I had a new Viessmann combination boiler fitted two weeks ago replacing a conventional boiler. Even during commissioning it took a long time (over an hour) before the radiators finally became hot. The boiler keeps cycling and first thing in the morning it can take two to three hours before the radiators get hot. The installer said it might be caused by the pipework in the loft where the cold water tank and header tank were located. Once the radiators do become hot there doesn't seem to be a problem regulating the room temperature through the day and evening. Viessmann sent an engineer to check the boiler and he said the boiler was working fine. I would welcome any suggestions as to what is causing the problem. The installer and Viessmann are basically saying it's not their fault, it's my old pipework.
New combi very slow to heat radiators from cold
- Thread starter leon21
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My son had this with a Worcester Bosch boiler, and it turned out to simply be the lock shield valves that needed setting.
Once to temperature the TRV's will control the flow through each radiator, however while heating up the lock shield valves control the flow, so if left wide open, it will heat up one radiator at a time, starting with the easiest for water to circulate through, and only when the TRV starts to close will the water be pushed through next easiest radiator.
Simple test, is the return water hot, if so the lock shields need setting. The basic idea is there should be around 15ºC temperature drop on each radiator when the TRV is wide open, but not an exact figure, so what I did was use the info from the TRV heads,
if the current exceeds the target then the lock shield needs closing a little, used this method at late mothers house and once set the TRV's kept the rooms spot on.
The problem is how a boiler works has changed, in the old days returning hot water did not really matter, it only turned the boiler off when it reached a set limit, but today boilers well at least gas, modulate that means turn down, so the warmer the return water the lower the flame hight.
So my old house has Myson fan assisted radiator in living room, this had no lock shield valve, output was controlled by turning fan on/off rather than regulating the flow, great system no need for geofencing and the like, as could heat living room from cold to 20°C in half an hour, with a 4.5kW gas fire, 3.5kW fan assisted radiator and a 4kW conventional radiator. But when my son had the two boilers removed and a combi boiler replace them both, he had to modify the old Myson, and needed to restrict the flow, he also added a thermostat across the resistor for speed control, so now works two speed, not quite as good as new Myson iVector which has a 5 speed fan, but an improvement on old system as high speed only used when room very cold.
But I know when I went to live with my mother to look after her, I found every lock shield valve wide open, I suspect the installers never set them to start with, but my dad could have opened them.
Once to temperature the TRV's will control the flow through each radiator, however while heating up the lock shield valves control the flow, so if left wide open, it will heat up one radiator at a time, starting with the easiest for water to circulate through, and only when the TRV starts to close will the water be pushed through next easiest radiator.
Simple test, is the return water hot, if so the lock shields need setting. The basic idea is there should be around 15ºC temperature drop on each radiator when the TRV is wide open, but not an exact figure, so what I did was use the info from the TRV heads,
if the current exceeds the target then the lock shield needs closing a little, used this method at late mothers house and once set the TRV's kept the rooms spot on.The problem is how a boiler works has changed, in the old days returning hot water did not really matter, it only turned the boiler off when it reached a set limit, but today boilers well at least gas, modulate that means turn down, so the warmer the return water the lower the flame hight.
So my old house has Myson fan assisted radiator in living room, this had no lock shield valve, output was controlled by turning fan on/off rather than regulating the flow, great system no need for geofencing and the like, as could heat living room from cold to 20°C in half an hour, with a 4.5kW gas fire, 3.5kW fan assisted radiator and a 4kW conventional radiator. But when my son had the two boilers removed and a combi boiler replace them both, he had to modify the old Myson, and needed to restrict the flow, he also added a thermostat across the resistor for speed control, so now works two speed, not quite as good as new Myson iVector which has a 5 speed fan, but an improvement on old system as high speed only used when room very cold.
But I know when I went to live with my mother to look after her, I found every lock shield valve wide open, I suspect the installers never set them to start with, but my dad could have opened them.
Thanks for the suggestions - I think maybe the way the boiler modulates has something to do with the problem and we're learning to live with it. The installer balanced and bled the radiators so I don't think cause of the problem is in the radiators. I'm setting the room thermostat on auto in the night to raise the room temperature up to 19 degrees at 4.00 am so that the radiators get hot by 6 or 7 in the morning. If the problem lies with old layout of piping in the house, my wife just doesn't want any more disruption to the home. Thanks again.
How long did they spend powerflushing the old pipes and radiators before fitting the boiler?
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Have you reduced the CH output to say 15kw (range rating), this won't affect the DHW output but may reduce the boilercycling until rads fully hot which should normally only take 20 minutes or so.
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