Understanding TRVs / Combi / underfloor heating

[matthew.larkin]

Hi

We're trying to get our new system (new to us anyway) working most optimally for the coming colder times, and I'm not sure I entirely understand the relationship of the devices we've got. If anyone could explain in nice easy terms I'd be grateful, so I'll put down my assumed understanding and then someone can tell me I'm wrong!

We have a Potterton Combi 100 boiler for HW and CH. (HW is fine, well understood). CH system has a (recently added) programmable thermostat / timer (Drayton Digistat 3) for the rads, TRVs on all (bar two) rads, and then an underfloor heating setup for the kitchen, controlled by a wall mounted stat, heated water for this provided by the CH system.

I think my understand boils down to what TRVs actually do, how efficient a combi can be if only a small amount of the rads need heat, and how to get it all nicely balanced.

So, what do TRVs do? My presumption is that they allow you to control the maximum heat of a rad. I don't think that they monitor the temperature of the room as they are too close to the source of heat to do that. Am I wrong?

In terms of the combi, all rads are fine, but we just want one room hotter, obviously the boiler kicks in. Presumably as that rad cools, it will call for more HW from the boiler, so provided the main stat minimum temperature isn't exceeded, the boiler will respond.

Does the combi have a small store of hotwater in the system? I ask as last night all of the radiators had their TRVs set low, and were all OK. However, the stat for the main system was demanding more heat, as was the stat for the underfloor heating. The pump for the underfloor heating was on, so I presume it was drawing some hot water from the system, however the combi wasn't fired. As the underfloor heating doesn't need particularly hot water (compared to the radiators), I was presuming that it was getting what it needed, and once the small reserve was emptied it would fire the combi (which seemed to happen). Does this sound right?

Thanks in advance for anyone that can help me understand things! I guess some trial and error will be required to get it just right, but if I felt that I understood the way TRVs, stats and combi boilers interacted I would be happier!

Cheers!

Matt

[John Rumm]

Yup... they are intended to set a maximum temperature for the room, not the rad. As the room gets to the preset temperature they will throttle the water flow through the rad to reduce its output.

If the boiler is running (this will be controlled by the programmable stat), then the TRV will maintain the room temp at the preset level (give or take some hysteresis). If the boiler is not running there is nothing the TRV can do about it.

Hence yo u need to ensure that the main prog stat is the last one to be satisfied. This is really down to balancing the system and having the stat in an appropriate place.

Some cobis have a store of hot water - however this is usually only used for the hot water side and not the CH.

It is entirely possible thought that there was enough hot water in the CH circuit to satisfy the needs of the UFH for a short while.

BTW, the fact that the boiler is a combi does not really have any bearing - it would be the same with a heating only boiler.

[matthew.larkin]

Glad I'm wrong!! But how can something right next to the hottest thing in the room actually detect the room temperature? Curious, but not disputing what you say! The effect does seem though that the rads don't get as hot when we have them turned down to 2 or 3.

Yes, of course - that was understood!

That shouldn't be a problem. The stat is in one of only two rooms in the house that is double glazed and has cavity walls (the remainder being 100 year old, and we like the old windows too much to consider replacing). Therefore having the TRV set to a low number in that room and pumping out hot like there's no tomorrow elsewhere should make the stat location the last to heat up.

That sounds right; I hadn't considered the CH as a circuit. Currently the new UFH system is only set to put through water at 35degC, which we will be upping in the next few days / weeks if a) I can get to the screw on the pump that sets this and b) we're doing this gradually as it is set in a new screed (though screed is now about 2 months since laid).

True; I imagine a hot water tank based system would pivot on whether it was pumping water around, whereas the combi is both pump and gas firing (of which the gas firing is the most immediately noticeable).

Do the rads on my system need balancing? I only ask as its something I've heard of, and since we've now got the UFH fitted I'm wondering it that throws out the balance; or do the TRVs provide the balancing?

Thanks for helping out!

Matt

[The Natural Philosopher]

You shuld be OK. IIRC 50C is the bleeding edge for screeds.

Mine was running flat out after a coule of months with no adverse effects.

YOU WILL NEED TO SEPARATELY TIME UFH. The lags are massive on a screeded floor. At best I get about 1 deg C per hour rise in the heated areas on UFH alone.

In winter its on 24x7, because experience shows it makes sod all difference to oil consumption.

They sort of do, but excessive pressure can affect their operation, and frankly I have NOT found they are very good at regualation at all even on a well balanced system.

The colder the day they more they need turning up.

They are at best, better than nothing, but they are not a patch on a room stat and local motorised valve.

[John Rumm]

Its partly because they are stuck out to the side, The hottest bit will be directly above the rad. Where the valve is should be in the path of the convected room air as it arrives at the rad.

That's partly a limitation of the technology. Most of these use some form of wax capsule that expands and contracts in sympathy with changing temperature. As it warms and contracts it pushes down on the valve pin that starts to cut off the flow. If you set the temperature quite low on the valve, then it will be pushing on the pin a little all the time (unless very cold) hence the maximum flow is slightly inhibited. The end result should still be ok - room gets to the right temperature, but the delay may be longer.

That is a problem in fact... you don't want a TRV on the rad that shares a room with the stat. You can arrive at a situation where you defeat the boiler interlock because the stat never gets satisfied.

Having the stat in the best insulated room is also not ideal. A more typical arrangement would be say stat in the hall on a radiator with lockshield valves on both ends (so it is deliberately difficult to adjust). The main stat will only then be satisfied when this room is up to temperature (probably one of the last to be). The TRVs on the other rads would prevent other rooms overheating.

What you could do for the moment, is take the TRV head off the valve on the rad in the room with the stat. This should leave that side fully open. Then experiment with the lockshield valve at the other end to throttle that rad back such that all the others get long enough to heat their respective rooms before the main stat stops calling for heat.

There is probably 100 litres of water or so in all the rads and pipework. Once the rad TRVs start restricting the flow through the rads the temperature of the return water to the boiler will start to increase. Modern modulating boilers will respond by reducing the gas rate and hence amount of heat injected. However there will come a point where the boiler will need to stop firing to prevent the CH water temperature exceeding its maximum pre-set temperature. So the boiler may be in the off state even though the mains stat is calling for heat. (this would indicate your system is not well balanced, or you have a TRV on the rad in the room with the main stat! ;-)

Not sure I follow that.... adding a hot water store does not really change anything - it is just a different heating zone. So depending on what valves are open as to where the CH circuit water from the boiler is directed.

(In fact you can run hot water cylinder on a combi as easily as on any other boiler. (something often overlooked, and a handy way to get decent mains pressure showers from the combis ability to directly heat water on demand, and decent bath filling rates from a stored water system)).

The TRVs certainly help, but will not totally fix a situation if there is a big disparity between different bits. You also need to look at the potential problems with TRV and main stat interactions.

[Ed Sirett]

The main FAQ, the wiki and maybe the resources below could help.

[matthew.larkin]

Interesting. The installation instructions have been followed etc in terms of the screed depth and so on, and the spec of the pump to feed the UFH allows for water up to 55degC to be pumped (that is set as its max limit, so even if you turn it higher it won't exceed that). I'd like to get it as hot as possible, so I'll head for 55 if that is what the manufacturers allow, and then they can replace my floor when it all cracks....

Unfortunately, we can't separately time the UFH (as I understand it). The combi is controlled by a stat, and the stat for the UFH is effectively acting something like a TRV for the UFH system alone, so even if the UFH wants heat it won't get it unless the main stat is triggered. We'll see, I'll go through this winter and if anything needs tweaking I'll explore options next spring.

THanks!

Matt

[The Natural Philosopher]

Do you have a separate UFH pump?

If not its relatively trivial to hook up zone valves on a seperate timer.

Its not too hard WITH a seperate pump.

[matthew.larkin]

Its a separate pump for the UFH. Excuse my ignorance, but if it was on 24/7 how energetic would the boiler be? Is its gas consumption likely to be either a fixed amount when on and zero when off? If that were the case, it would be very expensive to run UFH on a combi, but I may be misunderstanding!

When the plumber comes back to snag various other bits I'll put my new found understanding to the test and see what he has to say!

Thanks

Matt

[The Natural Philosopher]

A energetic as it needs to be. To keep the house warm!

The fact that the controller is on 24x7 doesn't mean the boiler is. It will go on and off as the temp falls below/rises above the master stat setting.