Last June (2019!) we moved to a house, built in the mid 1980's, which has a WB 37CDi combi boiler, installed about 10 years ago. I have estimated, based on the standards set when it was built, that the heat loss is about 10kW. There are about 10kW of radiators. The boiler can only modulate down to 9.5kW so it spends most of its time cycling on and off.
When it started to get very cold (early December) I found that the house would hardly rise above 20C. The boiler seemed to be cycling all the time with the flow temperature only rising to 60-65. When the pump overran the flow would drop below 40C before the boiler restarted.
I down-rated the CH output from 30kW to 13kW and set the room stat to maintain 20C overnight. (At my previous house it was set to 16C - just in case as the house never feill below 18C.) The house is now reasonably comfortable in the morning with a wake-up temperature of 21C. However we find that it will not rise any more. Last night I set the room stat to 24C; two hours later the roomtemperature was still 21C.
I have monitored the boiler, which is set to a CH flow of 75C. However the temperature never reaches this before it cuts out - 65C or thereabouts. The boiler then stays off for several minutes (anti-cycling?) while the water temperature drops about 20C before it relights.
I can't work out what is happening. The rads are all warm, with no cold spots to indicate blockage, but they never get really hot.
I have checked the parameters and they are all correct, according to the Engineer's Service Booklet for the CDi Range (apart from CH output, which I have lowered to 13kW).
1. Why does the boiler never reach set temperature - 75C?
2. Why does the water temperature drop so far before the boiler relights?
When it started to get very cold (early December) I found that the house would hardly rise above 20C. The boiler seemed to be cycling all the time with the flow temperature only rising to 60-65. When the pump overran the flow would drop below 40C before the boiler restarted.
I down-rated the CH output from 30kW to 13kW and set the room stat to maintain 20C overnight. (At my previous house it was set to 16C - just in case as the house never feill below 18C.) The house is now reasonably comfortable in the morning with a wake-up temperature of 21C. However we find that it will not rise any more. Last night I set the room stat to 24C; two hours later the roomtemperature was still 21C.
I have monitored the boiler, which is set to a CH flow of 75C. However the temperature never reaches this before it cuts out - 65C or thereabouts. The boiler then stays off for several minutes (anti-cycling?) while the water temperature drops about 20C before it relights.
I can't work out what is happening. The rads are all warm, with no cold spots to indicate blockage, but they never get really hot.
I have checked the parameters and they are all correct, according to the Engineer's Service Booklet for the CDi Range (apart from CH output, which I have lowered to 13kW).
1. Why does the boiler never reach set temperature - 75C?
2. Why does the water temperature drop so far before the boiler relights?