Heat meter - Fun & games

matt_drummer

michaels said:

On the flip side the 12kw will be much too powerful for the majority of the time when temps are 5c and higher and is unlikely to be able to modulate to a point where it isn't cycling on and off which is less efficient....

That really depends on the heat pump, some are really good at working in short cycles getting up to full efficiency almost immediately.

Others are not so good at this.

I know because I have one that is terrible in short cycles!

My Daikin, and all the others, take a long time to get into their stride (like hours!) whereas something like a Vaillant is good almost immediately.

FreeBear

NedS said: Most heat pumps are rated at a flow temp of 35C at an ambient temp of 7C. So your 6kW heat pump will generate 6kW with 35C flow temp at 7C ambient. But on the coldest days, you may need 50C flow temps, so it will be lower, maybe 5.5kW. And it's not 7C outside, but -2C so it may be more like 5kW under those conditions (heat pump manufacturers normally supply tabulated data for this).

Had a quick look at the technical specs for a random Daikin and Vaillant heat pump - Only Vaillant gave a modulation range, whilst both gave outputs for different flow temperatures. Found it rather difficult to make sense of the numbers, so may need to set up a spreadsheet to get a side by side comparison.

One thing I did note though, was minimum water volumes. Up to around 8kW, min volume is given as 30-40l. Go to 12kW, and minimum volume jumps to 60l. As my heating system currently contains ~50l, the smaller HP wouldn't need buffer/volumiser tanks.

Really need to do a proper detailed heat loss calculation - Maybe with Heatpunk..

michaels

FreeBear said:

NedS said: Most heat pumps are rated at a flow temp of 35C at an ambient temp of 7C. So your 6kW heat pump will generate 6kW with 35C flow temp at 7C ambient. But on the coldest days, you may need 50C flow temps, so it will be lower, maybe 5.5kW. And it's not 7C outside, but -2C so it may be more like 5kW under those conditions (heat pump manufacturers normally supply tabulated data for this).

Had a quick look at the technical specs for a random Daikin and Vaillant heat pump - Only Vaillant gave a modulation range, whilst both gave outputs for different flow temperatures. Found it rather difficult to make sense of the numbers, so may need to set up a spreadsheet to get a side by side comparison.

One thing I did note though, was minimum water volumes. Up to around 8kW, min volume is given as 30-40l. Go to 12kW, and minimum volume jumps to 60l. As my heating system currently contains ~50l, the smaller HP wouldn't need buffer/volumiser tanks.

Really need to do a proper detailed heat loss calculation - Maybe with Heatpunk..

If you are monitoring your heat input and your room temps then that is your detailed heat loss calc - or actually better because it is actuals rather than theocraticals.

NedS

FreeBear said:

NedS said: Most heat pumps are rated at a flow temp of 35C at an ambient temp of 7C. So your 6kW heat pump will generate 6kW with 35C flow temp at 7C ambient. But on the coldest days, you may need 50C flow temps, so it will be lower, maybe 5.5kW. And it's not 7C outside, but -2C so it may be more like 5kW under those conditions (heat pump manufacturers normally supply tabulated data for this).

Had a quick look at the technical specs for a random Daikin and Vaillant heat pump - Only Vaillant gave a modulation range, whilst both gave outputs for different flow temperatures. Found it rather difficult to make sense of the numbers, so may need to set up a spreadsheet to get a side by side comparison.

One thing I did note though, was minimum water volumes. Up to around 8kW, min volume is given as 30-40l. Go to 12kW, and minimum volume jumps to 60l. As my heating system currently contains ~50l, the smaller HP wouldn't need buffer/volumiser tanks.

Really need to do a proper detailed heat loss calculation - Maybe with Heatpunk..

Yes, larger heat pumps often require a greater volume. Remember, those are minimum recommended system volumes, and as such may be far from optimal. We have a 50L volumiser fitted, and in most circumstances it would make sense to fit a volumiser. Try to avoid buffers which are completely different.

A volumiser is simply a fat pipe adding volume. Typically a 50L tank fitted inline in the central heating circuit on the return flow pipework. It will serve two practical purposes - firstly it may help prevent short cycling as the heat pump now has a greater volume of water to warm up when it first fires up. Secondly, it means the heat pump has access to a far larger volume of warm water for defrost cycles which is critical in cold weather, otherwise there is a danger of the heat pump entering a "death spiral" whereby there is never enough warm water to effectively defrost the system and you are left without heat when it's freezing cold outside. Most installers tend to just fit a volumiser regardless, as they'd rather be safe than sorry, and don't want you phoning them at 5am in January when your heat pump is frosted up and can't heat the house. Our "system volume" easily exceeds the recommended 50L minimum, but we still have an additional 50L volumiser installed, and having seen how much heat gets sucked out of the system during a defrost cycle, I'm glad we do.

Also consider that your system volume can be reduced if you have TRVs fitted on your radiators. We run all our radiators on 5 (fully open), but if you start turning down radiators overnight, for example, then you also restrict system volume just when you may need it most.

FreeBear

NedS said: A volumiser is simply a fat pipe adding volume. Typically a 50L tank fitted inline in the central heating circuit on the return flow pipework. It will serve two practical purposes - firstly it may help prevent short cycling as the heat pump now has a greater volume of water to warm up when it first fires up. Secondly, it means the heat pump has access to a far larger volume of warm water for defrost cycles which is critical in cold weather

Thanks for that - These little snippets of information will help to guide my decisions when the time comes. I'd already decided against a buffer tank and hadn't considered defrost cycles. Somewhat constrained on space, so would need to look at where a 30-50l volumiser tank could be installed.

Reed_Richards

FreeBear said:

Somewhat constrained on space, so would need to look at where a 30-50l volumiser tank could be installed.

Since hot water cylinders are typically cylindrical but are generally housed within a rectangular cupboard, there should be space for a long fat pipe in one of the corners of that cupboard, I would have thought. 

NedS

FreeBear said:

NedS said: A volumiser is simply a fat pipe adding volume. Typically a 50L tank fitted inline in the central heating circuit on the return flow pipework. It will serve two practical purposes - firstly it may help prevent short cycling as the heat pump now has a greater volume of water to warm up when it first fires up. Secondly, it means the heat pump has access to a far larger volume of warm water for defrost cycles which is critical in cold weather

Thanks for that - These little snippets of information will help to guide my decisions when the time comes. I'd already decided against a buffer tank and hadn't considered defrost cycles. Somewhat constrained on space, so would need to look at where a 30-50l volumiser tank could be installed.

Ours is fitted above the DHW cylinder (and the two expansion tanks are fitted in the space to the side/corner), although we do have high ceilings (~2.7m IIRC). It was a tight fit, and the pressure/air relief valve on top of the volumiser goes up through the ceiling into the loft! Our DHW cylinder is also taller than a regular cylinder as it is a slimline (slimmer and taller) model, so maybe a 50L volumiser may fit above a regular DHW cylinder (they can also be fitted horizontally I believe).
Fitting oversized radiators will also help maximise system volume, as will 22mm piping throughout (and 28mm primary runs to/from the ASHP external unit)

FreeBear

The space I have in mind for tanks & expansion vessels is only 1.5m X 0.8m and ~2.3m high. Some of that space is taken up by a washing machine which I don't want to relocate. Enough space for a DHW tank plus expansion vessel.

If a volumiser is required, I can't seem to find any that qualify as being slim. Suppose worst case would be to fabricate one out of sched 10 ss pipe (used to work for a company that did pressure vessels).

NedS

FreeBear said:

The space I have in mind for tanks & expansion vessels is only 1.5m X 0.8m and ~2.3m high. Some of that space is taken up by a washing machine which I don't want to relocate. Enough space for a DHW tank plus expansion vessel.

If a volumiser is required, I can't seem to find any that qualify as being slim. Suppose worst case would be to fabricate one out of sched 10 ss pipe (used to work for a company that did pressure vessels).

They are pretty well insulated, so how about in the loft space above the DHW tank? The return pipework could go up into loft, through volumiser and straight back down again with minimal heat loss. As it's installed outside the heated envelope of the property, you could wrap it in additional lagging to further reduce any heat loss.

My neighbour had no space even for the DHW cylinder, to they built a small lean to wooden shed, insulated it with celotex sheet insulation and installed the DHW cylinder, volumiser and expansion tanks into that, with pipework into and out of the house through the adjoining wall.

NedS

FreeBear said:

The space I have in mind for tanks & expansion vessels is only 1.5m X 0.8m and ~2.3m high. Some of that space is taken up by a washing machine which I don't want to relocate. Enough space for a DHW tank plus expansion vessel.

If a volumiser is required, I can't seem to find any that qualify as being slim. Suppose worst case would be to fabricate one out of sched 10 ss pipe (used to work for a company that did pressure vessels).

@FreeBear You should be OK.

I have 1.0m x 0.75m and ~2.7m high, and I have a slimline 170L DHW tank, with 50L volumiser above, two expansion vessels and all associated pipework. The slimline tank was required due to the 0.75m depth (the tank and control unit to the front sit 0.7m from the rear wall allowing ~5cm clearance. A standard tank would not have fitted, or would have been too tight for comfort.

With 0.8m depth, hopefully you can accommodate a standard tank, and the extra 0.5m on the width should be sufficient to accommodate the volumiser to the side of the DHW tank, with plenty of space above for expansion vessels. My volumiser tank is around 330mm diameter, and occupies around 0.5m once the two connecting elbow pipework is allowed for.

If you pm me your email address, I'm happy to send you some photos for reference, as I'm not sure I can upload here.