I've been asked by my father inlaw to install 3 radiators for him. After reading the boiler manual (halstead finest gold combi) am I correct in assuming that we can still run the hot water after isolating the central heating at the boiler inlet and draining it off? Also is it usual practice to put inhibitor in via a radiator before/during refilling for a combi?
Do the same general rules (ie 15mm shouldn't serve more than 3 rads) apply for using 15mm or 22mm CH pipework for combis as they do for traditional systems? Any help and warnings very much appreciated.
This isn't so much an issue of whether it's a combi as whether it's a sealed system. For sealed systems you can obtain inhibitor in gel form that can be injected into a radiator (I do it when they are empty rather than against the pressure when full). Alternatively you could put in a tee, a valve (e.g. a lever ball valve) and a short stub of pipe vertically and deliver liquid inhibitor into the system. The liquid is a little less expensive.
You can't really assess it in that way. The limitation of what can be fed through a length of pipe is ultimately limited by the flow rate, and the design rule for that is that it should be less than 1.5 metres per second to avoid noise and excessive flow restriction.
The flow will be related to the pipe size and length and therefore you can say that there is a limit of volume per second for a given case. This in turn can be related to the required rate of heat transfer, and you can finally relate that back to the total radiator output fed through the pipes in question.
Obviously, radiators come in different sizes and can have outputs ranging from a few hundred watts up to 5-6kW or more. As a rough rule of thumb, for lengths of up to a few metres you can say that 15mm tube will handle 6kW. Clearly this works out OK if you had 3 x 1kW radiators, but won't if they were each 3kW.
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a useful design paper. You can plug your numbers into the tables and work out exactly what is needed.
I don't know about the Halstead, but most combis will not allow you to drain down the system and run the boiler, even just for water. If, however, you meant you cut out the radiator circuit, but the boiler still has pressurised water in it, you may be OK.
I could imagine that some boilers still require some sort of bypass in the circuit, but don't know if any do in practice. Internally, the boiler will probably use the water to circulate between the burner and the internal plate heat exchanger.
Normally combis do need a pressurized radiator loop of some kind for the HW to work at all. So the answer to the OP's 1st question is no. They will be not problem to try it but it my experience you just won't get any hot water.
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