Advice on central heating/boiler heat exchange

[agentgonzo]

My central heating seems to take a while to get the radiators hot. 30-40 minutes for the rads to get hot is normal. I'm in a 4-bedroom house. The central heating is the original one installed and so 30 years old.

I don't want to poke and prod around the system too much, but here's what I've done to diagnose the system so far.

I got back this evening (before the heating had been turned on) and turned off all the radiators except the one in the kitchen (next to the boiler). Then turned the radiator on. Within a couple of minutes, the radiator was starting to heat up. After about 5 minutes, the radiator was warm, but not hot. I then started feeling around the back of the boiler. There is a pipe that comes out the back of the heat exchanger and then goes down into the floor. I suspect that this is what feeds the radiator loops (over the course of 5 minutes it seemed to stay the same temperature as the radiator). During the first 5-10 mins after the boiler being on, this pipe warmed up gradually from cold to be hot, but I could still keep my hand clasped around the copper pipe). Is this normal? I would hope that this would be piping hot..

After this, I started opening up the radiators again. I could immediately feel the hot water flowing through the radiators as they heated up from one side to the other, so this leads me to suspect that this is not a problem with the pump or blocked pipes, otherwise the radiators wouldn't heat up immediately.

Once all the radiators were opened up, the pipe coming out of the heat exchanger and down into the floor cooled down (a load of cold water from the other 10 rads going into the system probably) and then gradually heated up again, but after an hour as I type this, the rads are still "hot but not piping hot" so that I can keep my hand on them.

I'm not about to take the boiler apart, but suspect that this isn't normal. I assume that it's a problem with the heat exchanger, but could do with some advice before I get a heating engineer out to look at it or try and sell me a new boiler. It's an old boiler, but here's the schematic of it in case that helps anyone.

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• agentgonzo
• 24 Feb 2013
• 1

 

[bolshy]

What's your boiler control thermostat set to?

 

[agentgonzo]

Boiler thermostat is set it 4 (close to max). I haven't sat in front of the boiler for a long time but I haven't seen the boiler flames go off full all evening (front of boiler is off at the moment).

 

[bolshy]

What's the boiler model, what's the maximum setting? 6?

 

[agentgonzo]

It's a potterton kingfisher RS50. Thermostat settings are min 1 2 3 4 max

 

[bolshy]

Turn the thermostat up to max for starters. If you turn the boiler thermostat down, does the burner go out? The boiler is working, why not get it serviced by a RGI? When was it last serviced? Could be you have a thermostat fault, or your boiler isn't set up correctly. In any case, you need a registered professional in now to carry out some basic checks for you.

 

[agentgonzo]

Had it serviced by an RGI back in October, but can have him back to take a look at it. If I turn the boiler thermostat down to way below 'min' it goes out. I'll turn it up to max for the time being.

Either way, would you expect the pipe coming out of the heat exchanger to take over 10 minutes to get hot, and ~30-45 with all the rads on?

 

[bolshy]

If you turn your boiler thermostat down to 1 for instance, then your burner should fire up and go out to maintain that temperature. I have no idea how big your radiators are, but your heating load could be greater than your boiler's maximum, or it's got a fault, or it's incorrectly set up. There may be nothing wrong at all, you're perhaps just not giving it time to heat the volume of water in your system. It's not a high output boiler.
Were you in the house last winter? What was the heating like then? Have the radiators become noticeably colder in the past week, or is it because the weather is colder? What is your hot water temperature like? Have you added radiators to the system?
I would expect the flow pipe from the boiler to be hot enough not to be able to leave my hand on it for long, yes. But if the boiler is at it's maximum heat load, this could still take a while, even if it's working properly.

 

[agentgonzo]

Bolshy - thanks for the insight.

I've only been here for a year and can't remember what it was like last year. I don't remember it being this long to heat up the house, but can't be any-where near certain.

Hot water temperature is fine, but I don't know whether that's because it runs for a long time to get the hot water or not - I'll check this evening. I did a quick check and the boiler is rated as its output ~8-13kW, whereas some websites recommend 24kW for a house my size, so it may just be a weaker (and old thus even weaker) output from the boiler. But I'd still like to rule out other things as I'm surprised it takes so long to heat the house up (and get the rads hot).

 

[Malc]

If you want to be sure work out how much water you have in your system. Radiator maker's spec sheets will tell you how water they hold. You probably won't find old radiators so you'll have to make a guess, say 25% more water?? Same for pipes, guess water you have and work out. For my 5 bed with some rather long pipe runs I have Rads = 24l, boiler is 18l, piepwork 15l. My boiler is 14kW so with that amount of water the rise is 3.6C/min. So from cold, 15C (?) to full hot rads (80C) is 20min by those calcs.

BTW that is the time just to heat water, not the radiator steel nor does it include the heat given off by the rads as they are warming up, so it just an approximation to work out roughly how long it takes.