System Boiler - DHW & Rads at the Same Time

[ncdlloyd]

We've recently moved to a property with a Glow Worm Energy 30s. This is our first time in a property with a system boiler and up until now it's been fine. Now it's getting colder we've run out of hot water a couple of times and it seems like when hot water is being generated the rads don't come on.

I've changed the schedule so that hot water release is enabled all day, rather than just when the rads are on - this was suggested in the MiGo app FAQ. Is it normal behaviour for a system boiler to only do rads or DHW and not both at the same time?

Thanks,

Nick

 

[Madrab]

Need to understand what CH system you have, though it sounds like you have a priority hot water system and this exact subject is doing the rounds just now on the forum. https://www.diynot.com/diy/threads/remote-control-of-boiler-flow-temperature.598399/

If it is indeed a priority HW system then it needs to be setup correctly so the HW is only on for the shortest time at a high heat to replenish the HW cylinder quickly, so the central heating isn't off too long and then goes cold.

It's all a bit technical and wont really help you but that's exactly the behaviour I am trying to work around.

 

[oilhead]

As long as the controls are set up correctly, there is no reason why you should not have hot water and heating at the same time. What size is your DHW cylinder? How many people are using hot water? Do you have an all day requirement for DHW? How long do you have DHW programmed? What flow temperature have you set for your boiler?

 

[ncdlloyd]

Thanks @Madrab. Our issue had been that hot water was being used during the day and the schedule wasn't set to replenish it until the heating came on. The impact seemed to be that the tank got filled first, delaying the ability for the heating to come on.

Thanks @oilhead - I don't know what capacity the cylinder is, but it's roughly 110cm long and 160cm circumference. There are four people in the house. We don't have an all day requirement for DHW during the working week, just at the weekend.

DHW was set to only come on with the heating, so first thing and then from 7pm - I've now changed this to be on all the time, but should maybe adjust it again to only do that at the weekend. Hot water is set to 55 degrees.

 

[amcluesent]

>We don't have an all day requirement for DHW during the working week, just at the weekend.<

Do you have a 5+2 programmer with separate CH and DHW schedules?

> Is it normal behaviour for a system boiler to only do rads or DHW and not both at the same time?<

Yes, if the piping/valve are setup in what the trade calls a C-plan or W-plan configuration.

 

[ScottishGasMan]

Your system is working perfectly fine.

You have a weather compensated heating system, with Hot water priority. This means the system will prioritise hot water at higher flow temperature first, once hot water is recovered, then it will flow to the heating at "weather compensated" lower temperatures in order to maximise the boilers condensing efficiency.

Typically you wouldnt notice the house cool down as it will reheat the water fast. But if both come on at the same time then it will do the hot water first before allowing the heating to start.

 

[ncdlloyd]

Thanks @ScottishGasMan, good to have that confirmed. I'll adjust our schedules accordingly.

 

[ncdlloyd]

>We don't have an all day requirement for DHW during the working week, just at the weekend.<

Do you have a 5+2 programmer with separate CH and DHW schedules?

> Is it normal behaviour for a system boiler to only do rads or DHW and not both at the same time?<

Yes, if the piping/valve are setup in what the trade calls a C-plan or W-plan configuration.

Thanks @amcluesent the MiGo app is very flexible, I'll probably set hot water to come on more often when the family is at home and less often during the week.