Is 5kw ASHP too small?

[ProDave]

48 minutes ago, ruggers said:

@HughF I can't install it, to obtain the 5k grant you need to go through MCS, to get the full warranty you have to be a registered installer and Fgas certified. I can size everything up & I can fit the underfloor, the radiators and piping but not the plant room install or unvented cylinder.

The point is, if you buy the kit and install yourself, it will probably still be cheaper without the grant, than getting the £5K grant and paying for an MCS install.

 

A monoblock ASHP comes charged with gas and does not need an FGAS installer.

[HughF]

1 hour ago, ruggers said:

@HughF I can't install it, to obtain the 5k grant you need to go through MCS, to get the full warranty you have to be a registered installer and Fgas certified. I can size everything up & I can fit the underfloor, the radiators and piping but not the plant room install or unvented cylinder.

All these new regs, Part L and complying with things that require heat loss surveys for weather compensation, and then we don't stick to the figures used from the results. 

Why would you want to pay an extra £5k or more to an MCS certified monkey to then claim the grant? And no, you don’t need F-Gas for a monobloc… I have a pretty low opinion of MCS to be honest.

 

My 9kW inverter driven machine was £3100 delivered to my door…. £500 for a heat pump cylinder, underfloor pump/mixer was £200, and the balance on a full set of new rads. Managed to pick up a couple hundred meters of 16mm wras approved Alpex for £20 off facebook, change out of £5k for a new heating system.

 

Did my own heat loss and rad sizing using the Freedom toolkit and cross checked it with heatpunk.

Edited November 26, 2022 by HughF

[Dan F]

6 hours ago, Chanmenie said:

What ASHP do you have Dan ?

Vaillant AroTHERM 7kW.

[Dan F]

5 hours ago, ruggers said:

So I wanted to know does an ASHP rated at 7kw run closer to 7kw when outside is freezing, but on warmer days like +10°C, then ASHP would only run at 3Kw. So drawing 3kw of energy when in use as opposed to 7Kw?

Correct, although if you have low flow temperature you'd be drawing closer to 600W at COP of 5 to deliver 3kW heat.  The 7kW Vaillant can deliver 9kW at -2C if your flow temp <45C.  At 7C outside it can modulate down to 3.2kW.

 

5 hours ago, ruggers said:

Both Vaillant and the Vaillant local installer have said that when sizing for an ASHP, they have to exclude MVHR from the heat loss survey and base it on natural ventilation as thats the worst case situation of when someone returns from holiday to a house that hasn't had it's heating on and theres no heat for the MVHR to recover.

Its up to you how quickly the house heat up on return from holiday, not them.  Also you can internet conenct heating and MVHR and turn it on before you arrive if needed.  I wouldn't consider this sceanrio in sizing personally! Especially if it means bigger radiators. 

 

[MikeSharp01]

7 hours ago, Dan F said:

Its up to you how quickly the house heat up on return from holiday, not them.  Also you can internet conenct heating and MVHR and turn it on before you arrive if needed.  I wouldn't consider this sceanrio in sizing personally! Especially if it means bigger radiators.

Plus one to that - I also think having too big a machine will mean it won't modulate down far enough for average temperatures and will end up cycling! I wonder why people size for the extreme cases and not closer to the most common. If your house is well insulated then running a few towel rails might be a better way to deal with the extreme case or with UFH run a Willis in line to handle the three days a year when the outside temp get seriously out of whack.

[ProDave]

9 hours ago, Dan F said:

At 7C outside it can modulate down to 3.2kW.

Even fully modulated down it would be too much heat for our house even at -10 outside

[ProDave]

9 hours ago, Dan F said:

Its up to you how quickly the house heat up on return from holiday, not them.  Also you can internet conenct heating and MVHR and turn it on before you arrive if needed. 

You don't need internet for that.  Our heating programmer has a "holiday mode" where you tell it the dates you are away and the heating is off for those days.  We set the return day 1 day before we actually get back to give it time to heat up.

[joth]

11 hours ago, ProDave said:

You don't need internet for that.  Our heating programmer has a "holiday mode" where you tell it the dates you are away and the heating is off for those days.  We set the return day 1 day before we actually get back to give it time to heat up.

Also the controller will allow a "away" / eco temperature to be set during the holiday that maintains some reasonable level (we do about 15), meaning the house still had plenty of heat for the MVHR to continue to recover throughout the holiday and on return.

[ruggers]

@HughF @ProDave I've done my own heat loss survey on heat engineer and spent ages trying all different things, I've got it as accurate as can be from the details I input. I'll be able to shave a little bit more off it, but theres no way anyone coming to do another will spend as much time as I will, they'll just take as close to U-values & air changes where as I've got the exact ones.

there should be no reason for someone to do another. MCS seems a farce. 

 

If MVHR reduces my heat loss by 25%, then it should be factored in so I require smaller radiators, then if I run it at a lower flow rate to be more efficient, I have room to upsize them to suit. If using my none MVHR heat loss figure, I can't run the flow lower than 40C or the radiators are physically too big for 2 of the rooms.

If I'm to fit a heat pump I want to fit an arotherm plus, but I won't get the 7 year warranty if I fit it myself & would need an unvented cylinder. I've done full house combi's but never fit cylinders, expansion vessels & more. If shown I'd learn it easily but theres no source for me to learn it from it would take ages.

 

I think people fitting heat pumps are over charging by thousands, they are no more complex than a gas install for a competent installer. 

[ruggers]

On 27/11/2022 at 08:15, MikeSharp01 said:

Plus one to that - I also think having too big a machine will mean it won't modulate down far enough for average temperatures and will end up cycling! I wonder why people size for the extreme cases and not closer to the most common. If your house is well insulated then running a few towel rails might be a better way to deal with the extreme case or with UFH run a Willis in line to handle the three days a year when the outside temp get seriously out of whack.

I agree Mike, I'm not concerned about the coldest days of the year, I will have plenty to cover that. Although it doesn't feel like it, the average temperature for my area is 8°C so having a unit that can cope in the extreme cold but also modulates down low in the warmer months enough not to cycle is very important. I run my heating from October to April normally. 

 

On 27/11/2022 at 00:12, Dan F said:

Correct, although if you have low flow temperature you'd be drawing closer to 600W at COP of 5 to deliver 3kW heat.  The 7kW Vaillant can deliver 9kW at -2C if your flow temp <45C.  At 7C outside it can modulate down to 3.2kW.

 

Its up to you how quickly the house heat up on return from holiday, not them.

 

How can you draw 600w if the lowest modulation is 2.1 or 3kw pending which model is selected?
I know that with gas that the lowest modulation is 1900W & this would mean that the boiler would cycle from outside being 10.5°C. I've no idea how to work this out for electric modulation, haven't found anyone who can.

 

4.5Kw heat loss is 183W per degree of temperature change for my indoor and outside design temps.

[HughF]

33 minutes ago, ruggers said:

@HughF @ProDave I've done my own heat loss survey on heat engineer and spent ages trying all different things, I've got it as accurate as can be from the details I input. I'll be able to shave a little bit more off it, but theres no way anyone coming to do another will spend as much time as I will, they'll just take as close to U-values & air changes where as I've got the exact ones.

there should be no reason for someone to do another. MCS seems a farce. 

 

If MVHR reduces my heat loss by 25%, then it should be factored in so I require smaller radiators, then if I run it at a lower flow rate to be more efficient, I have room to upsize them to suit. If using my none MVHR heat loss figure, I can't run the flow lower than 40C or the radiators are physically too big for 2 of the rooms.

If I'm to fit a heat pump I want to fit an arotherm plus, but I won't get the 7 year warranty if I fit it myself & would need an unvented cylinder. I've done full house combi's but never fit cylinders, expansion vessels & more. If shown I'd learn it easily but theres no source for me to learn it from it would take ages.

 

I think people fitting heat pumps are over charging by thousands, they are no more complex than a gas install for a competent installer. 

I couldn’t agree more - it’s pure profiteering by the so called professionals.

 

Any reason why you’re so keen on an aerotherm plus, and why you’re so keen on a warranty? I genuinely feel you’ll do a better job and save yourself a ton of cash doing it yourself.

 

The plumbing couldn’t be simpler. Tank temperature sensor, 3 port diverter valve, power supply. Just watch YouTube 🤣

 

All jokes aside, all the g3 stuff is in an approved document, just follow that. Pay BC to come and inspect it, or don’t.

Edited November 28, 2022 by HughF

[ruggers]

1 hour ago, HughF said:

I couldn’t agree more - it’s pure profiteering by the so called professionals.

 

Any reason why you’re so keen on an aerotherm plus, and why you’re so keen on a warranty? I genuinely feel you’ll do a better job and save yourself a ton of cash doing it yourself.

 

The plumbing couldn’t be simpler. Tank temperature sensor, 3 port diverter valve, power supply. Just watch YouTube 🤣

 

All jokes aside, all the g3 stuff is in an approved document, just follow that. Pay BC to come and inspect it, or don’t.

I have a plumber friend who's gas safe and G3. He's getting old so not up to speed with weather comp and the latest tech but I've been learning as much as I can. He's happy to work & have me involved.

Based on everything gas, which i can get my head around a little easier, I was going to fit a low loss header & two pump sets, one mixed, one unmixed. I'm not sure if you still need the hydronic separation with heat pumps but the hex module indoors separates this. It's just with having radiators first floor and UFH down are they not different flow rates or all the same with a heat pump?

 

Arotherm is just because it generally gets a lot of good reviews and seems popular and their cylinders are really good & a tidy package. Impressive reheat times. It seems easier to learn once you've focussed on one brand. If self installing, is there much difference in using other models?

 

I work it out that a lot of these installs are making £1000 a day for 5 days work instead of £300-£400. Even them prices are a lot. 

 

I need to see a drawing for a piped system, it's just all the new terminology for component names.

Viessmann has a nice display unit with the heating curves, lots on youtube to see. Struggled to find any on Vaillant, only the senso comfort but it just shows program settings like times and temperatures.

[HughF]

Use the pump in the ashp to drive the rad circuit, use close coupled Ts on the return to hydraulically separate the ufh mixer/pump station. Use an ivar unimix3 pump station (eBay seems to have some on occasion) or if you want to go fancy, an esbe valve and a separate weather comp valve controller.

 

I probably sound like a stuck record by now, but have you considered Cool Energy? Their 9kW offering is very capable, sensibly priced, and they sell direct. The system is built on a Carel industrial refrigeration control platform and is very capable. And their installation instructions have plumbing diagrams for a variety of standard configurations. @dpmiller has had his machine running for a while now and seems pleased with the performance of the package. He self installed.

 

 

 

 

Edited November 29, 2022 by HughF

[HughF]

[Dan F]

19 hours ago, ruggers said:

How can you draw 600w if the lowest modulation is 2.1 or 3kw pending which model is selected?

It will cycle.  Cycling isn't a bad thing when you need very low output, you just want to avoid it in your main usage ranges (i.e. 2-6kW) by not buying a 12kW heat pump than can't module below 4-5kW.

[JohnMo]

A buffer will limit cycling, that's what they do.  The bigger the buffer, the longer the run time and off time.

[ReedRichards]

11 hours ago, HughF said:

 Does this work?  It looks to me as if when neither the cylinder or radiators are calling for heat then there is no supply of heated water to keep the UFH up to temperature.  Perhaps there is an invisible buffer in there somewhere or I am misinterpreting the drawing? 

[HughF]

1 hour ago, ReedRichards said:

 Does this work?  It looks to me as if when neither the cylinder or radiators are calling for heat then there is no supply of heated water to keep the UFH up to temperature.  Perhaps there is an invisible buffer in there somewhere or I am misinterpreting the drawing? 

No thermostat, heating is just on or off from the Cool Energy controller. If it’s set to dhw the tank is charged till it is satisfied, if it’s set to heating the weather compensated liquid flows through the rad and ufh circuit. Rad and ufh always together (ufh is one loop in the extension we’re building)… About as simple as I could make it. Will see how well it works when I get round to plumbing it up. Can’t see why it won’t though. 

 

EDIT: I'm rambling, sorry. The UFH and the rads work together, they are not zoned separately. 

Edited November 29, 2022 by HughF
Clarifying what I mean

[ReedRichards]

1 hour ago, HughF said:

No thermostat, heating is just on or off from the Cool Energy controller. If it’s set to dhw the tank is charged till it is satisfied, if it’s set to heating the weather compensated liquid flows through the rad and ufh circuit. Rad and ufh always together (ufh is one loop in the extension we’re building)… About as simple as I could make it. Will see how well it works when I get round to plumbing it up. Can’t see why it won’t though. 

 

EDIT: I'm rambling, sorry. The UFH and the rads work together, they are not zoned separately. 

 

Well it's nice and simple but can you do a night time setback?  I don't want my house to be at one constant temperature for 24 hours; perhaps you do?  Then if you did have a setback then the rads would recover faster than the UFH so that might be awkward. 

[HughF]

26 minutes ago, ReedRichards said:

 

Well it's nice and simple but can you do a night time setback?  I don't want my house to be at one constant temperature for 24 hours; perhaps you do?  Then if you did have a setback then the rads would recover faster than the UFH so that might be awkward. 

Yep, I can program a change to the WC for a nighttime setback. The UFH will be slow to respond because it will be in 100mm of concrete, so the pump timing for that will need some figuring out.

[ruggers]

14 hours ago, HughF said:

What are you running in the outside lines from the compressor to indoors? Whats separating this, theres usually a wall unit isn't there with the heat exchanger in? 

I've seen the Meiflow jackets housing esbe mixing valves with control units for controlling the heating curves, and grundfos or wilo pump inside.

 

Is that set up for priority hot water with the 3 port valve? The Unistor cylinders have a 15 minute reheat time or 11 mins to 70% for a 250L 

[ruggers]

6 hours ago, Dan F said:

It will cycle.  Cycling isn't a bad thing when you need very low output, you just want to avoid it in your main usage ranges (i.e. 2-6kW) by not buying a 12kW heat pump than can't module below 4-5kW.

I don't understand that part  saying it's not a bad thing when you need low output. I thought the low output is what causes it to cycle which is never great? I wouldnt need anything too big for a 4.5-6kw heat loss depending on which figure was used. Bigger pump can reheat a bigger cylinder though and quicker.

6 hours ago, JohnMo said:

A buffer will limit cycling, that's what they do.  The bigger the buffer, the longer the run time and off time.

Does the buffer not end up the same as temperature as the system water so then it just cycles again or is it set to take higher heat with it's own stat? How much efficiency do they remove, 45 litres is quite a lot of volume.

[Dan F]

55 minutes ago, ruggers said:

I don't understand that part  saying it's not a bad thing when you need low output. I thought the low output is what causes it to cycle which is never great? I wouldnt need anything too big for a 4.5-6kw heat loss depending on which figure was used. Bigger pump can reheat a bigger cylinder though and quicker.

Does the buffer not end up the same as temperature as the system water so then it just cycles again or is it set to take higher heat with it's own stat? How much efficiency do they remove, 45 litres is quite a lot of volume.

 

If your heat loss is 4kW then you may look to install a 5kW ASHP to ensure it covers you heat demand on the coldest day of the year.  But, in May the same house might have a heat demand of just 500W, but no 5kW will modulate down to 500W without needing to turn the compressor off part of the time, even with a buffer.  So what do you do; i) get a smaller ASHP (no because that'll be no good in the winter) ii) get a second smaller ASHP for spring (no because that's silly) iii) use alterntive method for satisfying 500W demand (why, when you have an ASHP) iv) use 5kW ASHP knowing that it the compessor will only be on 25% of the time but use a buffer, if required, to ensure the cycles aren't too short.

 

Point is, you need to right-size your ASHP, not try to find something that will always match your heat demand.  A buffer doesn't avoid cycling if heat transfer to emittters < min modulation, rather it helps ensure the cycles are longer (it's lots of short cycles which are bad).

 

 

 

Edited November 30, 2022 by Dan F
spelling

[HughF]

7 hours ago, ruggers said:

What are you running in the outside lines from the compressor to indoors? Whats separating this, theres usually a wall unit isn't there with the heat exchanger in? 

I've seen the Meiflow jackets housing esbe mixing valves with control units for controlling the heating curves, and grundfos or wilo pump inside.

 

Is that set up for priority hot water with the 3 port valve? The Unistor cylinders have a 15 minute reheat time or 11 mins to 70% for a 250L 

Monobloc heat pumps have everything inside, you get hot water out of them. No need for an indoor heat exchanger.

 

those pipes are the flow and return and carry the central heating circuit water. 

[JohnMo]

9 hours ago, ruggers said:

Does the buffer not end up the same as temperature as the system water so then it just cycles again or is it set to take higher heat with it's own stat? How much efficiency do they remove, 45 litres is quite a lot of volume.

 

A buffer is just an extension of the heating system, it give the system more volume.  The heat pump has to run longer to get it from temp A to temp B.  Once the return temp of water to the heat pump is at a level to hot for the heat pump, it stops its heating cycle.  The circulation pump continues running.  When the return temp is within limits again the heat pump starts. 

 

There are two ways to manage a buffer.

No thermostat

Within the period of time from heat pump stop to heat pump start, the heating system draws heat from the buffer, the buffer temp rides up and down within a few degrees of it's set point, the same as the heating system.

 

With thermostat 

Heat pump heats the buffer to a thermostat set point, this thermostat only managed the buffer temp, this is set hotter than heating system flow temp, works the same as a thermal store.  Heating system sucks the heat from the buffer.  Buffer is recharged by heat pump it doesn't know or care what the heating system is doing.

 

A bigger buffer gives a longer the run time and longer the off time, less cycles more efficient heat pump operation. Heating system 0.1kW to max HP output, it doesn't care.  The heating system is a seperate system, you could have a 10kW heat pump driving a 2kW heating system, with a correctly sized buffer.

 

Down side is slightly high flow temp with the thermostat system, some vessels heat loss also.