Weird wiring required for boiler

[waynemarlow]

We have an oil fired boiler which has a basic high level thermostat which is ideal for heating the water cylinder ( a few hours per day ) but we have also an underfloor heating system which requires a lower temp water ( most of the day).
Does anyone know of a dual temperature thermostat ie one that has two adjustable temps. It is a waste of energy to heat to high temperatures when a lower level would do.

 

[giblets]

Errr - mixing valve, seperate controls for the u/f heating maybe?

Have a wonderful mental vision of you hopping about on a floor heated to 80deg C.

 

[simond]

If your boiler is not a high efficiency (eg: Condensing) model it will not have much range adjustment on the boiler temp stat.

Your UFH will have a mixer on it to lower the water temp.

There is no remote boiler temperature stat available, unless your boiler was available with weather compensation (unlikely).

If you believe you are wasting significant amounts of energy running the boiler at higher temps - where do you think the wasted heat is going?

Grant market a retro fit weather comp system for oil and gas boilers, but it does not alter the boiler temperature. It acts rather like an electrically controlled UFH mixer. See www.grantuk.com for details.

 

[croydoncorgi]

An important factor IF the boiler is NON-condensing:

YOU MUST NOT REDUCE the operating temperature (ie. wind down the boiler thermostat) to the point where condensation might occur inside the boiler. Each boiler of this type will have a stated range of temperatures which are 'safe' for each design.

If you run it too cool, you risk 'back-end rot' due to condensation at the far end of the combustion chamber. This can wreck a boiler beyond repair in a matter of months.

As has already been suggested, you COULD use a blender valve to reduce the temperature of the Flow towards the radiators / UFH system but if the UFH manifold(s) already have individual mixing valves (as they should), where's the benefit from doing so? Little or no efficiency gain.

 

[Terrywookfit]

Extact from grant Vortex manual!

On underfloor systems it is essential that the return is
pre-heated by mixing flow water into the return before
it enters the boiler. The return temperature must be
maintained above 40°C to prevent internal corrosion
of the boiler water jacket.

 

[croydoncorgi]

...in which case, the best option will be a 4-port blending valve, which will reduce the temperature TO the UFH AND ensure that the Return temperature remains high.

(I'm surprised by Grant's statement that 40C degrees is hot enough to prevent condensation).

 

[simond]

Er, isn't that what I proposed three posts ago Croydon Corgi?

quote]If you believe you are wasting significant amounts of energy running the boiler at higher temps - where do you think the wasted heat is going?

Grant market a retro fit weather comp system for oil and gas boilers, but it does not alter the boiler temperature. It acts rather like an electrically controlled UFH mixer. See www.grantuk.com for details.[/quote]

 

[croydoncorgi]

errr - sorry Simond, your link led only to the Grant Homepage, so until now I didn't actually root around to find the 'Grant Efficiency Optimiser' which is presumably what you had in mind.

However, having had a look at the info about this gadget (such as it was) I get the distinct impression that it would NOT be suitable for a non-condensing boiler - the emphasis of the info is on EXTENDING condensing mode in the boiler, not preventing it. But a 4-port mixer is usually utterly useless for achieving this objective because in it's 'throttled' state (REDUCING temperature of the flow in the direction of rads, UFH), it will inevitably INCREASE the Return temperature to the boiler.

If you have further info on this device that clarifies this confusion, please post it here.

 

[waynemarlow]

Yes the boiler is a high efficiency ( oil boilers don't really condense )requiring allowing slightly lower than normal returns plus each under floor zone has its own temp blending valve but to run the boiler to 50c rather than say 80c for most of the time would save quite a lot of fuel over a long period of time, its just extracting the best efficiency as the boiler works more efficently heating cold water than heating to higher temp levels

 

[croydoncorgi]

You haven't told us the boiler model so it's difficult to assess the full situation. I'm assuming for now it's a Vortex of some sort.
Grant advice clearly states that Return temperature should remain ABOVE 40C degrees. In a typical situation (without any blending valve(s) and / or with Return water coming straight back to the boiler from UFH zone(s)), it would be hard to achieve this with a Flow temperature as low as 50C degrees.

What advice did Grant give? (seems obvious - I assume you DID ask!) Why did you reject it?

 

[waynemarlow]

Its a Warmflow HE, and yes did ask, return of 30 -35 OK by manufacturers, in practice the return is about 30 -32 so sought of working OK. Heat needs to be only 38 reaching the floor to give good even heat with a floor temp of about 25.

Would just be nice to keep the boiler working in the optimum most efficient way ie with low water temperatures and then boost it up to high levels when the water cylinder requires higher temps. Probably won't gain a lot of savings but over a period of time it could mount up.

Thanks for your interest.

 

[croydoncorgi]

There's no problem with any Return temperature below 50 or so degrees, so long as it's OK to push the boiler into condensing mode (it is in this case- it's an HE boiler) AND the difference between Flow and Return temps is not greater than recommended (usually up to 20C degrees).

There is one more thing you need to consider: the temperature drop across the worst-case UFH loop. If you have a thermostatic mixer and a separate pump on each UFH manifold, quite a high flow temperature may be needed to get enough heat into the manifold at the maximum flow rate from the boiler. Consider what could happen: boiler flow temperature set at (say) 55C degrees, mixer set to give a loop inlet temperature of (say) 50C degrees, temperature drop in the loops from 50C down to (say) 30C. The mixer will be wide open but because of the pressure differences, flow dynamics, etc., it can't actually blend-in enough water from the boiler to get the inlet manifold up to 50C. So maybe the floor doesn't get hot enough.

At the same time, because of the big demand for water VOLUME from the manifold(s), the main circulator is probably pumping at its maximum rate against virtually no dynamic head, so there's undesirable pipe/ pump noise.

If you increase the boiler Flow temperature to (say) 60 degrees, then for the same amount of heat, the UFH mixer now needs a smaller Flow volume, so it closes a bit. This reduces the demand on the main pump and the flow rate through the primary pipework, so less noise. The inlet manifold temperature will now reach 50C, so the loops will be at the right temperature and the floor will heat up correctly. Chances are that the Return to the boiler will still be well below 50C degrees, so the boiler will condense and deliver its maximum efficiency. Any efficiency gained from dropping the boiler Flow temperature much below 55C will be quite small anyway, compared with the big gain from condensing mode.

I happened to be talking to a UFH manufacturer today: their system spec requires at least 60C at the mixing valve inlet to give a blended temperature of 50C in the inlet manifold.

NOTE: if your UFH manifolds are the 'direct feed' type, with no local temperature control, then different design rules apply. You MUST have a 'whole house' UFH mixing valve to keep the Flow at the correct UFH inlet manifold temperature or controls on the boiler to achieve the same result, ideally a weather-compensator controller which directly matches floor temperature to outside temperature. If there's only one boiler, you also need to consider hot water cylinder heating: Flow temperature for this needs to be at least 75C to achieve an acceptable recovery time. Controls and motorised valves to make this all happen tend to get a bit complicated!

 

[simond]

Most oil boilers use a secondary heat exchanger made from stainless or aluminium - although I did see a Kroll the other day with a plastic secondary HE.

They have to ensure that by the time the water hits the primary HE it has reached sufficient temperature to avoid condensation occuring, or rapid corrosion and failure will result.

I am surprised that Warmflow advocate 32C.

One should bear in mind as a purchaser that many of the manufacturers making oil condensers have only been producing them for 24 months or so..... therefore no one really knows how reliable they are going to be.

There are a few manufacturers of oil condensers that have been making high efficiency boilers for much longer (my favourite being Kidd from 1982 onwards) and those who are risk averse may like to look at how long their boiler has been in production before intentionally designing a system to reduce the return temp to 30C.

Kidd will not advocate the return to go below 42C, by the way.

 

[waynemarlow]

From practical experiance the higher temp you put into the blending valve the lower return temp of the boiler return as most of the return feed is diverted into the blending valve which as the input is high temp then slows down the input flow which then means all the extra heat you have created is wasted in the cooling ( slow flow ) of the water in the feed pipes, its a bit of a vicous circle.

This then gets me back to my original question, I would like to have a flow rate of about 60 for the UFh and a flow rate of 75 - 80 for the high efficiency water tank, is there any easy way of doing this without resorting to two seperate thermostat controls ?

Thanks for the great answers by the way, I'm learning.

 

[Agile]

It may be to simplistic but what do the others think about a pipestat set to say 55 C on the flow pipe in series with the room stat.

That would cause the boiler to go off immediately the CH flow reached 55 C. That would cause increased cycling but seems to me to be the only simple way to achieve the requirements.

I would expect a blending valve to cause the boiler to go off pretty quickly unless the heating load is very high.

Part of the problem is that boilers are usually massively over sized for teh actual heating load and oil boilers dont modulate!

Tony