

Maybe the pump is not working. We do not having enough information to know the cause.

So i watched the boiler today when it fired up. It is a Glowworm 18. The boiler takes ages to get warm up to the temperature it's set at. It slowly goes up 35, 36, 37 etc until it finally reaches its target of 65 but it takes about an hour? Our boiler in our old house used to fly on up through the numbers. What could be causing that? @PigletJohn

perhaps it is underpowered, or perhaps it has a fault or setting that prevents it delivering full heat.

it is useful to look at the gas meter and see how much gas it is burning per hour. There are about 11 kWh in a cubic metre of gas.

"18" probably means it can deliver 18kW.

this means it can heat about 6 litres of water per minute

A medium house holds around 100 litres of water in the radiators. If it has a hot water cylinder, it could be anything from about 100 litres to about 300 litres. A 300 litre cylinder is likely to be white and very large. Older houses generally have larger radiators because they have more heat loss. Old-fashioned iron column radiators absorb a lot of heat. if you had a house with a large cylinder, you would set the timer to turn on the HW half an hour or so before the CH, and the cylinder would very seldom be fully cold.

modern boilers, especially combis, are often around 30kW because they have to be able to instantly heat a shower or bathtap. However combis do not heat the CH while they are heating the HW.