HW doesn't heat when CH pump on??

[mc_wardy]

Hi all, new to the forum, any advice much appreciated.

During these colder periods we have all most run out of hot water when both having a shower in the morning. Initially I thought the motorized valve feeding to the HW cylinder was dodgy but after further testing I don't think it is.

The system is set-up pretty much like this one here......
http://www.diydoctor.org.uk/project_images/boilers/Conven_s.gif (sorry couldn't find the correct diagram on this site :->). So the radiators are pumped and the HW water is convection feed.

After several days testing I've come to the conclusion that when the CH heating pump is working the hot water doesn't circulate in the HW cylinder. The hot feed into the cylinder gets cold even though the cylinder is cold and the valve is open, once the central heating clicks off the feed in to the cylinder gets hot.

My thoughts are that when the pump is on for the CH it is pulling all the water round the radiators and there is not enough flow in the convection system to get water round the cylinder.

I guess my first question is, does that make sense?
I did try turning the pump down from 3 (out of 4) to 2, I think it that helped but not enough make a difference?

Is there anything else I can try or am I completely off the mark the here?

Cheers Steve

 

[Slugbabydotcom]

During these colder periods we have all most run out of hot water when both having a shower in the morning.

I am not ruling out your ideas here just giving you some more.

There could be a problem with sludge in the system but think about this.
Mains water temp
Summer 15º ish
Winter 5ºish

In winter boiler is providing heat to a CH system and has to do more work to reheat a cylinder full of water at the same time.

Turning your heating on earlier on a morning would perhaps allow the system to be hot enough to allow it to cope with the extra winter demands.

It could be simply that your storage cylinder is undersized. A new cylinder will have more coils inside and will therefore reheat more quickly as well. Unfortunately new legislation will mean that your system will have to be upgraded to fully pumped and more controls should be fitted. A cheaper idea if you dont have much cash kicking about right now would be to use your immersion heater if you have one to assist your system during winter months.

A powerflush and inhibitor treatment will do your system the world of good if the main culprit is sludge as most on here would suspect.

You may be able to get a grant for work that makes your system more efficient

 

[DP]

Easiest fix to your problem is to fit a full programmer like Danfoss FP75 and programme for water to start heating couple of hours before CH kicks in (e.g. HW on 4.00 am to 6.30 am; heating 6.30 to 8.00 am). This will give you uninterupted natural convection through HW loop (not disturbed by the pump). When the heating kicks in, water will already be hot.

Another solution would be to convert to fully pumped or instal a pump in the 'gravity loop' ensuring feed and vent pipes do not have any components like pumps and valves in line.

 

[mc_wardy]

These comments and my suspicions are alined, there's not really anything wrong with the current system it's just reached it's limitations with the new pumped shower and very cold water and weather. When I removed the actuator to take a look there was a sticker on the back that stated the warranty for the part ran out in "Dec 1988" this gives an idea of the age and I think this is one of the newer parts!! It's still working though :->

I plan to upgrade the system when we have an extension next year or the year after. So I really only have this winter and maybe another to get through.
I've set the HW and CH to come on earlier and increased the boil boiler water temp slightly, which has made some improvement.

Although I will have a look at the fully programmed timmer as this will be useful for different week timings and I'm guessing this is easily transitioned to a new system.
Cheers

Steve

 

[weareleeds]

As a temporary fix (if updating next year) have you got an immersion back up that you could use?

 

[tryitandsee]

I'm probably missing the point, you say you turned the pump speed down. I would have thought you need to turn the pump speed up to highest setting (if pipework design can take it). This will bring the designed heat up to the radiators much quicker then allowing boiler to concentrate on hw circuit.