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 17 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 16 hours 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. 

[marshian]

7 minutes ago, -rick- said:

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. 

 

I think you can "back calculate" flow rate thro each rad if you know the flow temp and return temp??

[JohnMo]

Your boiler is a weather compensation boiler, so why not use it as one?

 

Does your system follow the rules in the manual 

 

Note. Systems incorporating zone valves which could completely cut off the flow through the system must also include a bypass.

The boiler does not normally need a bypass but at least some radiators on the heating circuit, of load of at least 10% of the minimum boiler output, must be provided with twin

lockshield valves so that this minimum heating load is always

available.

 

Balancing
1. Set the programmer to ON.
Close the manual or thermostatic valves on all radiators, leaving the twin lockshield valves (on the radiators referred to above) in the OPEN position. 
Turn up the room thermostat and adjust the lockshield valve to give an uninterrupted flow through the radiator.These valves should now be left as set.
2. Open all manual or thermostatic radiator valves and adjust the lockshield valves on the remaining radiators, to give around 20oC temperature drop at each radiator.
3. Adjust the room thermostat and programmer to NORMAL settings.

 

 

[-rick-]

11 hours ago, marshian said:

I think you can "back calculate" flow rate thro each rad if you know the flow temp and return temp??

 

Good idea!

 

This is what GPT came up with (which seems good to my non-mathsy eyes):

 

Flow rate for the specified radiator and temperatures

Given your inputs, we can estimate the radiator’s actual heat output at 60°C/50°C water and 21°C room air, then use an energy balance to get the flow rate.

 

Given data

• Radiator rating: 550 W at ΔT50 (mean water to air)

• Inlet water: 60°C

• Outlet water: 50°C

• Room air: 21°C

 

Radiator output at actual conditions

• Mean water temperature:

 

Tm = 60+502 = 55∘C

 

• Temperature difference to air:

 

ΔTair = 55−21 = 34∘C

 

• Output correction (ΔT method): Radiator output scales approximately as Q=Q50(ΔTair/50)^n. For typical steel panel radiators, n≈1.3.

 

Q ≈ 550×(3450)^1.3 ≈ 550×0.606 ≈ 333 W

 

Flow rate calculation

• Water-side temperature drop:

 

ΔTwater = 60−50 = 10∘C

 

• Mass flow (energy balance):

 

m˙ = Qcp⋅ΔTwater ≈ 333 W / (4180 J/(kg\cdotK)⋅10 K) ≈ 0.0080 kg/s

 

• Volumetric flow (water, ρ≈1000 kg/m3):

 

V˙ ≈ 0.00801000 m3/s = 8.0×10−6 m3/s ≈ 0.48 L/min ≈ 28.7 L/h

 

Notes and sensitivity

• Exponent sensitivity: If the radiator type uses a different exponent (e.g., n= 1.2 to 1.51.5), the output—and thus flow—varies modestly. Across n = 1.2–1.51.5, the flow would be roughly 0.44–0.53 L/min.

• Fluid properties: If you’re using glycol mix, adjust cp and ρ\rho accordingly; glycol will increase the required mass flow slightly.

• Direct answer: About 0.48 L/min (≈29 L/h) for water under these conditions.

 

If you know the exact radiator type, I can refine the exponent and give a tighter figure.

Edited 3 hours ago by -rick-
formatting

[-rick-]

3 hours ago, JohnMo said:

Your boiler is a weather compensation boiler, so why not use it as one?

 

Two reasons:

1. It wasn't installed as one and I don't have the kit it was supplied with (manual says it should have been included in the box)

2. While I'm sure I could get the kit, the boiler is installed in the centre of the property and installing an outside sensor would involve making lots of holes in plasterboard. As I needed to replace the timer anyway, my plan was to use load compensation instead of weather compensation. It might not be as good but it's a lot easier to set up.

 

Neither is going to be particularly useful with the size of radiators I have in the room with the highest heat loss and the overall ability of the installed radiators to dissapate the boiler output at minimum modulation (6kw)

 

3 hours ago, JohnMo said:

Does your system follow the rules in the manual 

 

Overall I think so (just wrote a lot on this) but then realise that I think you are just referring to this specific bit:

 

3 hours ago, JohnMo said:

Note. Systems incorporating zone valves which could completely cut off the flow through the system must also include a bypass.

The boiler does not normally need a bypass but at least some radiators on the heating circuit, of load of at least 10% of the minimum boiler output, must be provided with twin lockshield valves so that this minimum heating load is always available.

 

Balancing
1. Set the programmer to ON.
Close the manual or thermostatic valves on all radiators, leaving the twin lockshield valves (on the radiators referred to above) in the OPEN position. 
Turn up the room thermostat and adjust the lockshield valve to give an uninterrupted flow through the radiator.These valves should now be left as set.
2. Open all manual or thermostatic radiator valves and adjust the lockshield valves on the remaining radiators, to give around 20oC temperature drop at each radiator.
3. Adjust the room thermostat and programmer to NORMAL settings.

 

Firstly, there is a bypass installed. I have two towel radiators with manual flow valves only (not lockshields, so nothing to prevent anyone adjusting them). It's not entirely clear to me what they mean by uninterrupted flow. I've adjusted the valves so there is a dT across these two radiators of about 5C. One of these radiators is the closest to the boiler (<2m) so was bypassing a large portion of the flow until I started balancing. 10% of the minimum output of my boiler is 600W.

 

The radiators are 500x750 and looking at similar online they I'm finding outputs specified as ~250W per at dT 50. I struggle to believe that's right given the heat output I get from these (the one closest to the boiler seems to overwhelm the other radiators nearby (assuming doors are left open) putting out enough heat that the other radiators barely come on.

 

Even if these two towel radiators are turned down too much right now, there will still be plenty of flow elsewhere to satisfy the minimum specified above as I have the TRVs removed from 4 radiators with a total output (at dT 50) of approx 3500W, these rads were (at the start of this thread) controlled to a dT of about 8 but all still putting out decent heat.

[JohnMo]

So how many radiators do you have

[marshian]

Do that for all the rads and then sum up the total flow rate and you'll have a better idea of the total flow rate.

 

Rule of thumb I've used over the years is for every kW of boiler output (up to 15kW) you need a min of 1 litre/min for each kW to keep the boiler happy (this is probably out of date for modern boilers with good turndown ratio's.)

 

After 15 kW provided the boiler modulates down you can get away with an extra 1 litre/min for every 2 additional kW increase

 

Example - current boiler is 16kW but range rated down to min - most it hits on start up is 58% of max output so 60% of 16 is 9.6 kW 

 

Current Flow rate is 0.6 m3/hr = 10 litres / min is more than enough to keep the boiler happy under start up conditions and in normal running

 

Manual Says 633 Litres per hour is the expectation which is 10.5 litres/min

 

 

[-rick-]

2x Single Flat Panel 800x500

2x Single Flat Panel 1000x600

1x Single Flat Panel 800x600

1x Double Flat Panel 800x600

1x Single Flat Panel 400x500

2x Towel Rail 500x750

 

Total 9

 

All fed via what looks like Speedfit 15mm pipes homerun to manifolds at the boiler. 22mm copper at boiler.

 

Edit additional info:

 

Pipe is through the floor (presume embedded in screed, though can't feel heat through floor except in very limited area near manifolds. Furthest radiator is about 6m straight line away, closest <2m

Edited 1 hour ago by -rick-
More info

[marshian]

Looking at the installation manual it needs 21.5 Litres/Min at Max output pretty much in line with my ball park numbers.

 

I'm guessing as the boiler has control of the pump it's not possible to slow the flow rate down and with 9 smallish rads it's going to have a narrow DT unless the boiler sees that and slows the pump down as well as modulating down to min.

[-rick-]

1 minute ago, marshian said:

I'm guessing as the boiler has control of the pump it's not possible to slow the flow rate down and with 9 smallish rads it's going to have a narrow DT unless the boiler sees that and slows the pump down as well as modulating down to min.

 Best it can do is set the pump at 70%.

[-rick-]

1 hour ago, marshian said:

Do that for all the rads and then sum up the total flow rate and you'll have a better idea of the total flow rate.

 

Finding GPT really useful for this. It suggests:

 

2K water temp drop, combined flow rate 26.9 L/min

5K water temp drop, combined flow rate 10.2 L/min

10K water temp drop, combined flow rate 4.65 L/min

 

I guessed at some of the heat loss specs but they are roughly inline with what I've seen while browsing radiators.

 

So in summary, to ensure sufficient flow I need to balance to maybe a dT of 2-2.5C? (I doubt my boiler will ever fire at full modulation for long at all given the size of the radiators so maybe could target minimum flow and around 4K drop)?

Edited 1 hour ago by -rick-