Gas combi boiler

ThisIsWeird

moneytalks said:

Following on from keeping TRV at 5, if the flow rate was set at 50-55 and heating run for a couple of hours, would the boiler condense and therefore be efficient, but would the rooms heat quicker?

At 50-55oC flow, I'd expect the boiler to be very close to full condensing operation, with its correspondingly cooler return, so - yes - more efficient operation.

Having the TRVs at '5' (fully, permanently, open unless your house is on fire) as opposed to, say '3' (19-20oC) would have zero effect on anything, until the actual room temp began to reach the TRV-set temp. Obviously, set at '5', this wouldn't normally occur.

At any realistically achievable TRV-set temp - frost, 1, 2, 3, 4 - the rad will begin to shut off. Until then, the rad will effectively be fully open.

For example, if you set your TRV at '3' in a room that's currently at 15oC, then the TRV will effectively be fully open, and the rad will benefit from a full flow. The room will warm up at its max rate given the boiler's flow temp. As the temp reaches, say, 18oC (I'm guessing), then the TRV may begin to close off, so the rad will become less warm at that stage, so, yes, you could argue that the final degree or so of desired room temp would take longer to achieve. However, since the boiler will by now have modulated down (burner rate turned down) in order to maintain that now-much-easier-to-achieve ~55oC flow, it should still be in condensing mode as before.

The same room, same ~15oC starting point, but with TRV now set to '5'; the room will warm up at its max rate as before, given the same boiler flow temp. As the temp reaches, say, 18oC, nothing will change. At 19oC ditto. 20oC dittoditto. The room will require an external influence to turn off the heating, and everything until then will be a 'waste'; ie, there's nothing to stop the room from over-shooting the desired temp to above the required 19oC.

In both these cases, the boiler will modulate its burner down to maintain that user-set ~55oC flow rate with as little flame as possible. Once it's reached its lowest possible flame to do this - and it's still too much - its remaining option is to shut the burner off - it'll 'cycle' - to keep the flow at ~55oC. (Of course, the room 'stat will likely have turned the boiler off before then).

So, 3rd scenario - this time with a 'Smart' control with an Opentherm-style protocol. The actual boiler flow temp will be reduced in order to match the falling demand. Ok, you wouldn't have TRV in the same room as the 'stat, but in a similar scenario as above, as the boiler realises the CH demand is being met more easily, rather than just modulating down, it'll go a step further and drop the output flow temp in order to match what's needed. Ie, it'll provide the coolest flow it can to get the job done, always trying to achieve the max condensing mode.

moneytalks

As previously stated, some of my TRV whistle when heat on for approx. 45 mins. TRV set at 3 on whistling ones. 

 This morning, in a room upstairs I opened the TRV fully, and ran the heating for approx 90 mins. No whistling from any TRV. I don’t want to run at 5, I will experiment with TRV settings and see what setting stops the whistling.

FreeBear

moneytalks said:

Following on from keeping TRV at 5, if the flow rate was set at 50-55 and heating run for a couple of hours, would the boiler condense and therefore be efficient, but would the rooms heat quicker?

It is the return temperature that is important for the boiler to condense. Generally speaking, the return is usually 20°C cooler than the outward flow (assuming the system has been balanced properly) when set to 70°C. As the flow temperature drops, the difference between return also drops (i.e. it doesn't remain at 20°C). With my system, at 70°C flow, return might be 20°C cooler. At 55°C, 17°C cooler, and at 45°C, 12°C cooler.

Keeping the return temperature below 50°C should give you a theoretical efficiency of around 90%. Going down to 40°C return might push efficiency to 94%, but the gains in efficiency get increasingly smaller as you reduce the flow/return temperatures.

Reducing the flow temperature will certainly increase the time the house heats up - At 55-60°C, I'll get a temperature rise of around 1°C per hour. Drop down to 45-50°C, and it could double the time the house gets up to temperature (but the amount of gas used remains broadly similar).

At the end of the day, it is a balancing act between speed of heating the place up, gas usage, and boiler efficiency. I've settled on a maximum flow temperature of 55°C rising to 60°C if it is subzero outside and am happy with a 1°C per hour increase indoors. Boiler efficiency is around 92-94% as a result. BUT my radiators are deliberately oversized, so I can take advantage of lower flow temperatures.

As for the whistling TRVs - Check the arrow on the valve body and see if it matches the direction of flow. I made the mistake of installing a TRV the wrong way round once, and it made a right racket as it started to operate.

Grandad2b

This is all a learning experience for me: we have no TRVs and I don't know whether to bother. New boiler and wireless stat in June last, we certainly seem to be using less gas with flow temp set to 55˚C which gives about 1˚ per hour temp increase. Heating does tend to overshoot by about 0.3˚ by the time the boiler has gone off.

FreeBear

Grandad2b said:

This is all a learning experience for me: we have no TRVs and I don't know whether to bother. New boiler and wireless stat in June last, we certainly seem to be using less gas with flow temp set to 55˚C which gives about 1˚ per hour temp increase. Heating does tend to overshoot by about 0.3˚ by the time the boiler has gone off.

Building regs say that "each room should be provided with thermostatic room controls" (Part L, section 5.20) - This can either be a thermostat or a TRV. Many installers will not check for TRVs when doing a basic boiler swap, and a few won't even bother with fitting smarter controls (i.e. a programmable thermostat). When I had my boiler fitted, the installer didn't bother to check to see if TRVs were fitted (got a list of other things he should have done but didn't)..

55°C is the current recommended design flow temperature for new installations and where the entire heating system is being replaced. As for the overshoot, just knock the thermostat back by 0.5°C and save a little bit of gas in the process.