cold radiators problem

The water path round the indirect coil which heats the water is short and low resistance compared with the radiator circuit. If (a) the pump is knackered/going too slow or (b) the radiators/pipes are getting sludged up, the flow will drop in the radiators considerably when the path round the water circuit is open.

The pump may have a three position control switch on it - check it's not set too low

If the downstairs rads heat up quite quickly from cold when only heating is turned on, I would suspect the pump isn't pushing hard enough overall.

If the downstairs ones warm up slowly and the upstairs ones quick (one scenario) when just heating is asked for, it it still might be the pump OR it might be worth draining and flushing the system and maybe running it with cleaner for a while.

Thanks Bob for your comments. The water pump is on number 2 - Is 3 or 1 the higher setting?

Jon

If you wish to have the heating and domestic hot water heated simultaneously, the cylinder coil must be balanced with a series gate valve used as a LSV (Lock-Shield Valve), just as if it was a radiator. You can get 'locking' gate valves for this purpose. See

Heating either the DHW or the CH, rather than both together, avoids this problem. But balancing the rads may still prove beneficial if any of yours have cool return temperatures.

-- Phil Addison The uk.d-i-y FAQ is at

simultaneously,

I have no doubt this would have the described effect but I would quite like to know your reason for using the word "must" - is this now part of building regs?

I certainly wouldn't want my water circuit stopped down too much as I want it to take preference.

I guess that "must" here refers to complying with the laws of Physics - rather than those of Tony Blair!

In other words, if you want your radiators to get hot when the water is also being heated, it is necessary to ensure that they (the radiators) get their fair share of the flow.

Unless you have a fast recovery cylinder (in which case you may not notice the radiators getting cold in the time taken to reheat the water) the heating coil won't be able to absorb much of the boiler's output, so it does no harm to restrict the flow through it.

Quite so. I did not intentend to imply a building regulation, and am happy to substitute 'should'.

I'm not entirley sure about that. When the tank is full of cold water, I suspect that it can absorb quite a bit more tha when it is just re-heating a partial draw-off. But apart from that, the water velocity wants to be limited for noise and cavitation erosion reasons.

-- Phil Addison The uk.d-i-y FAQ is at

What I am saying is that the rate of heat transfer is determined by temperature difference between the primary and secondary water and the surface area of the heat exchanger. If you pump water through it at too high a rate of knots, you'll be sending it back to the boiler with very little temperature drop - and will also be depriving the radiators of flow.

Yes, I understand that. What I'm not sure of (without looking up data sheets) is whether the coil is spec'd for heating from cold or for partial re-heating, i.e. which temperature difference to work on.

-- Phil Addison The uk.d-i-y FAQ is at

I had a further thought about how to have your cake and (h)eat it!

Suppose you install a gate valve in the feed to the coil and tun it down to limit the flow when it is competing with the CH. Suppose you by-pass the gate valve with another pipe containing a zone valve - which is wired to be open whenever the CH zone valve is closed. You can then feed the full boiler output to the HW coil at any time when the CH is switched off or the demand satisfied.

Can anyone see any problems with this - apart from the cost?