Oil boiler sizing - is bigger better?

[Deleted User]

TiredGeek wrote: »

We're looking into a combined ASHP / oil setup at the moment.
Air to air isn't really viable now as the rads are in, and as we'd need one unit per room the costs would very quickly add up, including hallways that would be 16 units, at least
Plus there's the issue of getting the pipes through the 18" solid granite walls to consider....and the electrics. I'm not sure it would look all that nice either with loads of those things hanging off the walls.

It's got to be a wet radiator system, just don't know which yet

Thanks,

I guess you already know but you can get split systems whereby you can have two inside units connected to a single outside unit. So say two outside units driving four inside units all blowing warm air. If cleverly placed would that keep the bulk of the house warm? The existing rads would just be for oil. I do understand as I don't like our 14kw eyesore on the side of our house.

If sticking with air-water we have had great success with fan convectors [fancoils as some call them] as they run at a low temp [30c or thereabouts] like ufh helping to keep efficiency high only they are expensive. In some respects they are better then UFH as there is no huge slab to heat up so system provides instant heat... And with little water in the system little energy is used too...

IMO using a ASHP to heat a huge volume of water say in a rad system will be pricey to keep up at a highish temp. UFH may also have huge amount of water in but works at lower temps due to heat retention in the slab.

Lots to think about...

Keep us posted with progress/ideas as all very interesting.

Cheers

TiredGeek

I did an experiment with some cheap computer fans, a plug in timer and a small 12v power supply to make our rads blown convector's. I must admit it did work rather well, one room was just a little too big for the two rads with water at 35' but with 4 x 120mm fans blowing up through them the room was much better..... and a hell of a lot cheaper than proper fancoils

I'm not following your logic on less water = less energy used.
Surely once that great amount of water is up to temp the only energy used thereafter is whatever is transfered into the rooms as heat. Maybe I'm wrong, I'm no expert

[Deleted User]

TiredGeek wrote: »

I did an experiment with some cheap computer fans, a plug in timer and a small 12v power supply to make our rads blown convector's. I must admit it did work rather well, one room was just a little too big for the two rads with water at 35' but with 4 x 120mm fans blowing up through them the room was much better..... and a hell of a lot cheaper than proper fancoils

I'm not following your logic on less water = less energy used.
Surely once that great amount of water is up to temp the only energy used thereafter is whatever is transfered into the rooms as heat. Maybe I'm wrong, I'm no expert

What I'm basically saying is that it takes more energy to heat and maintain a large volume of water than a smaller amount. With the low grade heat from some [not all] ASHPs I can understand why Cardew bangs on about some systems having to run constantly to maintain house temp. This is something we have not found with our system which I put down to low running temps with a low volume of water when compared to oversized rads for example. Cheers

TiredGeek

Ah, yes, I see your point
Our system runs with about 250 litres of water in 34 large radiators, how does that compare to your volume?

I know 250 litres sounds a lot, but when companies are wanting to put 100+ litre buffer tanks in as well it starts to look better....

[Deleted User]

TiredGeek wrote: »

Ah, yes, I see your point
Our system runs with about 250 litres of water in 34 large radiators, how does that compare to your volume?

I know 250 litres sounds a lot, but when companies are wanting to put 100+ litre buffer tanks in as well it starts to look better....

Oh gosh, not sure....

13 fanconvectors each holding roughly a litre
UFH in small bathroom so only a few litres
Towel rail in bathroom so only another few litres
Flow and return pipes in 106sqm house, no idea...

Overall not much really...

Cheers

TiredGeek

This has got me thinking, it sounds like your system is coming in somewhere between 30 and 60 litres max. I'm wondering just how accurate the 250 litre estimate for my system actually is...

I have a spare 1200 x 600 double rad, and some 3mtre lengths of 28mm pipe, I think I'll fill each up and measure the contents. That should give me a better starting point to work from

[Deleted User]

TiredGeek wrote: »

This has got me thinking, it sounds like your system is coming in somewhere between 30 and 60 litres max. I'm wondering just how accurate the 250 litre estimate for my system actually is...

I have a spare 1200 x 600 double rad, and some 3mtre lengths of 28mm pipe, I think I'll fill each up and measure the contents. That should give me a better starting point to work from

Good to hear from you.

Nothing like a bit of tinkering eh

Cardew

jeepjunkie wrote: »

What I'm basically saying is that it takes more energy to heat and maintain a large volume of water than a smaller amount. With the low grade heat from some [not all] ASHPs I can understand why Cardew bangs on about some systems having to run constantly to maintain house temp. This is something we have not found with our system which I put down to low running temps with a low volume of water when compared to oversized rads for example. Cheers

By low grade heat, I assume you mean lower temperatures?

There are several points to make.

The fundamental issue(problem) with ASHPs is that the CH water must be at a low temperature to get good efficiency.

With ‘conventional’ CH with gas/oil/lpg with very high outputs, the water can be at 80+C and thus warm a property quickly. So if you are out at work during the day, you can set the heating to come on, say 20 minutes before you return and arrive at a warm house.

Exactly the same overnight, heating on 20 minutes before you get up. So during the day if you are at work, and at night, you are not using fuel.

This simply isn’t the case with heat pumps, with both low output and low water temperatures they simply cannot heat up a property quickly. So to overcome this they have to run far longer during the day and night –which is wasted energy.

In the ‘Green’ forum of MSE the Energy Saving Trust have conceded the above point. i.e an output of, say, 10,000kWh from a heat pump is considerably less effective than 10,000kWh from conventional CH.

[Deleted User]

Cardew wrote: »

This simply isn’t the case with heat pumps, with both low output and low water temperatures they simply cannot heat up a property quickly. So to overcome this they have to run far longer during the day and night –which is wasted energy.

In the nicest possible way - what utter nonsense...

Fancoils/fan-convetors are designed to heat from cold very fast which they do in practice.

Our system does not heat the house during working hours nor during the night and yet here we are with a nice warm house with bills of little over a £1 a day come rain or shine... Not sure how wasted energy FITs in here... Indeed once the change to LEDs, loft insulation etc is completed I might get down to below a £1 a day sometimes. That would be total electric. Indeed I don't know many on gas/elec that gets down this low, certainly no one with a house this size/age...

Oh and the house is over 100 years old

If I believed all the tripe on various forums about how ASHPs cost fortunes to run and don't work in anything except when installed in new fangled ecoboxes I'd still be paying fortunes for LPG with MUG tattoo'd on my forehead ...

Cardew

jeepjunkie wrote: »

In the nicest possible way - what utter nonsense...

Fancoils/fan-convetors are designed to heat from cold very fast which they do in practice.

Our system does not heat the house during working hours nor during the night and yet here we are with a nice warm house with bills of little over a £1 a day come rain or shine... Not sure how wasted energy FITs in here... Indeed once the change to LEDs, loft insulation etc is completed I might get down to below a £1 a day sometimes. That would be total electric. Indeed I don't know many on gas/elec that gets down this low, certainly no one with a house this size/age...

Oh and the house is over 100 years old

If I believed all the tripe on various forums about how ASHPs cost fortunes to run and don't work in anything except when installed in new fangled ecoboxes I'd still be paying fortunes for LPG with MUG tattoo'd on my forehead ...

You really can do better than that – all your posts seem designed to show how clever you are to have bought an ASHP for £10,000+? and you take as personal criticism any post that suggests Heat Pumps are less than the perfect solution.

I suggest before repeating that my post is rubbish you read the following from the EST:

https://forums.moneysavingexpert.com/discussion/2867980

= Cardew

However that wasted heat brings me nicely to a subject I have raised many times on this forum, and as far as I am aware is not covered by the EST. Namely a major limitation of heat pumps.

We are all aware that for heat pumps to work efficiently the water in radiators or Under Floor Heating(UFH) has to be much lower than conventional CH. 35C to 45C is a typical temperature and as the water temperature increases the COP falls.

With my gas CH, with water temperature at up to 82C, I can set the heating to come on 20 minutes before I get up, or come home from work and return to a warm house.

Not so with Heat pumps, with the low water temperature it is necessary to run the system for much longer – even continuously – to ensure a warm house.


So if we consider a system with a COP of, say, 2.5, a 40kWh input will produce 100kWh. However exactly in line with your caveat on Storage Heater efficiency, a good proportion of the 100kWh can be ‘wasted’ when the house is unoccupied or occupants in bed.

This lowers the overall efficiency considerably when comparing to other heating systems.

Given the very disappointing results – particularly for ASHPs – on the heat pump trial you conducted, if the above is taken into account, it makes them an even less viable heating system – until they get matters sorted.

= Energy Saving Trust Representative

It is true that heat pumps will take longer to heat a house up than a gas boiler system. They also operate more efficiently when running for extended periods at a steady output, rather than switching on and off like a boiler. This does mean that, in order to achieve a high COP, the heating programmer has to be set to longer hours than an equivalent boiler system would require. In turn, this means that the house will lose more heat and the total heat required from the heat pump will be greater than the total heat required from a boiler.



This is one reason why we recommend that heat pumps are only installed in well insulated houses. If the house is well insulated then the additional heat loss from heating the house for longer will not be too great, and the increased efficiency from a well designed heat pump will easily more than compensate for this.





We would still be cautious of recommending heat pump installations in an old house with gas heating, even if it had basic insulation and draught proofing measures installed, as it is not clear there would be financial or CO2 savings. However, in a new build property we would expect the thermal performance of the building to be good enough to enable good heat pump systems to deliver net savings. And any house without a gas supply could potentially benefit from a heat pump along with standard insulation and draught proofing measures.

I will try and find a likely date for the second stage field trial publication, and post a separate answer.









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