How long does it take for your house to heat up?

ilikecookies

macman wrote: »

The big variable here is the ambient temp, since heat loss increases with a greater differential between the two.
The warmer you make your house, and the colder it is outside, the faster it will lose heat.
It's not just about increasing the internal temp by 1C in a hour or whatever. Your calculation makes no allowance for the variable rate of heat loss.
In the current weather, achieving that 1C rise will take longer, due to the higher losses.

Interesting point macman - I hadn't considered that. The temp change is definitely a bit faster now the outside temp is a bit higher.

I still wonder though if there is a general design principle which states a correctly sized system should be able to raise the temperate by x degrees per hour in a standardised set of conditions ...

malc_b

So what does the temperature do overnight? That is how cold does it get? It could you have a massive thermal lag which would show up in taking a long time to get up and also a long time to cool down.

If it cools down rapidly then you don't have enough capacity in the system to balance the outgoing losses and warm up the fabric of the house.

From cold, do all the radiators warm up equally and at the same time? Or, especially if you have TRVs do you find some radiators warm up fast and the other catch up? If so then the radiators could need balancing to ensure the boiler heat is evenly shared with all radiators so that you are put maximum heat into the house.

Is the boiler thermostat set low? Is the boiler short cycling? That is cutting in and out? The hotter the radiators the more heat they give out but that can stop a condensing boiler condensing. However, if you have a condensing boiler on an undersized radiator system then you may have accept it only runs in condensing mode in spring/autumn. Or fit larger rads.

Is the pump set fast enough? Too low a flow and the boiler will short cycle or modulate to lower power (with a modulating gas boiler). There should be 12C (more form condensing) difference between boiler inlet and outlet.

Ectophile

I had a go at measuring the flow and return temperatures. I'm not sure how accurate my figures were - I used a thermocouple plugged into a multimeter, pressing the thermocouple against the pipes using an offcut of pipe lagging.

So far as I can tell, the flow temperature is set to (only) 58C and the return is 50C.

If I understand these things correctly, 50C is ideal for the boiler to operate in condensing mode, but the small difference between flow and return suggests that the radiators are undersized for the job.

The small radiator in the kitchen heats up very rapidly, because it's on normal 15mm pipe. The others are larger and connected by microbore pipe, and take much longer to warm up. But they do all get there eventually.

The boiler tends to run flat out for a few minutes when first started, but then modulates itself down to a low hum pretty much constantly until the house warms up and the room stat turns it off.

I'm not sure how much point there is in replacing old radiators with new more efficient ones if they are still connected to microbore pipes.

Cardew

Ectophile wrote: »

I had a go at measuring the flow and return temperatures. I'm not sure how accurate my figures were - I used a thermocouple plugged into a multimeter, pressing the thermocouple against the pipes using an offcut of pipe lagging.

So far as I can tell, the flow temperature is set to (only) 58C and the return is 50C.

If I understand these things correctly, 50C is ideal for the boiler to operate in condensing mode, but the small difference between flow and return suggests that the radiators are undersized for the job.

You have the classical dilema of comfort versus lower running costs.

You are correct that your boiler is operating in condensing mode, but the water at 58C is obviously not warm enough to bring your house up to temperature quickly enough with your current size radiators.

I can find little data about the difference in efficiency between the boiler operating in condensing mode or not condensing. My guess(and it is a guess) is there is not a big gap(5%-10%??) All the publications I read state that even when not in condensing mode, they perform better than non-condensing boilers.

Would it be an idea to switch up the water temperature when heating house from a low temperature, and then turning it down to your current setting?

Another suggestion is to have small(computer size) fans to blow air over radiators. Some people with Heat Pumps(that have very low water temperatures - 35C-45C) use them as a solution.

Ectophile

Thanks, Cardew.

The weather's been relatively warm these last few days, so the heating has been fine.

If we get another cold snap, I will try turning up the temperature a bit to see if it's more effective.

malc_b

Ectophile wrote: »

I had a go at measuring the flow and return temperatures. I'm not sure how accurate my figures were - I used a thermocouple plugged into a multimeter, pressing the thermocouple against the pipes using an offcut of pipe lagging.

A bit of grease might help get a better thermal linking

Ectophile wrote: »

So far as I can tell, the flow temperature is set to (only) 58C and the return is 50C.

If I understand these things correctly, 50C is ideal for the boiler to operate in condensing mode, but the small difference between flow and return suggests that the radiators are undersized for the job.

Not necessarily, the flow could be too fast or all mainly through one radiator.

Ectophile wrote: »

The small radiator in the kitchen heats up very rapidly, because it's on normal 15mm pipe. The others are larger and connected by microbore pipe, and take much longer to warm up. But they do all get there eventually.

The boiler tends to run flat out for a few minutes when first started, but then modulates itself down to a low hum pretty much constantly until the house warms up and the room stat turns it off.

I'm not sure how much point there is in replacing old radiators with new more efficient ones if they are still connected to microbore pipes.

What is the temperature drop across each radiator? That indicates the flow through each. It sounds to me that you may have most the water going through the kitchen radiator and next to nothing going through the others. Does the kitchen have a TRV? If so then what you could be seeing is all the flow going through the kitchen until the kitchen TRV cuts it which then pushes the flow to the other radiators. So the rest of the house won't start to warm up until the kitchen is first warm. Even if it doesn't have a TRV it sounds like the kitchen radiator is shorting the loop and stealing most of the water.

If you think about how CH works the boiler is adding heat to the water, X kWh to each litre. Each radiator needs Y litres per minute so it can give up that heat to the house. If a rad gets too little water its output is low, the room cold. If a rad gets too much water it has a high output, BUT, the return water doesn't have all the heat taken out of it so the boiler isn't putting its rated output. On a modulating boiler it cuts down output so it's less obvious than an on/off boiler which would be short cycling.

If you measure the temperature drop you'll get an idea of the flow and where the heat is going. The ideal is that each radiator has the same drop, then you increase or decrease the pump speed to get ~12C (for non condensing boilers, condensing might be different) across the boiler.

There is not a lot of difference between microbore and 15mm systems and you should see all radiators warm up together. In 15mm one pipe feeds many radiators. In microbore each radiator has it's own pair of pipes that feed back to a central hub fed with a large pipe. The advantage of microbore is it is easier to fit the piping but it shouldn't perform any different to 15mm because the pipe isn't shared between multiple rads.

Google balancing radiators. I would say you first step is to get all the radiators warming up all at the same time. If there is still a problem then your next step is to consider raising the water temperature. The formula (actually my curve fit) for radiator output is

OP = 0.0115 x TempDiff + 8.5E-5 x TempDiff^2

Where TempDiff is the radiator temperature above ambient and ^2 is to power of 2. So for your 54C (36C above 18C say) the radiators are 0.53, 53% of rated output. 60C above ambient is required for rated output. 45C would give you 70%, 40C ~60% so even a small increase would help.

Ectophile

All the radiators except one have TRVs. The radiator in the kitchen is tiny, so the room is always quite cold - so the TRV may never be operating.

I wonder if the hot water is just going round and round through the kitchen radiator and bypassing the others. It might be worth experimenting to see what happens if I shut down the TRV on that one.

I will also have a go at measuring the temperatures at each radiator.

malc_b

Ectophile wrote: »

All the radiators except one have TRVs. The radiator in the kitchen is tiny, so the room is always quite cold - so the TRV may never be operating.

I wonder if the hot water is just going round and round through the kitchen radiator and bypassing the others. It might be worth experimenting to see what happens if I shut down the TRV on that one.

I will also have a go at measuring the temperatures at each radiator.

In case you don't know the valve at the other end to the TRV is called a lockshield and its function is to balance the flows in the radiators (i.e. by restricting the flow on those with too fast a flow). But yes the simplest test would be to shut off the kitchen with the TRV and see if the other radiators get warmer faster. But note that'll work if the other radiators are more balanced in resistance. If you have one that is still significantly less than the others then you'll just get that one heating up and the rest lukewarm. That's you'll just move the problem.

Temperature drop across the radiators will tell you the flow, fast flow = low drop, slow flow = high drop. That is as long as the input temperature is around the boiler output temperature that is. If the input temperature is far lower then you have very slow flow (zero flow would be zero difference of course).