Condensing boilers: getting the return temperature low enough

We're expecting to replace our oil-fired boiler this summer and so will have a condensing one. I read that to be most efficient, the water return temp from the CH needs to be around 55C. Are there ways to achieve this without mucking with the CH pipework? Will just closing all the lock-shields a bit more do the trick? I couldn't see this anywhere on the Wiki but I mat have missed it.

Reply to
Tim Streater
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Um no, that will surely *increase* return temperatures as the radiators won't get as hot.

Ideally for maximum efficiency you want to increase the size of the radiators but I dare say that in the vast majority of retro fits this isn't done.

Reducing the pump speed would probably reduce return temperature but at the cost of increasing the time taken to bring the house up to temperature and restricting the ability of your boiler to produce its maximal output.

It occurs to me though that the efficiency gains from a condensing oil fired boiler are less than a gas one so the gains from all this mucking about are going to be small.

Tim

Reply to
Tim Downie

Do CH control systems ever vary the pump speed? Ours is three speed but it's only a manual control.

Mmmm.

Can oil-fired boilers be throttled back? Or are they, like Neapolitan car drivers, either producing the full 26kW or none at all?

Reply to
Tim Streater

Well for one thing I'd imagine that more careful balancing might help. The rad and towel rail in one of the bathrooms might be a problem, as it's pretty much a short-circuit on the CH system. I have to keep the LSVs pretty closed there. Without that, there's just going to be water almost at inlet temp mixing with all the other return water.

We already did some of that - replaced ageing rads with ones that throw out more heat. Hard to do much more there.

Reply to
Tim Streater

Grundfoss Alpha, varies it's speed based on back pressure and temperature (I think).

Not as easy to throttle back as gas and TBH not sure any do. You'd have to adjust the high pressure oil to the jet and if you drop the pressure too much you aren't going to get the right atomisation and spray pattern. Of course the jet/heat exchanger of a modulating oil burner would be designed with all that in mind but I doubt you'll have the output range that a gas jobbie can do.

Reply to
Dave Liquorice

It's not that simple, and you may need to add flow restriction in some cases if the boiler can't get the flow low enough because there's not enough water circuit resistance (mine has an indicator for that, which I managed to trigger during commissioning).

Radiators won't get as hot anyway when operating at peak efficiency. That doesn't necessarily mean the house is any colder - it depends on how oversized they were to start with (and they very often are oversized, as most heating installers don't have the skills to do accurate size calcs and err of the safe side of oversizing).

AFAIK, all condensing boilers (nowadays at least) control the pump speed themselves. Mine has a 3-speed Grundfoss pump in it, but the control board varies the pump speed to optimise the flow at any given moment - you don't have any manual control control of it.

Sorry, no idea on that one. My experience is just gas.

Reply to
Andrew Gabriel

I bought an infrared thermometer specially for measuring flow and return temperatures, with a view to accurate balancing.

Reply to
Fredxx

Well, "smaller" would probably be more accurate. It's all down to the carbon/hydrogen ratio in the fuel. Oil has much more carbon and less hydrogen so there's less latent heat to be reclaimed from cooling the exhaust gasses to the point of condensation.

I can't find any figures though so I can't say how much smaller.

Tim

Reply to
Tim

Yeah, got one of them. Seems to work OK indoors. Winter before last, I took it outside to try to measure outside temps of the house. I started at the front door, and worked my way around to finish there. I gave up after it started by measuring the front wall area at -5C, and temps went down as I went round. By the time I got back to the front door, it was claiming -15C for the front wall. Unlikely.

Reply to
Tim Streater

I have no idea what's going on there. Emissivity has a big effect, where a metallic surface will be reflective, even when corroded. A clear sky will look -50C, so you have to careful what you point it at, and at what angle. I felt with mine, the temperature given by a painted radiator was good enough.

Reply to
Fredxx

I have found them very reliable, providing you don't use them on copper, chrome, or other shiney polished metalic finish. (A dab of paint, or a sticky label solves that though.)

Finding the cold spots on the inside of the exterior surfaces is just as useful as finding the hot spots on the outside. In my case, the loft hatch was the worst, easily solved by putting some insulation over it (and since then remade using an offcut of kingspan and plasterboard, in a wooden frame).

Reply to
Andrew Gabriel

A clear sky will give -270C, but your hand-held device can't read that, and will normally just give an error. You'll get -40C to -60C if you manage to point it at some thin very high layer cloud cover (which might be too thin to see), and typically -10 to -20 when pointed at visible clouds.

Emissivity of most things is close enough to 0.95 that you can ignore the difference. Polished metal and bare copper are the two that will fail it though.

Reply to
Andrew Gabriel

I doubt Harry Bloomfield and harry are the same person.

Reply to
ARWadsworth

If there was no atmosphere I might agree with you, but I can assure you that's not the case and a few miles of atmosphere is not as transparent as you might think in the 8-14um waveband. If there is cloud cover it may look warmer depending on conditions.

Aluminium, even when dull is also extremely reflective. Metal surface that look dull in the visible are often reflective in the far IR.

Reply to
Fredxx

You adjust the boiler temp setting so the return is about 50C. It may mean the radiators don't get as hot and they may not cope with very cold days but you can turn the temp up when they occur. If its a new system you fit bigger rads to cope with the very cold days so it can stay in condensing mode.

Reply to
dennis

Co-incidentally I had occasion to balance my radiators. I have installed

12 new rads.

All the lock shields were opened fully and so were the TRV's

I the used a fluke 53 and a fluke 54. (borrowed from work, too expensive for me to buy) They both have two type K thermocouples allowing you to measure two different temps. They will also calculate T1 - T2 for you.

I then used the first fluke on the 1st rad, the second fluke on the 2nd rad, using pipe insulation to hold the two thermocouples.

I selected the T1 and the T1-T2 display. This gives me the flow temp and the temp difference across the rad.

I adjusted the lockshield valve till I got 10 degrees across the rad.

Once I had set up the first rad, I moved the fluke to the 3rd rad, and got to work on the 3rd rad (now stabilised with the second fluke. Balance the 2nd, move the fluke to the forth rad, balance the third rad, move the fluke to the 5th rad and so forth.

The biggest Rad originally had a temperature drop of 35 deg C. After having balance 5 towel rads (utility, Kitchen, d/stairs toilet, bathroom and ensuite) the biggest rad's temp diff started to decrease.

This meant the towel rads ended up being adjusted down to 1/4 turn from fully shut (after all the towel rads are several 15mm pipes in parallel and are effective a "thermal shunt"

Once I had worked round a few bedroom rads, the lounge rad is now showing a delta T of 15 degrees and the downstairs family room has 13 degrees C.

I have two more rads to add, so will rebalance the system again.

Seriously using two Flukes in a leapfrog fashion makes the job so much easier.

Regards

Stephen.

Reply to
Stephen H

[snip balancing info]

But can you rent the buggers?

Reply to
Tim Streater

My experience is similar, where the largest radiators are running full throttle so to speak, and the smallest radiator with a lockshield all but closed. Its a shame they don't make lockshields more progressive.

Problem with thermocouples is the time taken to attach, remove and attach to another radiator. That's why I went the non-contact route. You can much cheaper meters with a thermocouple input.

I felt I was going round in loops honing in on the ideal settings for each lockshield as things stabilised. Its a painful process but worth it.

Reply to
Fredxx

Is the above what your average heating installer does;?...

Reply to
tony sayer

In article , tony sayer writes

Normally the pumps themselves have a 3 position switch on them that can be used to change the speed so it is certainly possible at that point (stator coil switching, changing the number of poles?)

If Andy still has the same boiler as mine, they use a modified pump that takes the switch wiring off to the control board and they do the switching with relays. Don't know about other manufacturers.

Reply to
fred

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