New Boiler slow to heat radiators

[scottpetrie]

Hi
I recently had my gravity fed system replaced and new combi installed in the garage. I have noticed it takes about 15 mins longer for radiators to get hot. I had a 2 channel Hive previously and the sparky has wired it up so I can still use it. In the mornings I get up and press boost on my way in to the shower. Normally the radiators are hot by the time im out the shower but ive noticed they are only just getting warm by that time. Ive been playing about with the lockshields trying to get them heating correctly as a few of them were cold at the bottom and took ages to heat fully. Maybe I still need to tweak them?

 

[Collectors]

Sounds like it just wants a good bleeding. Remember to turn the hot rads down half a turn to push it towards the others.

 

[helplessowner]

My understanding of combi boilers is that hot water takes precedence over heating.
So if your shower is running directly from the combi, there will not be any heat going to the radiators while your shower is running.

 

[foxhole]

What does the “Boost” do?

 

[scottpetrie]

My understanding of combi boilers is that hot water takes precedence over heating.
So if your shower is running directly from the combi, there will not be any heat going to the radiators while your shower is running.

Ah that would make sense. How frustrating!

 

[scottpetrie]

What does the “Boost” do?

Boost just boosts the heating at a high temperature for 30 mins

 

[foxhole]

Boost just boosts the heating at a high temperature for 30 mins

Maybe boost for 20mins before you shower , warm bathroom means less condensation .

 

[scottpetrie]

Maybe boost for 20mins before you shower , warm bathroom means less condensation .

That means getting up 20 mins earlier haha. I generally only do it if its chilly but seems to be more often recently and coming into winter maybe need to adjust the timer so it comes on earlier

 

[Madrab]

I presume you didn't have a combi before? As suggested when a combi supplies Hot Water, the central heating won't work until the HW demand is finished and the diverter clicks over to the central heating

Programme your HIVE to automatically run the CH 20mins before you get up, that's kinda what it's there for??

 

[scottpetrie]

Yeh was just a standard boiler. I suppose I was just used to hitting boost on my way to the bathroom then it would be warm by the time Im out.

 

[Delta925]

My understanding of combi boilers is that hot water takes precedence over heating.
So if your shower is running directly from the combi, there will not be any heat going to the radiators while your shower is running.

I only found this out after having my basic 12kW boiler, hot water cylinder and Y-plan 3-position valve replaced by a combination boiler.

For the old system I set the timer so the cylinder heating and radiator heating did not start at the same time so heating circuit always had the full boiler output from cold.

Initially when quoting me the potential installers were proposing 25 - 30kW boilers. I did not want to find in winter when the incoming water is at it's coldest the hot tap feed was only lukewarm unless run slowly. At work the washbasins have 9kW instantaneous electric heaters and at full flow the water is not that hot. After discussion that there was no downside other than cost I chose a 32kW boiler thinking it could, at least theoretically, deliver 12kW heating and 18kW hot water.

I will only find out the maximum actual input into tap hot water when incoming feed becomes colder but so far the boiler will not input more than around 75%. The incoming 15mm feed is also a limitation where as before at least initially drawing hot water did not reduce the cold flow rate. Only when the header tank level had dropped significantly then did the cold flow drop as it refilled.

The cost difference was only around £200 but had I known that when hot water was drawn the heating output would be cut off I would have kept with the 25kW boiler or considered retaining the cylinder and 3-position valve.

 

[Harry Bloomfield]

The cost difference was only around £200 but had I known that when hot water was drawn the heating output would be cut off I would have kept with the 25kW boiler or considered retaining the cylinder and 3-position valve.

Even the later boilers, with a 3-port valve arrangement, using modern control systems - stop proving heating, when ever there is a demand for hot water. They serve up the central heating, usually at much lower temperatures, to that needed when heating the hot water cylinder, so they operate the boiler, at two conflicting temperatures, hence, one at a time, with hot water priority.

The workaround, is to set timings up, to heat the water up first, before CH is needed.

 

[desktop987456]

I only found this out after having my basic 12kW boiler, hot water cylinder and Y-plan 3-position valve replaced by a combination boiler.

Are you saving any gas with Combi as compared to previous set up?

Is combi serving just one shower.

 

[Delta925]

Even the later boilers, with a 3-port valve arrangement, using modern control systems - stop proving heating, when ever there is a demand for hot water. They serve up the central heating, usually at much lower temperatures, to that needed when heating the hot water cylinder, so they operate the boiler, at two conflicting temperatures, hence, one at a time, with hot water priority.

The workaround, is to set timings up, to heat the water up first, before CH is needed.

This would have add complexity.
The combination of 2 channel timer, cylinder thermostat, room thermostat and frost thermostat near boiler simply told the boiler to run if any demand then a separate module set the valve position based on room heating only, hot water cylinder heating only or both.
The previous boiler just had a knob with no scale of nominal temperature never mind a display. Once the temperature of outlet water reached the set-point temperature it shut off until this dropped several degrees as preset by the control board. Simply from feeling the common input pipe to the valve I guess around I had it set around 60C.

Are you saving any gas with Combi as compared to previous set up?

Too early to tell but I should be simply because the previous boiler was 20 years old and non-condensing. Even over 12months much will depend on temperature over this winter compared to recent years.
As to in reality if a gas saving with the combi over had I retained the hot water cylinder based on usage readings I now wonder. Having said that I accepted / tolerated water being warm rather than hot at times particularly after the price hike 2 years ago when I cut the timings when the cylinder heated. Occasionally I boosted the temperature with the ' run for 1 hour button on the timer '.

Is combi serving just one shower.

Yes just one shower or bath plus washbasin and kitchen sink.

 

[Johntheo5]

I will only find out the maximum actual input into tap hot water when incoming feed becomes colder but so far the boiler will not input more than around 75%. The incoming 15mm feed is also a limitation where as before at least initially drawing hot water did not reduce the cold flow rate. Only when the header tank level had dropped significantly then did the cold flow drop as it refilled.

Most boiler makers show the expected flowrate at a dT of 35C which is fair enough IMO because the very coldest mains temp in winter is occasionally 5C maybe, so a average showering temp of 40C, so the published flowrate will be the very lowest.

You can calculate quite easily what flowrate to expect from your 32kw combi in the debths of winter or whenever.
At 35C dT, a 32kw combi will flow, 32*860/60/(40-5), 13.1LPM. (assuming mains at 5C & showering temp oif 40C)
At a more realictic say 8C in winter it will flow, 32*860/60/(40-8), 14.3LPM
For most of the summer at around a average mains temp of 15C, it will flow, 32*860/60/(40-15), 18.4LPM. (If your mains can support that flowrate)

You can do your own sums in the attached simple spreadsheet.