Radiators balanced at full load, but not at low demand

[chatmandu]

Earlier today, I spent a couple of hours balancing my radiators. I did it via the Drayton TRVs using their balancing key.

I managed to get them fairly well balanced with a flow temp of 65C. The radiators were really pumping out the heat and the return temperature was well below the condensing threshold.

Now the house has cooled down a bit, and the Honeywell T4 thermostat is calling for heat, things seem to have gone a bit wonky. The temperature is set to a steady 21C and the boiler is performing short burns to top up the heat (my boiler can only modulate down to 6.5kW), so the target temperature is achieved before the flow temp reaches 65C. This results in the radiators getting warm, but not hot. I think that is fairly standard when maintaining temperatures unless you have a Viessman boiler.

All the radiators are double panel, double convectors - apart from a double column radiator in the kitchen.

That column radiator feels a bit cooler than the panel radiators. When I was balancing, I set it to '1' and, even then, the return temp was a bit higher than the other radiators (Delta T of 12 vs 20 for the other radiators) . I didn't want to set it any higher because I was worried the return temp might increase and lead to boiler cycling. Now that things are running at a lower flow temperature, it appears to be getting too little flow to reach the same temperature as the other radiators.

Any thoughts on how to balance that one radiator so that it 'works' at both high and low flow temperatures?

 

[Johntheo5]

A setting of 1, Kv of 0.1 is a flowrate of 0.1m3/hr, 1.67LPM @ 1bar dP, if one assumes a "average"1M dP across the rad then the flowrate is 0.032m3/hr, 0.53LPM?, this flowrate, theoretically, will give a dT of 27C/kw of rad output, you say you are getting a dT of 12C across the column rad which infers the rad is outputting ~ 2.3kw normally?
What are the settings roughly of the other rads?.
When a boiler is cycling rapidly especially when the flowtemp exceeds the setpoint temp by +5C (which is what I take you are saying above) then very difficult to figure out how to deal with this. Vaillant boilers are very prone to this.

 

[chatmandu]

The column radiator is double column 2000h x 490w. According to the Screwfix website, the output is 1380W.

I have changed the TRV to pre-setting 3 to roughly double the flow. I think the volume of water in the column radiators is much greater than the Type 22 rads?

It is a Vaillant boiler, but I don't think the cycling is due to the return temperature. The boiler didn't cycle when I was balancing the rads. The system ran for 2 hours with a flow temp of 65C. After I had finished balancing, and the boiler switched to pump overrun. The boiler then displayed a flow temp of around 45C, so I think return temps are OK?

Do Vaillant boilers target a Delta T of 20C?
Will they cycle if the Delta T drops below 20C - e.g. flow=50C and return=35C?

I think the cycling is being caused by the Honeywell thermostat. It supposedly 'learns' the system and is calling for a small heating top-up to maintain the temperature at 21C. The boiler initially pumps, ignites, modulates down and the flow temp will slowly rise. When maintaining a temperature, the flow temp never reaches the configured maximum flow temp. This is because the target room temperature is quickly achieved and the boiler switches to pump overrun.

The Honeywell thermostat works via relay, rather than ebus/opentherm, and appears to be attempting some form of 'smart' heating. From what I can tell from the manual, it learns how long it takes to heat the house and then uses that information to request heat for x minutes. This prevents the room temperature overshooting. Unfortunately, the temperature range tolerated by the Honeywell is too narrow for my boiler and can result in cycling.

Maybe I need to limit the temperature setting on the TRV near the thermostat (e.g. change from max to 3.5) so that the boiler runs a bit longer and the kitchen gets a chance to warm up? But that might introduce excessive heat issues in my living room - this houses the radiator furthest from the boiler and is currently configured as the 'bypass' (fully open lock shield and TRV set to max).

 

[Mister Banks]

Try setting the thermostat to 3 cycles per hour, the default setting is 6. This should allow the boiler to fire longer.

 

[Johntheo5]

I have a digital (non TPI), one roomstat controls our main livingroom/dining rooms which we keep at 20/21C, these rooms have their TRVs open fully, the other 8 TRVs have setpoints ranging from 16C to 18C and work quite well as their SPs are below the roomstat SP. The roomstat has a adjustable hysteresis of 0.3C to 2.0C, I have it set to 0.3C and the living/dining rooms temperatures vary by the stat SP +0.5C to -05C, not bad, maybe switch off the TPI logic part of the stat (if you can) and just use the hysteresis settings?. I have a oilfired boiler which cycles continuously anyway since obviously non modulating.

 

[chatmandu]

Try setting the thermostat to 3 cycles per hour, the default setting is 6. This should allow the boiler to fire longer.

Thanks I'll give that a try.

I have a digital (non TPI), one roomstat controls our main livingroom/dining rooms which we keep at 20/21C, these rooms have their TRVs open fully, the other 8 TRVs have setpoints ranging from 16C to 18C and work quite well as their SPs are below the roomstat SP. The roomstat has a adjustable hysteresis of 0.3C to 2.0C, I have it set to 0.3C and the living/dining rooms temperatures vary by the stat SP +0.5C to -05C, not bad, maybe switch off the TPI logic part of the stat (if you can) and just use the hysteresis settings?. I have a oilfired boiler which cycles continuously anyway since obviously non modulating.

I've actually ordered a Tado ebus thermostat. I'm hoping that might improve things from a modulation perspective, but I suspect my boiler just can't modulate low enough.

 

[Johntheo5]

Earlier today, I spent a couple of hours balancing my radiators. I did it via the Drayton TRVs using their balancing key.

View attachment 289156

What do the (1K) & (2K) in the 3rd & 4th columns and the (2K) in the 7th column mean.

 

[Johntheo5]

I have a digital (non TPI), one roomstat controls our main livingroom/dining rooms which we keep at 20/21C, these rooms have their TRVs open fully, the other 8 TRVs have setpoints ranging from 16C to 18C and work quite well as their SPs are below the roomstat SP. The roomstat has a adjustable hysteresis of 0.3C to 2.0C, I have it set to 0.3C and the living/dining rooms temperatures vary by the stat SP +0.5C to -05C, not bad, maybe switch off the TPI logic part of the stat (if you can) and just use the hysteresis settings?. I have a oilfired boiler which cycles continuously anyway since obviously non modulating.

Checked it over a few hours this evening.

Ambient: 0.5C. roomstat SP temp: 20C. minimum observed roomtemp 19.7C. maximum observed temperature 20.3C.
20kw oilboiler, cycletime 60mins. ontime 18mins. offtime 42mins. % ontime 30%. Average heating demand 6.0kw. (20*30%))

One or two unusual things I observed that even with a hysteresis of 3C that the stat fires the boiler at a indicated 19.9C, the temp then falls to a indicated 19.7C, the stat shuts the boiler down at a indicated 20C but the indicated temperature then continues to climbs to 20.3.C, so control, this evening, SP +&- 0.3C.
It seems that the stat may be switching at ~ a 1.5C differential on either side of its SP??, the display has "only" one decimal point.

 

[chatmandu]

What do the (1K) & (2K) in the 3rd & 4th columns and the (2K) in the 7th column mean.

It apparently refers to the 'proportional deviation'. I don't think my system was 'designed', so I've got no idea what my system's proportional deviation is. I presume it is 2k, because 1k appears to be better and I think the system fitter wasn't very good.

The important role of radiators in a heat pump system

For a heat pump system to operate efficiently, as for all systems in fact, all components must be fully coordinated with each other. Only then will the system be able to reach its full potential and guarantee optimal efficiency.

global.purmo.com