Condensing boiler efficiency with TRVs

[teak]

Looking to have a new condensing boiler installed and have been pointed in the direction of band B boilers and avoid TRVs as they stick and are just trouble. Are Band A boilers worth the extra or not? Also thought TRVs are now part of the building regs same as condensing boilers? So might be an issue when selling the house?

I have read that too many TRVs can hurt condensing boiler efficiency, as in if too many rads are off at the same time, the return temp will increase and too high for condensing mode. With the suggestion being only fit 1/3-1/2 of rads with them, in rooms that overheat and bedrooms where you want it cooler than the rest of the home. Yet I have also read they actually help keep a low return temperature by reducing flow rate initially rather than just switching the rad off. When off the return temp might rise but a fully modulating boiler would lower the output with the reduced demand and keep the return temp low enough to condense still? Which view about TRVs is correct?

Would you need a boiler that could modulate low enough, like you see them listed as 7.7kw-30kw. Would you need a flow switch, modulating pump (Grundfos Alpha?) or do modulating boilers not need that? Do they just turn down the burner or can they lower the flow rate in response to the TRVs, spending more time in the rads that are on and returns cooler. Or would a modern condensing boiler just detect the higher return temp, modulate down and at some point switch off with the anti-cycle control preventing it from coming back on too quickly?

 

[teak]

*Moved question to a seperate topic*

 

[cjb1971]

For current regs dont trv`s have to be fitted to rads when fitting a condensing boiler?

 

[teak]

Could be wrong, but my impression was that any new system install has to have controls which at the very least alllow a different temperature in the bedrooms compared to the rest of the home. Fitting TRVs isn't the only way to comply with that, but is the one suggested maybe as it's the simplest? Other way might be to split upstairs and downstairs into seperate heating zones, each having a seperate room thermostat (and timer?).

Which might explain why one place suggested just fitting bedrooms with TRVs. Not sure if any updated regs now require all rads (except the one near the room thermostat) to have TRVs. Or if that is just more efficient and makes sense to upgrade all at the same time. Was told since the new system would be under pressure, we'd probably have to replace all the old rad valves anyway. Have read going all-TRV adds complications, like needing a bypass (to keep required boiler flow when all are closed?) and still unsure if too many TRVs reduce boiler efficiency or not.

Although considering the emphasis is on efficiency these days, I guess it would be strange they'd suggest TRVs on new systems if it reduced boiler efficiency. Why bother making condensing boilers a requirement then suggest use of a control that prevents it condensing. But would like some clarification from those in the know.

 

[teak]

Found a post by someone in 2000 claiming to have got a reply from Central Heating Information Council (CHIC) about TRVs. Basically saying that suggesting TRVs prevent condensing is misleading as condensing boilers only condense anyway below 35oC return temp and that only happens on first firing from cold or if an "Energy Management System" is fitted, which automatically controls the return water temp.

Thought the dew point of NG flue gases was more in the region of 55-60oC, so only below that is required for condensing? Even then, aren't modern condensing boilers being fully modulating designed to go into condensing mode as often as possible? Maybe that CHIC info is out-of-date. Or is the "Energy Management System" what makes boilers modulating, so modern boilers come with it anyway?

I'm starting to wonder if TRVs are only a problem with non-modulating boilers, that cannot adapt to the increase in return temp. Where as fully modulating boilers will be fine as they can reduce output to keep the return temp low enough to condense for longer, until it's switched off completely by the room thermostat/timer?

Would balancing the rads for a 20oC drop normally done to help achieve a low enough return temp for condensing, still be required/useful for a fully modulated, all-TRV (except near thermostat) setup?

 

[Slugbabydotcom]

According to the latest CHeSS specification 2005 TRV's must be fitted to all rads except the one where a room stat or programmable room stat is fitted.
An automatic bypass must also be fitted for occasions where there is no or little flow through the rads.
Any boiler should be adjusted as per the MI's so that the output matches as closely as possible the heat requirement of the building.
There is a calculator here. When you get to that site click the link to 'Recommended boiler size' to work out the output to which your boiler should be set.
This will reduce the amount of whats known as sawtoothing that your boiler will do. To translate that:- Instead of your boiler being switched on and off lots of times because the output is set too high, Less gas is used but over a longer period. This reduces the number of operations of the gas valve increasing its lifespan and more importantly the gradual burn is condusive to more efficient operation.

 

[Agile]

Band B boilers are only 2-3 % less efficient than Band A and are much less sensitive to mis setting.

My expectation is that Band A will need much more frequent servicing by competent engineers with flue gas analysers which counts out 90% of the independents.

Band B uses proven technology with the addition of a second heat exchanger to get the condensing. They have conventional burners and common easily available parts instead of the expensive variable speed DC motors on Band A premix boilers.

I do have to say that my current suggestion is that for most people a Band B is likely to be a cheaper boiler to both install and own. I only have a Band A in my own home because I can service and repair it myself.

You can have a combi for the kitchen hot water and keep a cylinder for the bathroom.


Tony

 

[teak]

Just found this on the Worcester Bosh website:

The pre-mix burner automatically adjusts to the set level. The flow temperature of the boiler is then maintained at the customer setting by the facia mounted variable control. Should the system requirements reduce during operation (TRVs closing down, etc.) and the flow temperature exceed the customer setting then the burner will modulate downwards to match the system demand level. Should the flow temperature continue to rise then the burner will be de-energised and the control system will go into an anti-cycle mode and not allow the appliance to re-fire for a set period.

Which seems to suggest my guess that modern modulating condensing boilers have no trouble with TRV was right. Maybe all the negative comments about TRVs come from the non-modulating boiler days and not really an issue anymore.

Thanks for the input. Will probably start a seperate thread on cylinder vs combi suggestions.

 

[teak]

Just reading the Domestic Heating Compliance Guide May 2006 (from Office of Deputy PM):

http://www.planningportal.gov.uk/uploads/br/BR_PDF_PTL_DOMHEAT.pdf

Table 2 (p15) lists the minimum provisions for control of gas fired central heating systems, with a section for new systems (ie no existing system) and replacement systems.

Under Space zone heating, new systems up to 150m2 should be divided into 2 space heating zones with independent heating control. One being assigned to living area. For replacement systems is the same, except when only replacing the boiler where it's acceptable to only control one heating zone.

It then goes onto state Temperature control of heating space. Saying seperate temperature control of zones should be provided using either room stat (or programmable) in all zones, or room stat (or programmable) and TRVs on all radiators in the other zones. Or a combination of both.

So does look like there is a requirement for TRVs on all radiators (except the room stat one) if it's a new system and not using room stats in all zones. If replacing more than just the boiler (ie rads, etc), it also seems a requirement. But not if you are only replacing the boiler, since you are allowed to keep it a single zone? Wonder if it's this loophole that the fitter is able to get away with not suggesting TRVs with just replacing the boiler. But supposedly we need more than just the boiler..converting to fully pumped, some pipe work, etc. So loophole doesn't apply?

 

[croydoncorgi]

Despite Agile's comments about 'complexity' of Band A versus Band B, I would only saythat IMHO Band B boilers based on 'proven technology' MUST have an additional heat exchanger to achieve sufficient efficiency for Band B. In some examples, this CANNOT be opened in any way for cleaning. Whether this turns out to be a long-term problem, only time will tell. I'm also unconvinced about Agile's implicit assumption that 'proven' means 'reliable'. For gas valves and atmospheric burners, this is true but for the cheap and nasty fans fitted to most lower cost boilers, it isn't. I have changed a number of variable speed DC fans but NEVER because the fan motor or driver circuits/ tacho on the PCB had failed. It was in all cases due to a manufacturing problem which meant the impeller fell off the motor shaft!!! That particular fault no longer happens.

It is true that without some form of pump modulation and TRVs fitted, there can be situations where the Return temperature frequently goes above about 56C (NOT 35!!!) and the boiler cannot condense. The (BRE, government, etc.) argument is that this doesn't matter because the boiler is anyway much more efficient than most of the old non-condensing types, even when not condensing. Probably true, for most comparisons. Personally, I prefer to build systems configured with weather compensators, correctly-sized rads, modulating pumps,etc, which together ensure that the total energy input to the building remains proportional to the actual heatloss, including the effect of changing outside temperature. Such designs allow a condensing boiler to condense ALL the time when it's providing heating and SOME of the time when it's heating domestic hot water. For example, I was at a property in Westminster with about 45 TRVs yesterday where (in January!!) the Flow temperature FROM the boiler was down to about 50C degrees, due to an outside temperature of 9C. degrees.

 

[micky p]

I can't see how band B boilers are time proven. Without the second HE on they were band C and majority worked fine. But it is only recently they have started with this add on aluminium HE. time will only tell as the second HE are not servicable and I wouldn't like to guess how long an aluminium HE will last being in contact with condensing water vapour that is acidic.

Premix Band A boilers have been on the market for approx 25 years and some were poor quality but most were ok .For the likes of Viessmann and Broag to name a couple have been in the game years and built only quality Band A boilers

On my judgement i would go for a good quality band A condensing boiler with a weather compensator that will automatically adjust the required heat that is needed to heat your home.

Just make sure you get a decent engineer who knows what he is doing with them to install/ service it.

As for the TRV you need a way of controlling individual rooms via a thermostat so the easiest and cheapest way is a trv. It is the building regs im afraid.