Grant Combi 90 mk2 poor hot water output

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Hi, sorry if this topic as been covered a million times before but I haven't found any definite answer on searches?
The water remains luke warm or cold most of the time, the hot water tap as to be run for about 5 minutes before anything like warm water comes through. The burner does not cut in when a hotwater tap is opened, I have removed the diverter valve micro switch and the pushrod is moving by 11mm when hot tap opened. The diverter valve m/switch switches on the pump but does not fire up the boiler. Any suggestions would be much appreciated.
 
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The burner doesn't fire on these until the store stat calls. To check operation, turn off HW tap & CH. Boiler should fire to heat primary store. If not check primary store is hot. If it is, boiler works and your problem may be with the plate heat exhanger failing to transfer, If diverter is working it should push microswitch but this will only put it in HW demand mode, burner should only fire if primary store below stat call.

If all seems ok the next thing to check is the thermostatic blending valve, which could be faulty. This blends HW with cold main to regulate HW temp but requires annual inspection/cleaning.
 
The burner doesn't fire on these until the store stat calls. To check operation, turn off HW tap & CH. Boiler should fire to heat primary store. If not check primary store is hot. If it is, boiler works and your problem may be with the plate heat exhanger failing to transfer, If diverter is working it should push microswitch but this will only put it in HW demand mode, burner should only fire if primary store below stat call.

If all seems ok the next thing to check is the thermostatic blending valve, which could be faulty. This blends HW with cold main to regulate HW temp but requires annual inspection/cleaning.
Many thanks for your reply, how do I check primary store temp and is the blend valve combined with the diverter valve?
 
Put your hand on top of the heat exhanger jacket, if hot with CH off then it's priming for HW transfer use as normal. The blending valve is separate and has a rad valve like rotating top with numbers on it. If it hasn't been maintained it'll be worth doing by someone who understands oil & combis.
 
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There are two microswitches in the unit on top of the valve. Is the burner one not switching. Swap the wires over and see if the burner comes on without the pump. If so, then you have a faulty switch. If it is still the same, then heatstore stat is the suspect.
Also, feel the outlet from the heat exchanger to the blending valve. If this is hot, then blending valve is at fault.
 
T hankyou both I will have another look tonight, it was serviced about a month ago by bthe guy that installed it 8 yrs ago but he said he could not find the fault? Judging by the time taken to service I dont think that he could of removed the blend valve.
 
Not always required to touch the blending valve as part of the boiler service, but water flow & temperature should be checked, which if poor would lead to checking the blending valve.
 
There are two microswitches in the unit on top of the valve. Is the burner one not switching. Swap the wires over and see if the burner comes on without the pump. If so, then you have a faulty switch. If it is still the same, then heatstore stat is the suspect.
Also, feel the outlet from the heat exchanger to the blending valve. If this is hot, then blending valve is at fault.
Does the boiler top cover have to be removed to access the micro switches? The blend valve pipework appears to be hot on the top and side pipes.
 
There are two microswitches in the unit on top of the valve. Is the burner one not switching. Swap the wires over and see if the burner comes on without the pump. If so, then you have a faulty switch. If it is still the same, then heatstore stat is the suspect.
Also, feel the outlet from the heat exchanger to the blending valve. If this is hot, then blending valve is at fault.
Does the boiler top cover have to be removed to access the micro switches? The blend valve pipework appears to be hot on the top and side pipes.
I just ran the hot water and the pipe between the heat/ex and blend valve became to hot to touch but the top blend valve pipework is only slightly warm. The strange thing is sometimes the water will run hot?
 
Have you tried turning the blending valve up and down? This may help, but it sounds as though it is faulty. The sensing capsule gets scaled up, or just tired. New valve approx £65. Turn off the incoming water and it's just 3 connections.
 
I tried moving the blend valve adjustment and it did indeed improve the water temp but later in the day it was back to cold/luke warm again. Is that the usual outcome when the valves scaled up?
 

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