Balancing rads - increased cycling/failed starts after balancing

[-rick-]

As you may have seen from other threads, I recently got a Wiser heating controller and have been spending time looking at my heating system. I've known there have been problems with it for a long time and had basically been ignoring them by only limited a subset of my rooms and not worrying about the rest. Now I have the controller, I have some tools to monitor function an also wanted to attempt again to heat my whole flat, with the hope that I can optimise things and the overall cost won't be hugely worse than existing.

 

Very quickly, I've realised I needed to balance the radiators. I've done a couple of rounds of balancing. First round, I dealt with the radiators that were heating first and obviously stealing flow from the other 4 (which are located in my higher heat loss rooms that I've not been actively heating).

 

At this stage, I'd left the 4 wide open (in hindsight not ideal). This made a difference in the speed of getting to temperature (2-3 hours, not 10) but the overall dT was still about 6C which is not great. Chart below from roughly steady state once to temp (I know the temps are fairly high, that's another thread that I will write at some point*).

 

 

Today I did another round of balancing, I've now restricted the flow on all the radiators now and have managed to widen the dT to nearer 9C and had planned to go back tomorrow and tweak some more.

 

I have some concerns though. Firstly, the locksheilds on most rads are now barely open and I'm concerned about minimum flows and noise in the pipework (noise so far is minimal but I'm clearly very close to more noticable noise). Is this what I should be doing or have I misunderstood something?

 

Secondly, since I've done this I noticed the boiler is now really short cycling (the cycling before at steady state was not great but now it's clearly problematic). My thesis is that before when the boiler cycled the initial burst of heat from the ignition got absorbed by the high flowing water fairly easily, with the more restricted flow caused by additional balancing work this burst of heat during ignition is not being lost in the flow and is causing the temp limit to trip and the boiler to lockout. During the lockout the return temp drops further giving more space for the heat from the next ignition. In theory you could deal with this via the controller but Wiser doesn't appear to have an easy way to do this (there is a hystersis setting in the debug output but no obvious way to change this).

 

 

Is my thinking right? Is there a way to address this with balancing / doing something different on the physical side?

 

* My system as installed (when the flats were built) is a mess. The radiators in my high heat loss areas appear undersized even when dealing with traditional dT of 50 and obviously totally inadequate for any sort of low temp heating. In the first screenshot here, the state seems steady but in reality the room was slowly losing heat as the OAT dropped and the heat input was capped but the minimum boiler modulation + max flow temp as I had it set. The second graph has a much higher max flow temp, but you can still see the minimum modulation is an issue (even though the property could likely absorb that heat ok).

Edited 2 hours ago by -rick-

[marshian]

My advice is simple

 

Keeping the boiler happy with a decent flow rate thro it is far more important than squeezing the DT out of individual rads to drop the return temp

 

Squeeze too much out of the rads and you drop the flow rate thro the boiler

 

Drop the flow rate thro the boiler and it'll go out on temperature much faster

 

Every time the boiler fires you'll send a load of energy out the flue before it modulates down this is a bigger waste than running with a narrower delta at the boiler with a good flow rate

 

 

[-rick-]

Yeh, that matches my thoughts. Any suggestions for the best strategy to achieve this while maintaining as high a DT as possible?

[marshian]

1 minute ago, -rick- said:

Yeh, that matches my thoughts. Any suggestions for the best strategy to achieve this while maintaining as high a DT as possible?

 

Why do you want a high DT?

 

 

[-rick-]

1 minute ago, marshian said:

Why do you want a high DT?

 

It's looking like I can't maintain room temp at OAT 5C unless I allow flow temps >60C. With a DT of 5 that means operation will be entirely non-condensing, unless i've misunderstood something.

 

As I understand it, if I can get my flow temps below 50C then DT doesn't matter anywhere near as much.

[marshian]

4 minutes ago, -rick- said:

 

It's looking like I can't maintain room temp at OAT 5C unless I allow flow temps >60C. With a DT of 5 that means operation will be entirely non-condensing, unless i've misunderstood something.

 

As I understand it, if I can get my flow temps below 50C then DT doesn't matter anywhere near as much.

 

Your home is losing heat at OAT 5C - if you need flow temps of 60 deg C to maintain it at target temp then you don't really have a choice

 

Well you do - have a colder house and get condensing efficiency - great or probably not 🙂

 

Boiler flow rate is really bloody crucial - read the installation manual it should tell you what it's min flow rate needs to be for it to be happy

 

Screwing the flow rate down thro the rads and ending up with a really unhappy boiler isn't a great way to minimise energy usage

 

MWT in the rads is where you need to look 

 

Hypothetical Example Rad 

 

Flow 60

Return 50

 

Mean water temp 55 but this is achieved at a flow rate of 104 l/hr and the heat output is 1214W

 

Same rad

 

Flow 62,5

Return 52.5

 

Mean Water temp still 55 but flow rate would be 114 l/hr 

 

Multiply that round all the rads and you quickly add all the increases of flow to make the boiler happier - it stops doing the overheat short cycle and just does equal burns every cycle

 

[marshian]

Looks to me like the only way to widen the DT at the boiler and the rads is increase the size of the rads

 

What boiler is it and what pump (If not in the boiler)

[-rick-]

31 minutes ago, marshian said:

Looks to me like the only way to widen the DT at the boiler and the rads is increase the size of the rads

 

Yeh I agree. Unfortunately, they are not bog standard ones and colour matched to the interior so getting replacements that match the rest is not cheap. I've ordered some cheap Chinese fan kits to attach to the small rads to see if that buys me enough output but they won't be here for a while. If I'd known what I know now a few years ago I would have replaced the rads but given I want to move I doubt I'll be here long enough to get any sort of payback on the rads. Or at least, reverting to my previous heating strategy of ignoring the temp in that room seems like a better plan to me.

 

The other rooms are fine and radiators seem adequate. Maybe not enough for low temp at -5 OAT but should be good at 0.

 

31 minutes ago, marshian said:

What boiler is it and what pump (If not in the boiler)

 

Keston System S30 (A rebranded Ideal Logic with extra gubbins to support 50mm flues). Pump is built in Grundfoss UPM3 15-75 (according to the photo in the manual), boiler modulates pump between 70 + 100%. 

Edited 1 hour ago by -rick-

[-rick-]

15 minutes ago, marshian said:

Boiler flow rate is really bloody crucial - read the installation manual it should tell you what it's min flow rate needs to be for it to be happy

 

Screwing the flow rate down thro the rads and ending up with a really unhappy boiler isn't a great way to minimise energy usage

 

Agree, it's why I started this thread, to check I hadn't misunderstood something.

 

If I open up the lockshields on the cold room rads again, dT will on those rads will drop to 2-3C, overall dT 5-6C. So just wondering if you/anyone has any suggestions for how to find a middle ground, ie, keeping the boiler happy while maximising dT. I don't think I have any way of measuring the flow rate.